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GL_EXT_framebuffer_sRGB
homeprevnext Name
  
    EXT_framebuffer_sRGB  
  
homeprevnext Name Strings
  
    GL_EXT_framebuffer_sRGB  
    GLX_EXT_framebuffer_sRGB  
    WGL_EXT_framebuffer_sRGB  
  
homeprevnext Contributors
  
    Herb (Charles) Kuta, Quantum3D  
  
    From the EXT_texture_sRGB specification...  
  
    Alain Bouchard, Matrox  
    Brian Paul, Tungsten Graphics  
    Daniel Vogel, Epic Games  
    Eric Werness, NVIDIA  
    Kiril Vidimce, Pixar  
    Mark J. Kilgard, NVIDIA  
    Pat Brown, NVIDIA  
    Yanjun Zhang, S3 Graphics  
    Jeremy Sandmel, Apple  
       
homeprevnext Contact
  
    Mark J. Kilgard, NVIDIA Corporation (mjk 'at' nvidia.com)  
  
homeprevnext Status
  
    Complete.  
  
homeprevnext Version
  
    Date: October 21, 2006  
    Revision: 1  
  
homeprevnext Number
  
    ???  
  
homeprevnext Dependencies
  
    OpenGL 1.1 required  
  
    This extension is written against the OpenGL 2.0 (September 7,  
    2004) specification.  
  
    WGL_EXT_extensions_string is required for WGL support.  
  
    WGL_EXT_pixel_format is required for WGL support.  
  
    ARB_color_buffer_float interacts with this extension.  
  
    EXT_framebuffer_object interacts with this extension.  
  
    EXT_texture_sRGB interacts with this extension.  
  
    ARB_draw_buffers interacts with this extension.  
  
homeprevnext Overview
  
    Conventionally, OpenGL assumes framebuffer color components are stored  
    in a linear color space.  In particular, framebuffer blending is a  
    linear operation.  
  
    The sRGB color space is based on typical (non-linear) monitor  
    characteristics expected in a dimly lit office.  It has been  
    standardized by the International Electrotechnical Commission (IEC)  
    as IEC 61966-2-1. The sRGB color space roughly corresponds to 2.2  
    gamma correction.  
  
    This extension adds a framebuffer capability for sRGB framebuffer  
    update and blending.  When blending is disabled but the new sRGB  
    updated mode is enabled (assume the framebuffer supports the  
    capability), high-precision linear color component values for red,  
    green, and blue generated by fragment coloring are encoded for sRGB  
    prior to being written into the framebuffer.  When blending is enabled  
    along with the new sRGB update mode, red, green, and blue framebuffer  
    color components are treated as sRGB values that are converted to  
    linear color values, blended with the high-precision color values  
    generated by fragment coloring, and then the blend result is encoded  
    for sRGB just prior to being written into the framebuffer.  
  
    The primary motivation for this extension is that it allows OpenGL  
    applications to render into a framebuffer that is scanned to a monitor  
    configured to assume framebuffer color values are sRGB encoded.  
    This assumption is roughly true of most PC monitors with default  
    gamma correction.  This allows applications to achieve faithful  
    color reproduction for OpenGL rendering without adjusting the  
    monitor's gamma correction.  
  
homeprevnext New Procedures and Functions
  
    None  
  
homeprevnext New Tokens
  
    Accepted by the <attribList> parameter of glXChooseVisual, and by  
    the <attrib> parameter of glXGetConfig:  
  
        GLX_FRAMEBUFFER_SRGB_CAPABLE_EXT             0x20B2  
  
    Accepted by the <piAttributes> parameter of  
    wglGetPixelFormatAttribivEXT, wglGetPixelFormatAttribfvEXT, and  
    the <piAttribIList> and <pfAttribIList> of wglChoosePixelFormatEXT:  
  
        WGL_FRAMEBUFFER_SRGB_CAPABLE_EXT             0x20A9  
  
    Accepted by the <cap> parameter of Enable, Disable, and IsEnabled,  
    and by the <pname> parameter of GetBooleanv, GetIntegerv, GetFloatv,  
    and GetDoublev:  
  
