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GL_ARB_pixel_buffer_object
homeprevnext Name
  
    ARB_pixel_buffer_object  
  
homeprevnext Name Strings
  
    GL_ARB_pixel_buffer_object  
  
homeprevnext Status
  
    Proposed.  
  
homeprevnext Contributors
  
    Ralf Biermann  
    Nick Carter  
    Derek Cornish  
    Matt Craighead  
    Mark Kilgard  
    Dale Kirkland  
    Jon Leech  
    Brian Paul  
    Thomas Roell  
    Ian Romanick  
    Jeremy Sandmel  
  
homeprevnext Contact
  
    Mark J. Kilgard, NVIDIA Corporation (mjk 'at' nvidia.com)  
    Ralf Biermann, NVIDIA Corporation (rbiermann 'at' nvidia.com)  
    Derek Cornish, NVIDIA Corporation (dcornish 'at' nvidia.com)  
  
homeprevnext IP Status
  
    Unknown.  
  
homeprevnext Version
  
    NVIDIA Date: December 8, 2004 (version 1.0)  
    $Date$ $Revision$  
    $Id: //sw/main/docs/OpenGL/specs/proposed/GL_ARB_pixel_buffer_object.txt#17 $  
  
homeprevnext Number
  
    ???  
  
homeprevnext Dependencies
  
    Written based on the wording of the OpenGL 2.0 specification.  
  
    Assumes support for (at least) OpenGL 1.5 or the  
    ARB_vertex_buffer_object extension.  
  
    NV_pixel_data_range affects the definition of this extension.  
  
    EXT_pixel_buffer_object interacts with this extension.  
  
homeprevnext Overview
  
    This extension expands on the interface provided by the  
    ARB_vertex_buffer_object extension (and later integrated into OpenGL  
    1.5) in order to permit buffer objects to be used not only with vertex  
    array data, but also with pixel data.  The intent is to provide more  
    acceleration opportunities for OpenGL pixel commands.  
  
    While a single buffer object can be bound for both vertex arrays and  
    pixel commands, we use the designations vertex buffer object (VBO)  
    and pixel buffer object (PBO) to indicate their particular usage in  
    a given situation.  
  
    Recall that buffer objects conceptually are nothing more than arrays  
    of bytes, just like any chunk of memory.  ARB_vertex_buffer_object  
    allows GL commands to source data from a buffer object by binding the  
    buffer object to a given target and then overloading a certain set of  
    GL commands' pointer arguments to refer to offsets inside the buffer,  
    rather than pointers to user memory.  An offset is encoded in a  
    pointer by adding the offset to a null pointer.  
  
    This extension does not add any new functionality to buffer objects  
    themselves.  It simply adds two new targets to which buffer objects  
    can be bound: GL_PIXEL_PACK_BUFFER and GL_PIXEL_UNPACK_BUFFER.  When a  
    buffer object is bound to the GL_PIXEL_PACK_BUFFER target, commands  
    such as glReadPixels pack (write) their data into a buffer object.  
    When a buffer object is bound to the GL_PIXEL_UNPACK_BUFFER target,  
    commands such as glDrawPixels and glTexImage2D unpack (read) their  
    data from a buffer object.  
  
    There are a several approaches to improve graphics performance  
    with PBOs.  Some of the most interesting approaches are:  
  
    - Streaming texture updates:  If the application uses  
      glMapBuffer/glUnmapBuffer to write its data for glTexSubImage into  
      a buffer object, at least one of the data copies usually required  
      to download a texture can be eliminated, significantly increasing  
      texture download performance.  
  
    - Streaming draw pixels: When glDrawPixels sources client memory,  
      OpenGL says the client memory can be modified immediately after the  
      glDrawPixels command returns without disturbing the drawn image.  
      This typically necessitates unpacking and copying the image prior  
      to glDrawPixels returning.  However, when using glDrawPixels with  
      a pixel pack buffer object, glDrawPixels may return prior to image  
      unpacking because future modification of the buffer data requires  
      explicit commands (glMapBuffer, glBufferData, or glBufferSubData).  
  
    - Asynchronous glReadPixels:  If an application needs to read back a  
      number of images and process them with the CPU, the existing GL  
      interface makes it nearly impossible to pipeline this operation.  
      The driver will typically send the hardware a readback command  
      when glReadPixels is called, and then wait for all of the data to  
      be available before returning control to the application.  Then,  
      the application can either process the data immediately or call  
      glReadPixels again; in neither case will the readback overlap with  
      the processing.  If the application issues several readbacks  
      into several buffer objects, however, and then maps each one to  
      process its data, then the readbacks can proceed in parallel with  
      the data processing.  
  
    - Render to vertex array:  The application can use a fragment  
      program to render some image into one of its buffers, then read  
      this image out into a buffer object via glReadPixels.  Then, it can  
      use this buffer object as a source of vertex data.  
  
homeprevnext Issues
  
    1)  How does this extension relate to ARB_vertex_buffer_object?  
  
        It builds on the ARB_vertex_buffer_object framework by adding  
        two new targets that buffers can be bound to.  
  
    2)  How does this extension relate to NV_pixel_data_range?  
  
        This extension relates to NV_pixel_data_range in the same way  
        that ARB_vertex_buffer_object relates to NV_vertex_array_range.  
        To paraphrase the ARB_vertex_buffer_object spec, here are the  
        main differences:  
  
        - Applications are no longer responsible for memory management  
          and synchronization.  
  
        - Applications may still access high-performance memory directly,  
          but this is optional, and such access is more restricted.  
  
        - Buffer changes (glBindBuffer) are generally expected to be  
          very lightweight, rather than extremely heavyweight  
          (glPixelDataRangeNV).  
  
        - A platform-specific allocator such as wgl/glXAllocateMemoryNV  
          is no longer required.  
  
    3)  Can a given buffer be used for both vertex and pixel data?  
  
        RESOLVED: YES.  All buffers can be used with all buffer bindings,  
        in whatever combinations the application finds useful.  Consider  
        yourself warned, however, by the following issue.  
  
    4)  May implementations make use of the target as a hint to select  
        an appropriate memory space for the buffer?  
  
        RESOLVED: YES, as long as such behavior is transparent to the  
        application.  Some implementations may choose, for example, that  
        they would rather stream vertex data from AGP memory, element  
        (index) data from video memory, and pixel data from video memory.  
        In fact, one can imagine arbitrarily complicated heuristics for  
        selecting the memory space, based on factors such as the target,  
        the "usage" argument, and the application's observed behavior.  
  
        While it is entirely legal to create a buffer object by binding  
        it to GL_ARRAY_BUFFER and loading it with data, then using it  
        with the GL_PIXEL_UNPACK_BUFFER_ARB or GL_PIXEL_PACK_BUFFER_ARB  
        binding, such behavior is liable to confuse the driver and may  
        hurt performance.  If the driver implemented the hypothetical  
        heuristic described earlier, such a buffer might have already  
        been located in AGP memory, and so the driver would have to choose  
        between two bad options: relocate the buffer into video memory, or  
        accept lower performance caused by streaming pixel data from AGP.  
  
    5)  Should all pixel path commands be supported, or just a subset  
        of them?  
  
        RESOLVED: ALL.  While there is little reason to believe that,  
        say, glConvolutionFilter2D would benefit from this extension,  
        there is no reason _not_ to support it.  The complete list of  
        commands affected by this extension is listed in issues 17 and 18.  
  
