| GL_ARB_pixel_buffer_object |
ARB_pixel_buffer_object
GL_ARB_pixel_buffer_object
Proposed.
Ralf Biermann
Nick Carter
Derek Cornish
Matt Craighead
Mark Kilgard
Dale Kirkland
Jon Leech
Brian Paul
Thomas Roell
Ian Romanick
Jeremy Sandmel
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)
Unknown.
NVIDIA Date: December 8, 2004 (version 1.0)
$Date$ $Revision$
$Id: //sw/main/docs/OpenGL/specs/proposed/GL_ARB_pixel_buffer_object.txt#17 $
???
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.
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.
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.
   New Procedures and Functions |
None.
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
| 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."
| 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."
| 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."
| Additions to Chapter 5 of the 1.2.1 Specification (Special Functions) |
None
| 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."
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.
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.
| 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.
| 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.
(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
| New Implementation Dependent State |
(none)
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);
| 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.
}
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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