        FRAMEBUFFER_SRGB_EXT                         0x8DB9  
  
    Accepted by the <pname> parameter of GetBooleanv, GetIntegerv,  
    GetFloatv, and GetDoublev:  
  
        FRAMEBUFFER_SRGB_CAPABLE_EXT                 0x8DBA  
  
homeprevnext Additions to Chapter 2 of the 2.0 Specification (OpenGL Operation)
  
    None  
  
homeprevnext Additions to Chapter 3 of the 2.0 Specification (Rasterization)
  
    None  
  
homeprevnext Additions to Chapter 4 of the 2.0 Specification (Per-Fragment Operations and the Frame Buffer)
  
    DELETE the following sentence from section 4.1.8 (Blending) because  
    it is moved to the new "sRGB Conversion" section:  
  
    "Each of these floating-point values is clamped to [0,1] and  
    converted back to a fixed-point value in the manner described in  
    section 2.14.9."  
  
    If ARB_color_buffer_float is supported, the following paragraph  
    is modified to eliminate the fixed-point clamping and conversion  
    because this behavior is moved to the new "sRGB Conversion" section.  
  
    "If the color buffer is fixed-point, the components of the source  
    and destination values and blend factors are clamped to [0, 1]  
    prior to evaluating the blend equation, the components of the  
    blending result are clamped to [0,1] and converted to fixed-  
    point values in the manner described in section 2.14.9. If the  
    color buffer is floating-point, no clamping occurs.  The  
    resulting four values are sent to the next operation."  
  
    The modified ARB_color_buffer_float paragraph should read:  
  
    "If the color buffer is fixed-point, the components of the source  
    and destination values and blend factors are clamped to [0, 1]  
    prior to evaluating the blend equation.  If the color buffer is  
    floating-point, no clamping occurs.  The resulting four values are  
    sent to the next operation."  
  
    Replace the following sentence:  
  
    "Destination (framebuffer) components are taken to be fixed-point  
    values represented according to the scheme in section 2.14.9 (Final  
    Color Processing), as are source (fragment) components."  
  
    with the following sentences:  
  
    "Destination (framebuffer) components are taken to be fixed-point  
    values represented according to the scheme in section 2.14.9 (Final  
    Color Processing).  If FRAMEBUFFER_SRGB_EXT is enabled and the boolean  
    FRAMEBUFFER_SRGB_CAPABLE_EXT state for the drawable is true, the R,  
    G, and B destination color values (after conversion from fixed-point  
    to floating-point) are considered to be encoded for the sRGB color  
    space and hence need to be linearized prior to their use in blending.  
    Each R, G, and B component is linearized by some approximation of  
    the following:  
  
            {  cs / 12.92,                 cs <= 0.04045  
       cl = {  
            {  ((cs + 0.055)/1.055)^2.4,   cs >  0.04045  
  
    where cs is the component value prior to linearization and cl is  
    the result.  Otherwise if FRAMEBUFFER_SRGB_EXT is disabled, or the  
    drawable is not sRGB capable, or the value corresponds to the A  
    component, then cs = cl for such components.  The corresponding cs  
    values for R, G, B, and A are recombined as the destination color  
    used subsequently by blending."  
  
    ADD new section 4.1.X "sRGB Conversion" after section 4.1.8 (Blending)  
    and before section 4.1.9 (Dithering).  With this new section added,  
    understand the "next operation" referred to in the section 4.1.8  
    (Blending) to now be "sRGB Conversion" (instead of "Dithering").  
  
    "If FRAMEBUFFER_SRGB_EXT is enabled and the boolean  
    FRAMEBUFFER_SRGB_CAPABLE_EXT state for the drawable is true, the R,  
    G, and B values after blending are converted into the non-linear  
    sRGB color space by some approximation of the following:  
  
             {  0.0,                          0         <= cl  
             {  12.92 * c,                    0         <  cl < 0.0031308  
        cs = {  1.055 * cl^0.41666 - 0.055,   0.0031308 <= cl < 1  
             {  1.0,                                       cl >= 1  
  
    where cl is the R, G, or B element and cs is the result  
    (effectively converted into an sRGB color space).  Otherwise if  
    FRAMEBUFFER_SRGB_EXT is disabled, or the drawable is not sRGB  
    capable, or the value corresponds to the A element, then cs = cl  
    for such elements.  
  