    6)  Should glPixelMap and glGetPixelMap be supported?  
  
        RESOLVED: YES.  They're not really pixel path operations, but,  
        again, there is no good reason to omit operations, and they _are_  
        operations that pass around big chunks of pixel-related data.  
        If we support glPolygonStipple, surely we should support this.  
  
    7)  How does the buffer binding state push/pop?  
  
        RESOLVED: As part of the pixel store client state.  This is  
        analogous to how the ARB_vertex_buffer_object bindings  
        pushed/popped as part of the vertex array client state.  
  
    8)  Should NV_pixel_data_range (PDR) be used concurrently with pixel  
        buffer objects ?  
  
        RESOLVED: NO. While it would be possible to allocate a memory  
        range for PDR, using a pointer into this memory range with one  
        of the commands affected by PBOs will not work if a pixel buffer  
        object other than zero is bound to the buffer binding point  
        affecting the command.  
  
        Pixel buffer objects always have higher precedence than PDR.  
  
    9)  Should the INVALID_OPERATION error be generated if a pixel  
        command would access data outside the range of the bound PBO?  
  
        RESOLVED:  YES.  This requires considering the command parameters  
        (such as width/height/depth/format/type/pointer), the current  
        pixel store (pack/unpack) state, and the command operation itself  
        to determine the maximum addressed byte for the pixel command.  
  
        Brian Paul strongly recommends this behavior.  
  
        This behavior should increase the reliability of using PBO and  
        guard against programmer mistakes.  
  
        This is particularly important for glReadPixels where returning  
        data into a region outside the PBO could cause corruption of  
        application memory.  
  
        Such bounds checking is substantially more expensive for VBO  
        accesses because bounds checking on a per-vertex element basis  
        for each of multiple enabled vertex arrays prior to performing  
        the command compromises the performance justification of VBO.  
  
    10) If a pixel command with a bound PBO accesses data outside the  
        range of the PBO, thereby generating a GL_INVALID_OPERATION error,  
        can the pixel command end up being partially processed?  
  
        RESOLVED:  NO.  As for all GL errors excepting GL_OUT_OF_MEMORY  
        situations, "the command generating the error is ignored so that  
        it has no effect on GL state or framebuffer contents."  
  
        This means implementations must determine before the pixel command  
        is performed whether the resulting read or write operations on  
        the bound PBO will exceed the size of the PBO.  
  
        This means an implementation is NOT allowed to detect out of  
        bounds accesses in the middle of performing the command.  
  
    11) How expensive is it to predetermine whether a pixel command  
        accessing a PBO would have an out of bounds access?  
  
        See the "Appendix on Pack/Unpack Range" to see the computations  
        involved in computing the access limit.  
  
        Implementations can further specialize and optimize the check  
        to make this out of bounds checking negligible for any sizable  
        pixel payload.  
  
    12) Should feedback and select buffers output results into a  
        buffer object?  
  
        RESOLVED:  That might be useful for a future extension but is  
        not appropriate for this extension.  New targets (other than  
        PIXEL_PACK_BUFFER_ARB and PIXEL_UNPACK_BUFFER_ARB) make sense.  
  
    13) Should NV_pixel_data_range interactions be documented in  
        this specification?  
  
        RESOLVED:  YES.  Interactions with NV_pixel_data_range are  
        important to document to facilitate developers migrating to  
        the multi-vendor ARB_pixel_buffer_object extension.  Discussion of  
        interactions is limited to the issues and example usage sections.  
  
        Other ARB specifications follow this policy, and Jon Leech agrees  
        with this policy.  
  
    14) Should an INVALID_OPERATION error be generated if the offset  
        within a pixel buffer to a datum comprising of N basic machine  
        units is not a multiple of N?  
  
        RESOLVED:  YES.  This was stated for VBOs but no error was  
        defined if the rule was violated.  Perhaps this needs to be  
        better specified for VBO.  
  
        For PBO, it is reasonable and cheap to enforce the alignment rule.  
        For pixel commands it means making sure the offset is evenly  
        divisible by the component or group size in basic machine units.  
  
        This check is independent of the pixel store state because the  
        pixel store state is specified in terms of pixels (not basic  
        machine units) so pixel store addressing cannot create an  
        unaligned access as long as the base offset is aligned.  
  
        Certain commands (specifically glPolygonStipple,  
        glGetPolygonStipple, glBitmap, glCompressedTexImage1D,  
        glCompressedTexImage2D, glCompressedTexImage3D,  
        glCompressedTexSubImage1D, glCompressedTexSubImage2D,  
        glCompressedTexSubImage3D, and glGetCompressedTexImage) are not  
        affected by this error because the data accessed is addressed  
        at the granularity of basic machine units.  
  
    15) Various commands do not make explicit reference to supporting  
        packing or unpacking from a pixel buffer object but rather specify  
        that parameters are handled in the same manner as glDrawPixels,  
        glReadPixels, or the glCompressedTexImage commands.  So do such  
        commands (example: glCompressedTexSubImage2D) use pixel buffers?  
  
        RESOLVED:  YES.  Commands that have their behavior defined based  
        on commands that read or write from pixel buffers will themselves  
        read or write from pixel buffers.  Relying on this reduces the  
        amount of specification language to be updated.  
  
    16) What is the complete list of commands that can unpack (read)  
        pixels from the current pixel unpack buffer object?  
  
            glBitmap  
            glColorSubTable  
            glColorTable  
            glCompressedTexImage1D  
            glCompressedTexImage2D  
            glCompressedTexImage3D  
            glCompressedTexSubImage1D  
            glCompressedTexSubImage2D  
            glCompressedTexSubImage3D  
            glConvolutionFilter1D  
            glConvolutionFilter2D  
            glDrawPixels  
            glPixelMapfv  
            glPixelMapuiv  
            glPixelMapusv  
            glPolygonStipple  
            glSeparableFilter2D  
            glTexImage1D  
            glTexImage2D  
            glTexImage3D  
            glTexSubImage1D  
            glTexSubImage2D   
            glTexSubImage3D   
  
    17) What is the complete list of commands that can pack (write)  
        pixels into the current pixel pack buffer object?  
  
            glGetCompressedTexImage  
            glGetConvolutionFilter  
            glGetHistogram  
            glGetMinmax  
            glGetPixelMapfv  
            glGetPixelMapuiv  
            glGetPixelMapusv  
            glGetPolygonStipple  
            glGetSeparableFilter,  
            glGetTexImage  
            glReadPixels  
  
    18) How does support for pixel buffer objects affect the GLX protocol?  
  
        UNRESOLVED:  See the "GLX Protocol" section.  
  
    19) Prior to this extension, passing zero for the data argument of  
        glTexImage1D, glTexImage2D, and glTexImage3D defined a texture  
        image level without supplying an image.  How does this behavior  
        change with this extension?  
  
        RESOLVED:  The "unspecified image" behavior of the glTexImage  
        calls only applies when bound to a zero pixel unpack buffer  
        object.  
  
        When bound to a non-zero pixel unpack buffer object, the data  
        argument to these calls is treated as an offset rather than  
        a pointer so zero is a reasonable and even likely value that  
        corresponds to the very beginning of the buffer object's data.  
  
        So to create a texture image level with unspecified image data,  
        you MUST bind to the zero pixel unpack buffer object.  
  
        See the ammended language at the end of section 3.8.1.  
  
    20) How does this extension support video frame grabbers?  
  
        RESOLVED:  This extension extends buffer objects so they can  
        operate with pixel commands, rather than just vertex array  
        commands.  
  
        We anticipate that a future extension may provide a mechanism  
        for transferring video frames from video frame grabber hardware  
        or vertices from motion capture hardware (or any other source  
        of aquired real-time data) directly into a buffer object to  
        eliminate a copy.  Ideally, such transfers would be possible  
        without requiring mapping of the buffer object.  But this  
        extension does not provide such functionality.  
  
        We anticipate such functionality to involve binding a buffer  
        object to a new target type, configuring a source (or sink) for  
        data (video frames, motion capture vertex sets, etc.), and then  
        commands to initiate data transfers to the bound buffer object.  
  
    21) Can this ARB extension share the same enumerants with the EXT  
        version of this functionality?  
  
        RESOLVED:  YES.  The ARB extension is functionally compatible  
        with EXT_pixel_buffer_object except that the ARB version adds  
        additional error checks for alignment and buffer bounds checking.  
  
        The EXT behavior in the case of alignment violations and buffer  
        bounds overflow are technically undefined.  The ARB extension  
        simply defines the EXT extension's undefined behavior to be an  
        OpenGL error.  
  
        Using the same enumerants with firmed up error checking (that  
        would otherwise indicate buggy usage) is preferable to two sets  
        of enumerants where the older EXT set simply allows sloppy usage.  
  
    22) The expected usage parameters (GL_STREAM_DRAW, etc.) for  
        glBufferData are not clearly specified.  How can they be improved?  
  
        RESOLVED:  To improve the clarity, replace the phrase "specified  
        once" with "specified once per repetition of the usage pattern" so  
        that it is clear for the STREAM_* usage modes (and the STATIC_*  
        usage modes too, just much less frequently) that the repeated  
        specification is part of a pattern and it is expected that the  
        buffer can be, and will be for the STREAM_* usage patterns,  
        specified again after being used and this is likely to repeat.  
  
        Additionally, the *_COPY and *_DRAW usage patterns can source  
        the data with "a GL drawing command" but also with image  
        specification commands so change this phrase to "a GL drawing  
        or image specification command."  
  
    23) Is this the "right" way to expose render-to-vertex-array?  
  
        DISCUSSION:  You can use this extension to render an image  
        into a framebuffer, copy the pixels into a buffer object with  
        glReadPixels, and then configure vertex arrays to source the pixel  
        data as vertex attributes.  This necessarily involves a copy  
        from the framebuffer to the buffer object.  Future extensions  
        may provide mechanisms for copy-free render-to-vertex-array  
        capabilities but that is not a design goal of this extension.  
  
homeprevnext New Procedures and Functions
  
    None.  
  
  
homeprevnext New Tokens
  
    Accepted by the <target> parameters of BindBuffer, BufferData,  
    BufferSubData, MapBuffer, UnmapBuffer, GetBufferSubData,  
    GetBufferParameteriv, and GetBufferPointerv:  
  
        PIXEL_PACK_BUFFER_ARB                        0x88EB  
        PIXEL_UNPACK_BUFFER_ARB                      0x88EC  
  
    Accepted by the <pname> parameter of GetBooleanv, GetIntegerv,  
    GetFloatv, and GetDoublev:  
  
        PIXEL_PACK_BUFFER_BINDING_ARB                0x88ED  
        PIXEL_UNPACK_BUFFER_BINDING_ARB              0x88EF  
  
  
homeprevnext Additions to Chapter 2 of the GL Specification (OpenGL Operation)
  
    None  
  
 -- Section 2.9 "Buffer Objects"  
  
    Replace the first two paragraphs with:  
  
    "The vertex data arrays described in section 2.8 are stored in  
    client memory.  It is sometimes desirable to store frequently accessed  
    client data, such as vertex array and pixel data, in high-performance  
    server memory.  GL buffer objects provide a mechanism for clients to  
    use to allocate, initialize, and access such memory."  
  
    The name space for buffer objects is the unsigned integer, with zero  
    reserved for the GL.  A buffer object is created by binding an unused  
    name to a buffer target.  A buffer object is bound by calling  
  
       void BindBuffer(enum target, uint buffer);  
  
    /target/ must be one of ARRAY_BUFFER, ELEMENT_ARRAY_BUFFER,  
    PIXEL_UNPACK_BUFFER_ARB, or PIXEL_PACK_BUFFER_ARB.  The ARRAY_BUFFER  
    target is discussed in section 2.9.1  The ELEMENT_ARRAY_BUFFER target  
    is discussed in section 2.9.2.  The PIXEL_UNPACK_BUFFER_ARB and  
    PIXEL_PACK_BUFFER_ARB targets are discussed later in sections 3.6,  
    4.3.2, and 6.1.  If the buffer object named /buffer/ has not been  
    previously bound or has been deleted since the last binding, the  
    GL creates a new state vector, initialized with a zero-sized memory  
    buffer and comprising the state values listed in table 2.6."  
  
    Replace the 5th paragraph with:  
  
    "Initially, each buffer object target is bound to zero.  There is  
    no buffer object corresponding to the name zero so client attempts  
    to modify or query buffer object state for a target bound to zero  
    generate an INVALID_OPERATION error."  
  
    Replace the phrase listing the valid targets for BufferData in the  
    9th paragraph with:  
  
    "with target set to one of ARRAY_BUFFER, ELEMENT_ARRAY_BUFFER,  
    PIXEL_UNPACK_BUFFER_ARB, or PIXEL_PACK_BUFFER_ARB,"  
  
    In the 10th paragraph describing buffer object usage modes, replace  
    the phrase "specified once" with "specified once per repetition of  
    the usage pattern" for the STREAM_* and STATIC_* usage values.  
  
    Also in the 10th paragraph describing buffer object usage modes,  
    replace the phrases "of a GL drawing command." and "for GL drawing  
    commands." with "for GL drawing and image specification commands." for  
    the *_DRAW and *_COPY usage values.  
  
    Replace the phrase listing the valid targets for BufferSubData in  
    the 15th paragraph with:  
  
    "with target set to one of ARRAY_BUFFER, ELEMENT_ARRAY_BUFFER,  
    PIXEL_UNPACK_BUFFER_ARB, or PIXEL_PACK_BUFFER_ARB."  
  
    Replace the phrase listing the valid targets for MapBuffer in the  
    16th paragraph with:  
  
    "with target set to one of ARRAY_BUFFER, ELEMENT_ARRAY_BUFFER,  
    PIXEL_UNPACK_BUFFER_ARB, or PIXEL_PACK_BUFFER_ARB."  
  
    Replace the phrase listing the valid targets for UnmapBuffer in the  
    21st paragraph with:  
  
    "with target set to one of ARRAY_BUFFER, ELEMENT_ARRAY_BUFFER,  
    PIXEL_UNPACK_BUFFER_ARB, or PIXEL_PACK_BUFFER_ARB."  
  
 -- Section 2.9.2 "Array Indices in Buffer Objects"  
  
    Delete the 3rd paragraph that explains how the ELEMENT_ARRAY_BUFFER  
    target is acceptable for the commands specified in section 2.9.  
    The updated section 2.9 language already says this.  
  
 -- NEW Section 2.9.3 "Buffer Object Required State"  
  
    "The state required to support buffer objects consists of binding  
    names for the array buffer, element buffer, pixel unpack buffer, and  
    pixel pack buffer.  Additionally, each vertex array has an associated  
    binding so there is a buffer object binding for each of the vertex  
    array, normal array, color array, index array, multiple texture  
    coordinate arrays, edge flag array, secondary color array, fog  
    coordinate array, and vertex attribute arrays.  The initial values for  
    all buffer object bindings is zero.  
  
    The state of each buffer object consists of a buffer size in basic  
    machine units, a usage parameter, an access parameter, a mapped  
    boolean, a pointer to the mapped buffer (NULL if unmapped), and the  
    sized array of basic machine units for the buffer data."  
  
  
homeprevnext Additions to Chapter 3 of the 1.2.1 Specification (Rasterization)
  
 -- Section 3.6 "Pixel Rectangles"  
  
    Replace the 1st sentence in the 2nd paragraph:  
  
    "A number of parameters control the encoding of pixels in buffer  
    object or client memory (for reading and writing) and how pixels  
    are processed before being placed in or after being read from the  
    framebuffer (for reading, writing, and copying)."  
  
 -- RENAME Section 3.6.1 "Pixel Storage Modes and Pixel Buffer Objects"  
  
    Add to the end of the section:  
  
    "In addition to storing pixel data in client memory, pixel data  
    may also be stored in buffer objects (described in section 2.9).  
    The current pixel unpack and pack buffer objects are designated  
    by the PIXEL_UNPACK_BUFFER_ARB and PIXEL_PACK_BUFFER_ARB targets  
    respectively.  
  
    Initially, zero is bound for the PIXEL_UNPACK_BUFFER_ARB, indicating  
    that image specification commands such as DrawPixels source their  
    pixels from client memory pointer parameters.  However, if a non-zero  
    buffer object is bound as the current pixel unpack buffer, then  
    the pointer parameter is treated as an offset into the designated  
    buffer object."  
  
 -- Section 3.6.3 "Pixel Transfer Modes", page 116.  
  
    Replace the last phrase in the 2nd paragraph with:  
  
    "and /values/ refers to an array of size map values."  
  
    [values is no longer necessarily a pointer.]  
  
    Add the following paragraph after the third paragraph:  
  
    "If a pixel unpack buffer is bound (as indicated by a non-zero  
    value of PIXEL_UNPACK_BUFFER_BINDING_ARB), /values/ is an offset  
    into the pixel unpack buffer; otherwise, /values/ is a pointer to a  
    block client memory.  All pixel storage and pixel transfer modes are  
    ignored when specifying a pixel map.  n machine units are read where  
    n is the /size/ of the pixel map times the size of a float, uint,  
    or ushort datum in basic machine units, depending on the respective  
    PixelMap version.  If a pixel unpack buffer object is bound and data+n  
    is greater than the size of the pixel buffer, INVALID_OPERATION  
    results.  If a pixel unpack buffer object is bound and /values/ is  
    not evenly divisible into the number of basic machine units needed  
    to store in memory a float, uint, or ushort datum depending on their  
    respective PixelMap version, INVALID_OPERATION results."  
  
 -- Section 3.6.4 "Rasterization of Pixel Rectangles", page 126.  
  
    Change the 1st sentence of the 1st paragraph to read:  
  
    "The process of drawing pixels encoded in buffer objects or client  
    memory is diagrammed in figure 3.7."  
  
    Change the 4th sentence of the 2nd paragraph to read:  
  
    "/data/ refers to the data to be drawn."  
  
    [data is no longer necessarily a pointer.]  
  
    Change the initial phrase in the 1st sentence of the 1st paragraph  
    after "Unpacking" to read:  
  
    "Data are taken from the currently bound pixel unpack buffer or  
    client memory as a sequence of..."  
  
    Insert this paragraph after the 1st paragraph after "Unpacking":  
  
    "If a pixel unpack buffer is bound (as indicated by a non-zero  
    value of PIXEL_UNPACK_BUFFER_BINDING_ARB), /data/ is an offset  
    into the pixel unpack buffer and the pixels are unpacked from the  
    buffer relative to this offset; otherwise, /data/ is a pointer to  
    a block client memory and the pixels are unpacked from the client  
    memory relative to the pointer.  If a pixel unpack buffer object  
    is bound and unpacking the pixel data according to the process  
    described below would access memory beyond the size of the pixel  
    unpack buffer's memory size, INVALID_OPERATION results.  If a pixel  
    unpack buffer object is bound and /data/ is not evenly divisible  
    into the number of basic machine units needed to store in memory the  
    corresponding GL data type from table 3.5 for the /type/ parameter,  
    INVALID_OPERATION results."  
  
 -- Section 3.8.1 "Texture Image Specification", page 150.  
  
    Replace the last phrase in the 2nd to last sentence in the 1st  
    paragraph with:  
  
    "and a reference to the image data in the currently bound pixel unpack  
    buffer or client memory."  
  
    Replace the 1st sentence in the 13th paragraph with:  
  
    "The image itself (referred to by /data/) is a sequence of groups  
    of values."  
  
    Replace the last paragraph with:  
  
    "If the data argument of TexImage1D, TexImage2D, or TexImage3D  
    is a null pointer (a zero-valued pointer in the C implementation)  
    and the pixel unpack buffer object is zero, a one-, two-, or three-  
    dimensional texture array is created with the specified target, level,  
    internalformat, width, height, and depth border, but with unspecified  
    image contents.  In this case no pixel values are access in client  
    memory, and no pixel processing is performed.  Errors are generated,  
    however, exactly as though the data pointer were valid.  Otherwise if  
    the pixel unpack buffer object is non-zero, the data argument is  
    treatedly normally to refer to the beginning of the pixel unpack  
    buffer object's data."  
  
 -- Section 3.8.3 "Compressed Texture Images", page 163.  
  
    Replace the 3rd sentence of the 2nd paragraph with:  
  
    "/data/ refers to compressed image data stored in the compressed  
    image format corresponding to internalformat.  If a pixel  
    unpack buffer is bound (as indicated by a non-zero value of  
    PIXEL_UNPACK_BUFFER_BINDING_ARB), /data/ is an offset into the  
    pixel unpack buffer and the compressed data is read from the buffer  
    relative to this offset; otherwise, /data/ is a pointer to a block  
    client memory and the compressed data is read from the client memory  
    relative to the pointer."  
  
    Replace the 2nd sentence in the 3rd paragraph with:  
  
    "Compressed texture images are treated as an array of /imageSize/  
    ubytes relative to /data/.  If a pixel unpack buffer object is bound  
    and data+imageSize is greater than the size of the pixel buffer,  
    INVALID_OPERATION results."  
  
homeprevnext Additions to Chapter 4 of the 1.2.1 Specification (Per-Fragment Operations and the Frame Buffer)
  
 -- Section 4.3.2 "Reading Pixels", page 219.  
  
    Replace 1st sentence of the 1st paragraph with:  
  
    "The method for reading pixels from the framebuffer and placing them in  
    pixel pack buffer or client memory is diagrammed in figure 4.2."  
  
    Add this paragraph after the 1st paragraph:  
  
    "Initially, zero is bound for the PIXEL_PACK_BUFFER_ARB, indicating  
    that image read and query commands such as ReadPixels return  
    pixels results into client memory pointer parameters.  However, if  
    a non-zero buffer object is bound as the current pixel pack buffer,  
    then the pointer parameter is treated as an offset into the designated  
    buffer object."  
  
    Rename "Placement in Client Memory" to "Placement in Pixel Pack  
    Buffer or Client Memory".  
  
    Insert this paragraph after the newly renamed "Placement in Pixel  
    Pack Buffer or Client Memory" heading:  
  
    "If a pixel pack buffer is bound (as indicated by a non-zero value  
    of PIXEL_PACK_BUFFER_BINDING_ARB), /data/ is an offset into the  
    pixel pack buffer and the pixels are packed into the  
    buffer relative to this offset; otherwise, /data/ is a pointer to a  
    block client memory and the pixels are packed into the client memory  
    relative to the pointer.  If a pixel pack buffer object is bound and  
    packing the pixel data according to the pixel pack storage state  
    would access memory beyond the size of the pixel pack buffer's  
    memory size, INVALID_OPERATION results.  If a pixel pack buffer object  
    is bound and /data/ is not evenly divisible into the number of basic  
    machine units needed to store in memory the corresponding GL data type  
    from table 3.5 for the /type/ parameter, INVALID_OPERATION results."  
  
  
homeprevnext Additions to Chapter 5 of the 1.2.1 Specification (Special Functions)
  
    None  
  
  
homeprevnext Additions to Chapter 6 of the 1.2.1 Specification (State and State Requests)
  
 -- Section 6.1.3 "Enumerated Queries".  
  
    After the sentence in the last paragraph describing GetPixelMap, add:  
  
    "The GetPixelMapfv, GetPixelMapuiv, and GetPixelMapusv commands  
    write all the values in the named pixel map to /data/.  If a  
    pixel pack buffer is bound (as indicated by a non-zero value of  
    PIXEL_PACK_BUFFER_BINDING_ARB), /data/ is an offset into the pixel  
    pack buffer; otherwise, /data/ is a pointer to a block client memory.  
    All pixel storage and pixel transfer modes are ignored when returning a  
    pixel map.  n machine units are written where n is the size of the  
    pixel map times the size of FLOAT, UNSIGNED_INT, or UNSIGNED_SHORT  
    respectively in basic machine units.  If a pixel pack buffer object  
    is bound and data+n is greater than the size of the pixel buffer,  
    generate INVALID_OPERATION."  
  
 -- Section 6.1.4 "Texture Queries".  
  
    Remove the mention of img in the last phrase in the last sentence  
    of the 1st paragraph so the sentence reads:  
  
    "lod is a level-of-detail number, format is a pixel format from  
    table 3.6, and type is a pixel type from table 3.5."  
  
    Replace the 3rd sentence of the 2nd paragraph with:  
  
    "These groups are then packed and placed in client or pixel buffer  
    object memory.  If a pixel pack buffer is bound (as indicated by a  
    non-zero value of PIXEL_PACK_BUFFER_BINDING_ARB), /img/ is an offset  
    into the pixel pack buffer; otherwise, /img/ is a pointer to a block  
    client memory."  
  
    Add to the end of the 4th paragraph:  
  
    "If a pixel pack buffer object is bound and packing the texture  
    image into the buffer's memory would exceed the size of the buffer,  
    generate INVALID_OPERATION."  
  
    Replace the 2nd sentence of the 5th paragraph with:  
  
    "When called, GetCompressedTexImage writes n ubytes of compressed  
    image data to the pixel pack buffer or client memory pointed to by  
    ptr, where n is the texture image's TEXTURE_COMPRESSED_IMAGE_SIZE  
    value.  
  
    Add to the end of the 6th paragraph:  
  
    "If a pixel pack buffer object is bound and ptr+n is greater than  
    the size of the buffer, generate INVALID_OPERATION."  
  
 -- Section 6.1.5 "Stipple Query".  
  
    "The pattern is packed into client or pixel pack buffer memory  
    according to the procedures given in section 4.3.2 for ReadPixels;  
    ..."  
  
 -- Section 6.1.7 "Color Table Query".  
  
    "The one-dimensional color table image is returned to client or  
    pixel pack buffer memory starting at table."  
  
 -- Section 6.1.8 "Convolution Query".  
  
    "The one-dimensional or two-dimensional image is returned to client  
    or pixel pack buffer memory starting at image."  
  
    "The row and column images are returned to client or pixel pack  
    buffer memory starting at row and column respectively."  
  
 -- Section 6.1.9 "Histogram Query".  
  
    "The one-dimensional histogram table image is returned to client or  
    pixel pack buffer memory starting at values."  
  
 -- Section 6.1.10 "Minmax Query".  
  
    "A one-dimensional image of width 2 is returned to client or pixel  
    pack buffer memory starting at values."  
  
 -- Section 6.1.13 "Buffer Object Queries".  
  
    Change the 2nd sentence of the 2nd paragraph to read:  
  
    "target is ARRAY_BUFFER, ELEMENT_ARRAY_BUFFER, PIXEL_PACK_BUFFER_ARB,  
    or PIXEL_UNPACK_BUFFER_ARB."  
  
    Change the last phrase in the 1st sentence of the 4th paragraph to:  
  
    "with target set to ARRAY_BUFFER, ELMENT_ARRAY_BUFFER,  
    PIXEL_PACK_BUFFER_ARB, or PIXEL_UNPACK_BUFFER_ARB and pname set  
    to BUFFER_MAP_POINTER."  
  
  
homeprevnext GLX Protocol
  
    XXX still-in-progress  
  
    (ARB_vertex_buffer_object has similar issues and lacks specified  
    GLX protocol for its functionality.  This discussion just addresses  
    the issues created by pixel buffer objects, not buffer objects  
    in general.)  
  
    Pixel buffers, like texture objects and display lists, are server-side  
    state.  
  
    Prior to pixel buffer objects, pixel storage state for image packing  
    and unpacking was considered client-side state.  However, pixel  
    buffers create the new situation where the server performs packing  
    and unpacking into server-side pixel buffers.  
  
    The GLX protocol is designed so that the amount of unpacking done  
    by the client is parameterized with the request.  In other words,  
    the client can do as much unpacking as it wants, and then tell the  
    server what unpacking remains to be done by sending the appropriate  
    pixel storage parameters along with the image.  
  
    This means the GLX protocol for rendering commands involving pixel  
    data includes pixel store state for unpacking.  
  
    This means, in theory, the existing protocol for rendering commands  
    with pixel data is sufficient for manipulating pixel buffers.  
    A command (for example, glDrawPixels) could build a protocol request  
    containing the current pixel unpack state and specify zero bytes of  
    image payload when operating on a pixel buffer object.  
  
    In practice, while this addresses command requiring unpacking of  
    pixel data, commands that require packing of pixel data (for example,  
    glReadPixels) to return pixel data do not have protocol fields for  
    pixel store pack state.  
  
    Fortunately, the GLX protocol, through foresight or oversight,  
    has GLX protocol and non-rendering command opcodes (109 and 110)  
    assigned for glPixelStoref and glPixelStorei respectively.  
  
    It is better to use the existing protocol to send glPixelStorei and  
    glPixelStoref GLX commands.  This solves the problem of server-side  
    pixel state the same way for both pack and unpack state.  It may also  
    allow implementations to minimize validation overhead for pixel  
    commands because the pixel store modes are stateful rather than  
    being parameters sent with every pixel command.  
  
    To avoid creating useless protocol overhead for applications not using  
    pixel buffer objects, and hence not requiring server-side knowledge  
    of pixel store state, the GLX client library is free to defer pixel  
    store commands until just prior to pixel commands operating on pixel  
    buffer objects that require server-side pixel store state.  
  
    There is no GLX protocol however for glPushClientAttrib and  
    glPopClientAttrib.  New protocol should be specified for these  
    commands.  These commands are also needed for vertex buffer objects  
    because the vertex array state becomes server-side.  
  
    When bound to an pixel unpack buffer object, the pixel payload for a  
    non-reply pixel command (for example, glTexImage2D) can be ignored.  
    In fact, GLX client implementations are expected to send zero bytes  
    of pixel payload in this case.  
  
    When bound to a pixel pack buffer object, the reply for pixel commands  
    that return pixel data (for example, glReadPixels) is not required  
    since the pixel data is actually transferred to the server-side pixel  
    pack buffer object.  Indeed, forcing an unnecessary reply would hinder  
    the performance advantages of using pixel buffer objects  
  
    Therefore, protocol for "no reply" version of the following commands  
    is specified:  
  
        GetCompressedTexImage_noreply  
        GetConvolutionFilter_noreply  
        GetHistogram_noreply  
        GetMinmax_noreply  
        GetPixelMapfv_noreply  
        GetPixelMapuiv_noreply  
        GetPixelMapusv_noreply  
        GetPolygonStipple_noreply  
        GetSeparableFilter,_noreply  
        GetTexImage_noreply  
        ReadPixels_noreply  
  
    If a "no reply" command is sent when the current pixel pack  
    buffer object binding is zero, a GLXBadContextState error should  
    be generated by the server.  
  
  
homeprevnext Errors
  
    INVALID_ENUM is generated if the <target> parameter of  
    BindBuffer, BufferData, BufferSubData, MapBuffer, UnmapBuffer,  
    GetBufferSubData, GetBufferParameteriv, or GetBufferPointerv is not  
    one of ARRAY_BUFFER, ELEMENT_ARRAY_BUFFER, PIXEL_PACK_BUFFER_ARB,  
    or PIXEL_UNPACK_BUFFER_ARB.  
  
    INVALID_OPERATION is generated if Bitmap, ColorSubTable, ColorTable,  
    CompressedTexImage1D, CompressedTexImage2D, CompressedTexImage3D,  
    CompressedTexSubImage1D, CompressedTexSubImage2D,  
    CompressedTexSubImage3D, ConvolutionFilter1D, ConvolutionFilter2D,  
    DrawPixels, PixelMapfv, PixelMapuiv, PixelMapusv, PolygonStipple,  
    SeparableFilter2D, TexImage1D, TexImage2D, TexImage3D, TexSubImage1D,  
    TexSubImage2D, or TexSubImage3D would unpack (read) data from the  
    currently bound PIXEL_UNPACK_BUFFER_ARB buffer object such that  
    the memory reads required for the command would exceed the memory  
    (data store) size of the buffer object.  
  
    INVALID_OPERATION is generated if GetColorTable,  
    GetCompressedTexImage, GetConvolutionFilter, GetHistogram, GetMinmax,  
    GetPixelMapfv, GetPixelMapuiv, GetPixelMapusv, GetPolygonStipple,  
    GetSeparableFilter, GetTexImage, or ReadPixels would pack (write) data  
    to the currently bound PIXEL_PACK_BUFFER_ARB buffer object such that  
    the memory writes required for the command would exceed the memory  
    (data store) size of the buffer object.  
  
    INVALID_OPERATION is generated by GetColorTable, GetConvolutionFilter,  
    GetHistogram, GetMinmax, GetSeparableFilter, GetTexImage and ReadPixels  
    if the current PIXEL_PACK_BUFFER_BINDING_ARB value is non-zero and the  
    table/image/values/span/img/data parameter is not evenly divisible  
    into the number of basic machine units needed to store in memory a  
    datum indicated by the type parameter.  
  
    INVALID_OPERATION is generated by ColorTable, ColorSubTable,  
    ConvolutionFilter2D, ConvolutionFilter1D, SeparableFilter2D,  
    TexImage1D, TexImage2D, TexImage3D, TexSubImage1D,  
    TexSubImage2D, TexSubImage3D, and DrawPixels if the current  
    PIXEL_UNPACK_BUFFER_BINDING_ARB value is non-zero and the data  
    parameter is not evenly divisible into the number of basic machine  
    units needed to store in memory a datum indicated by the type  
    parameter.  
  
    INVALID_OPERATION is generated by GetPixelMapfv if the current  
    PIXEL_PACK_BUFFER_BINDING_ARB value is non-zero and the data parameter  
    is not evenly divisible into the number of basic machine units needed  
    to store in memory a float datum.  
  
    INVALID_OPERATION is generated by GetPixelMapuiv if the current  
    PIXEL_PACK_BUFFER_BINDING_ARB value is non-zero and the data parameter  
    is not evenly divisible into the number of basic machine units needed  
    to store in memory a uint datum.  
  
    INVALID_OPERATION is generated by GetPixelMapusv if the current  
    PIXEL_PACK_BUFFER_BINDING_ARB value is non-zero and the data parameter  
    is not evenly divisible into the number of basic machine units needed  
    to store in memory a ushort datum.  
  
    INVALID_OPERATION is generated by PixelMapfv if the current  
    PIXEL_UNPACK_BUFFER_BINDING_ARB value is non-zero and the data  
    parameter is not evenly divisible into the number of basic machine  
    units needed to store in memory a float datum.  
  
    INVALID_OPERATION is generated by PixelMapuiv if the current  
    PIXEL_UNPACK_BUFFER_BINDING_ARB value is non-zero and the data  
    parameter is not evenly divisible into the number of basic machine  
    units needed to store in memory a uint datum.  
  
    INVALID_OPERATION is generated by PixelMapusv if the current  
    PIXEL_UNPACK_BUFFER_BINDING_ARB value is non-zero and the data  
    parameter is not evenly divisible into the number of basic machine  
    units needed to store in memory a ushort datum.  
  
  
homeprevnext Dependencies on EXT_pixel_buffer_object
  
    When this extension is supported, the EXT_pixel_buffer_object  
    functionality adopts the tighter alignment and buffer bounds overflow  
    error generation behavior of ARB_pixel_buffer_object (previously,  
    EXT_pixel_buffer_object was not explicit about what happened in  
    these situations).  This is because the two extensions share the  
    same enumerants.  
  
homeprevnext Dependencies on NV_pixel_data_range
  
    A non-zero pixel pack buffer binding takes priority over the  
    READ_PIXEL_DATA_RANGE_NV  enable.  
  
    A non-zero pixel unpack buffer binding takes priority over the  
    WRITE_PIXEL_DATA_RANGE_NV enable.  
  
  
homeprevnext New State
  
(table 6.20, Pixels, p. 235)  
  
                                                         Initial  
    Get Value                        Type   Get Command  Value    Sec     Attribute  
    -------------------------------  ----   -----------  -------  ------  -----------  
    PIXEL_PACK_BUFFER_BINDING_ARB    Z+     GetIntegerv  0        4.3.5   pixel-store  
    PIXEL_UNPACK_BUFFER_BINDING_ARB  Z+     GetIntegerv  0        6.1.13  pixel-store  
  
  
homeprevnext New Implementation Dependent State
  
    (none)  
  
  
homeprevnext Usage Examples
  
    Convenient macro definition for specifying buffer offsets:  
  
        #define BUFFER_OFFSET(i) ((char *)NULL + (i))  
  
    Example 1: Render to vertex array:  
  
        const int numberVertices = 100;  
  
        // Create a buffer object for a number of vertices consisting of  
        // 4 float values per vertex  
        glGenBuffers(1, vertexBuffer);  
        glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, vertexBuffer);  
        glBufferData(GL_PIXEL_PACK_BUFFER_ARB, numberVertices*4,  
                     NULL, GL_DYNAMIC_DRAW);  
  
        // Render vertex data into 100x1 strip of framebuffer using a  
        // fragment program  
        glBindProgram(FRAGMENT_PROGRAM_ARB, fragmentProgram);  
        glDrawBuffer(GL_BACK);  
        renderVertexData();  
        glBindProgramARB(FRAGMENT_PROGRAM_ARB, 0);  
  
        // Read the vertex data back from framebuffer  
        glReadBuffer(GL_BACK);  
        glReadPixels(0, 0, numberVertices, 1, GL_BGRA, GL_FLOAT,  
                     BUFFER_OFFSET(0));  
  
        // Change the binding point of the buffer object to  
        // the vertex array binding point  
        glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);  
  
        glEnableClientState(VERTEX_ARRAY);  
        glVertexPointer(4, GL_FLOAT, 0, BUFFER_OFFSET(0));  
        glDrawArrays(TRIANGLE_STRIP, 0, numberVertices);  
  
  
    Example 2: Streaming textures  
  
    Streaming textures using NV_pixel_data_range:  
  
        const int texWidth = 256;  
        const int texHeight = 256;  
        const int texsize = texWidth * texHeight * 4;  
        void *pdrMemory, *texData;  
  
        pdrMemory = glAllocateMemoryNV(texsize, 0.0, 1.0, 1.0);  
  
        glPixelDataRangeNV(GL_WRITE_PIXEL_DATA_RANGE_NV, texsize,  
                           pdrMemory);  
  
        glEnableClientState(GL_WRITE_PIXEL_DATA_RANGE_NV);  
  
        // Define texture level (without an image)  
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, texWidth, texHeight, 0,  
                     GL_BGRA, GL_UNSIGNED_BYTE, NULL);  
        // Setup texture environment  
        ...       
  
        texData = getNextImage();  
  
        while (texData) {  
  
            memcpy(pdrMemory, texData, texsize);  
  
            glFlushPixelDataRangeNV(GL_WRITE_PIXEL_DATA_RANGE_NV);  
  
            glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, texWidth, texHeight,  
                            GL_BGRA, GL_UNSIGNED_BYTE, pdrMemory);  
  
            // Draw textured geometry  
            glBegin(GL_QUADS);  
            ...  
            glEnd();  
  
            texData = getNextImage();  
        }  
  
        glDisableClientState(GL_WRITE_PIXEL_DATA_RANGE_NV);  
  
        glFreeMemoryNV(pdrMemory);  
      
      
    Streaming textures using pixel buffer objects:  
  
        const int texWidth = 256;  
        const int texHeight = 256;  
        const int texsize = texWidth * texHeight * 4;  
        void *pboMemory, *texData;  
  
        // Define texture level zero (without an image); notice the  
        // explicit bind to the zero pixel unpack buffer object so that  
        // pass NULL for the image data leaves the texture image  
        // unspecified.  
        glBindBuffer(GL_PIXEL_UNPACK_BUFFER_ARB, 0);  
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, texWidth, texHeight, 0,  
                     GL_BGRA, GL_UNSIGNED_BYTE, NULL);  
  
        // Create and bind texture image buffer object  
        glGenBuffers(1, &texBuffer);  
        glBindBuffer(GL_PIXEL_UNPACK_BUFFER_ARB, texBuffer);  
  
        // Setup texture environment  
        ...       
  
        texData = getNextImage();  
  
        while (texData) {  
  
            // Reset the contents of the texSize-sized buffer object  
            glBufferData(GL_PIXEL_UNPACK_BUFFER_ARB, texSize, NULL,  
                         GL_STREAM_DRAW);  
  
            // Map the texture image buffer (the contents of which  
            // are undefined due to the previous glBufferData)  
            pboMemory = glMapBuffer(GL_PIXEL_UNPACK_BUFFER_ARB,  
                                    GL_WRITE_ONLY);  
              
            // Modify (sub-)buffer data  
            memcpy(pboMemory, texData, texsize);  
  
            // Unmap the texture image buffer  
            glUnmapBuffer(GL_PIXEL_UNPACK_BUFFER_ARB);  
  
            // Update (sub-)teximage from texture image buffer  
            glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, texWidth, texHeight,  
                            GL_BGRA, GL_UNSIGNED_BYTE, BUFFER_OFFSET(0));  
  
            // Draw textured geometry  
            glBegin(GL_QUADS);  
            ...  
            glEnd();  
  
            texData = getNextImage();  
        }  
  
        glBindBuffer(GL_PIXEL_UNPACK_BUFFER_ARB, 0);  
  
  
    Example 3: Asynchronous glReadPixels  
  
    Traditional glReadPixels:  
  
        const int imagewidth = 640;  
        const int imageheight = 480;  
        GLubyte readBuffer[imagewidth*imageheight*4];  
  
        // Render to framebuffer  
        glDrawBuffer(GL_BACK);  
        renderScene()  
          
        // Read image from framebuffer  
        glReadBuffer(GL_BACK);   
        glReadPixels(0, 0, imagewidth, imageheight, GL_BGRA,  
                     GL_UNSIGNED_BYTE, readBuffer);  
  
        // Process image when glReadPixels returns after reading the  
        // whole buffer  
        processImage(readBuffer);  
  
  
    Asynchronous glReadPixels:  
  
        const int imagewidth = 640;  
        const int imageheight = 480;  
        const int imageSize = imagewidth*imageheight*4;  
  
        glGenBuffers(2, imageBuffers);  
  
        glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, imageBuffers[0]);  
        glBufferData(GL_PIXEL_PACK_BUFFER_ARB, imageSize / 2, NULL,  
                     GL_STREAM_READ);  
  
        glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, imageBuffers[1]);  
        glBufferData(GL_PIXEL_PACK_BUFFER_ARB, imageSize / 2, NULL,  
                     GL_STREAM_READ);  
  
        // Render to framebuffer  
        glDrawBuffer(GL_BACK);  
        renderScene();  
  
        // Bind two different buffer objects and start the glReadPixels   
        // asynchronously. Each call will return directly after  
        // starting the DMA transfer.  
        glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, imageBuffers[0]);  
        glReadPixels(0, 0, imagewidth, imageheight/2, GL_BGRA,  
                     GL_UNSIGNED_BYTE, BUFFER_OFFSET(0));  
  
        glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, imageBuffers[1]);  
        glReadPixels(0, imageheight/2, imagewidth, imageheight/2, GL_BGRA,  
                     GL_UNSIGNED_BYTE, BUFFER_OFFSET(0));  
          
        // Process partial images.  Mapping the buffer waits for  
        // outstanding DMA transfers into the buffer to finish.  
        glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, imageBuffers[0]);  
        pboMemory1 = glMapBuffer(GL_PIXEL_PACK_BUFFER_ARB,  
                                 GL_READ_ONLY);  
        processImage(pboMemory1);  
        glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, imageBuffers[1]);  
        pboMemory2 = glMapBuffer(GL_PIXEL_PACK_BUFFER_ARB,  
                                 GL_READ_ONLY);  
        processImage(pboMemory2);  
  
        // Unmap the image buffers  
        glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, imageBuffers[0]);  
        glUnmapBuffer(GL_PIXEL_PACK_BUFFER_ARB);  
        glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, imageBuffers[1]);  
        glUnmapBuffer(GL_PIXEL_PACK_BUFFER_ARB);  
  
  
homeprevnext Appendix on Pack/Unpack Range
  
    The complexity of OpenGL's pixel pack/unpack state makes it difficult  
    to express succinctly what range of a pixel buffer object will be  
    accessed by a pixel command.  
  
    The following code, following the conventions of the SGI OpenGL  
    Sample Implementation, returns the limit (one byte more than the  
    maximum allowed offset into the buffer object) for the memory a  
    pixel command will read/write.  
  
    /*  
    ** Compute offset limit into user's data considering all pixel  
    ** store modes.  This offset limit is ONE MORE than the largest byte  
    ** offset for the image.  
    */  
    static GLsizeiptr OffsetLimitImage3D(__GLpixelStoreMode *pixelStoreMode,  
                                         GLsizei width, GLsizei height,  
                                         GLsizei depth,  
                                         GLenum format, GLenum type,  
                                         const GLvoid *userdata,  
                                         GLint skip_images)  
    {  
        const GLint line_length = pixelStoreMode->lineLength;  
        const GLint image_height = pixelStoreMode->imageHeight;  
        const GLint alignment = pixelStoreMode->alignment;  
        const GLint skip_pixels = pixelStoreMode->skipPixels;  
        const GLint skip_lines = pixelStoreMode->skipLines;  
  
        GLsizeiptr offsetLimit = (GLsizeiptr) userdata;  
  
        GLint rowsize;  
        GLint padding;  
        GLint imagesize;  
  
        assert(width > 0);  
        assert(height > 0);  
        assert(depth > 0);  
  
        assert(line_length >= 0);  
        assert(image_height >= 0);  
  
        assert(skip_pixels >= 0);  
        assert(skip_lines >= 0);  
        assert(skip_images >= 0);  
  
        assert((alignment == 1) ||  
               (alignment == 2) ||  
               (alignment == 4) ||  
               (alignment == 8));  
  
        /* All formats except GL_BITMAP fall out trivially */  
        if (type == GL_BITMAP) {  
            const GLint groups_per_line = (line_length > 0) ?  
                                          line_length : width;  
            const GLint rows_per_image = (image_height > 0) ?  
                                         image_height : height;  
  
            assert(1 == __glElementsPerGroup(format, type));  
  
            rowsize = (groups_per_line + 7) / 8;  
            padding = rowsize & (alignment-1);  
            if (padding) {  
                rowsize += alignment - padding;  
            }  
            imagesize = rows_per_image * rowsize;  
  
            offsetLimit += imagesize    * (skip_images + depth-1);  
            offsetLimit += rowsize      * (skip_lines  + height-1);  
            offsetLimit += (skip_pixels + width+7)/8;  
        } else {  
            const GLint components = __glElementsPerGroup(format, type);  
            const GLint element_size = __glBytesPerElement(type);  
            const GLint group_size = element_size * components;  
  
            if (0 == (line_length | image_height | skip_pixels |  
                      skip_lines | skip_pixels)) {  
                // Fast path: when above pixel store modes are all zero.  
                rowsize = width * group_size;  
                // Default alignment is 4 so allow arbitrary alignment  
                // on fast path.  
                padding = rowsize & (alignment-1);  
                if (padding) {  
                    rowsize += alignment - padding;  
                }  
                imagesize = depth * height * rowsize;  
                offsetLimit += imagesize;  
            } else {  
                // General path: when one or more non-zero pixel store modes.  
                const GLint groups_per_line = (line_length > 0) ?  
                                              line_length : width;  
                const GLint rows_per_image = (image_height > 0) ?  
                                             image_height : height;  
  
                rowsize = groups_per_line * group_size;  
                padding = rowsize & (alignment-1);  
                if (padding) {  
                    rowsize += alignment - padding;  
                }  
                imagesize = rows_per_image * rowsize;  
  
                offsetLimit += imagesize    * (skip_images  + depth-1);  
                offsetLimit += rowsize      * (skip_lines   + height-1);  
                offsetLimit += group_size   * (skip_pixels  + width);  
            }  
        }  
        return offsetLimit;  
    }  
  
    GLsizeiptr __glOffsetLimitImage3D(__GLpixelStoreMode *pixelStoreMode,  
                                      GLsizei width, GLsizei height,  
                                      GLsizei depth,  
                                      GLenum format, GLenum type,  
                                      const GLvoid *userdata)  
    {  
        return OffsetLimitImage3D(pixelStoreMode,  
                                  width, height, depth, format, type,  
                                  userdata,  
                                  pixelStoreMode->skipImages);  
    }  
  
    GLsizeiptr __glOffsetLimitImage(__GLpixelStoreMode *pixelStoreMode,  
                                    GLsizei width, GLsizei height,  
                                    GLenum format, GLenum type,  
                                    const GLvoid *userdata)  
    {  
        /* NOTE: Non-3D image max offset computations ignore (treat as zero)  
           the unpackModes.skipImages state! */  
        return OffsetLimitImage3D(pixelStoreMode,  
                                  width, height, 1, format, type,  
                                  userdata,  
                                  0);  // Treat skipImages as zero.  
    }  
  
  
homeprevnext Revision History
  
    revision 0.3: mjk  
  
        Numbered issues.  
  
        Add issues 14 through 18.  
  
        Remove all gl/GL prefix/suffixing in specification sections.  Use  
        gl/GL prefix/suffixing in sections other than the specification  
        sections. Leaving off prefixes in non-specification sections is  
        ambiguous, particularly within example source code.  
  
        Base specification language updates on OpenGL 2.0 specification.  
  
        Add buffer object required state section.  
  
        Added GL_INVALID_OPERATION when an offset accessed (read or  
        written) for a pixel command from/to a pixel buffer object would  
        exceed the size of the buffer object.  
  
        Added GL_INVALID_OPERATION when for misaligned offsets.   
  
        Added "Appendix on Pack/Unpack Range".  
  
        Add GLX protocol discussion.  
  
    revision 0.4: mjk  
  
        Fixed grammar issues from Brian Paul.  
  
        Improved example code and fixed grammar from Nick Carter.  
  
        Explain how a NULL data parameter to glTexImage commands works.  
  
    revision 0.5: mjk  
  
        Clarify that glBufferData usage modes apply to drawing _and_  
        image specification commands.  
  
    revision 0.6: mjk  
          
        Add "streaming draw pixels" to the list of interesting approaches  
        for this extension in the Overview.  
  
        Add issue discussing the relationship of this extension to data  
        aquisition hardware.  
  
    revision 0.7: mjk  
  
        Assign enumerant values to match the EXT_pixel_buffer_object values.  
  
        Add issue explaining why the ARB extension shares enums with  
        EXT_pixel_buffer_object.  
  
        Apply Dale's suggestion to improve the clarity of the usage  
        pattern parameters to glBufferData.  
  
    revision 0.8 mjk  
          
        Typo fixes from Ian Romanick and Nick Carter.  
  
    revision 1.0 mjk  
  
        Add issue 23 for Jeremy about render-to-vertex-array.  Move  
        render-to-vertex-array justification in overview to bottom of  
        the list.  
        the list.  
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