    The resulting cs values form a new RGBA color value.  If the color  
    buffer is fixed-point, the components of this RGBA color value are  
    clamped to [0,1] and then converted to a fixed-point value in the  
    manner described in section 2.14.9.  The resulting four values are  
    sent to the subsequent dithering operation."  
  
homeprevnext Additions to Chapter 5 of the 2.0 Specification (Special Functions)
  
    None  
  
homeprevnext Additions to Chapter 6 of the 2.0 Specification (State and State Requests)
  
    None  
  
homeprevnext Additions to the OpenGL Shading Language specification
  
    None  
  
homeprevnext Additions to the GLX Specification
  
    None  
  
homeprevnext Dependencies on ARB_color_buffer_float
  
    If ARB_color_buffer_float is not supported, ignore the edits to  
    ARB_color_buffer_float language.  
  
homeprevnext Dependencies on EXT_texture_sRGB and EXT_framebuffer_object
  
    If EXT_texture_sRGB and EXT_framebuffer_object are both supported, the  
    implementation should set FRAMEBUFFER_SRGB_CAPABLE_EXT to false when  
    rendering to a color texture that is not one of the EXT_texture_sRGB  
    introduced internal formats.  An implementation can determine whether  
    or not it will set FRAMEBUFFER_SRGB_CAPABLE_EXT to true for the  
    EXT_texture_sRGB introduced internal formats.  Implementations are  
    encouraged to allow sRGB update and blending when rendering to sRGB  
    textures using EXT_framebuffer_object but this is not required.  
    In any case, FRAMEBUFFER_SRGB_CAPABLE_EXT should indicate whether  
    or not sRGB update and blending is supported.  
  
homeprevnext Dependencies on ARB_draw_buffers, EXT_texture_sRGB, and EXT_framebuffer_object
  
  
    If ARB_draw_buffers, EXT_texture_sRGB, and EXT_framebuffer_object  
    are supported and an application attempts to render to a set  
    of color buffers where some but not all of the color buffers  
    are FRAMEBUFFER_SRGB_CAPABLE_EXT individually, the query of  
    FRAMEBUFFER_SRGB_CAPABLE_EXT should return true.  
  
    However sRGB update and blending only apply to the color buffers  
    that are actually sRGB-capable.  
  
homeprevnext GLX Protocol
  
    None.  
  
homeprevnext Errors
  
    Relaxation of INVALID_ENUM errors  
    ---------------------------------  
  
    Enable, Disable, IsEnabled, GetBooleanv, GetIntegerv, GetFloatv,  
    and GetDoublev now accept the new token as allowed in the "New  
    Tokens" section.  
  
homeprevnext New State
  
    Add to table 6.20 (Pixel Operations)  
  
    Get Value             Type  Get Command  Initial Value  Description      Sec.   Attribute  
    --------------------  ----  -----------  -------------  ---------------  -----  -------------------  
    FRAMEBUFFER_SRGB_EXT  B     IsEnabled    False          sRGB update and  4.1.X  color-buffer/enable  
                                                            blending enable  
  
    Add to table 6.33 (Implementation Dependent Values)  
  
    Get Value                     Type  Get Command  Initial Value  Description           Sec.   Attribute  
    ----------------------------  ----  -----------  -------------  --------------------  -----  ---------  
    FRAMEBUFFER_SRGB_CAPABLE_EXT  B     IsEnabled    -              true if drawable      4.1.X  -  
                                                                    supports sRGB update  
                                                                    and blending  
  
homeprevnext New Implementation Dependent State
  
    None  
  
homeprevnext Issues
  
    1)  What should this extension be called?  
  
        RESOLVED: EXT_framebuffer_sRGB.  
  
        The "EXT_framebuffer" part indicates the extension is in  
        the framebuffer domain and "sRGB" indicates the extension is  
        adding a set of sRGB formats.  This mimics the naming of the  
        EXT_texture_sRGB extension that adds sRGB texture formats.  
  
        The mixed-case spelling of sRGB is the established usage so  
        "_sRGB" is preferred to "_srgb".  The "s" stands for standard  
        (color space).  
  
        For token names, we use "SRGB" since token names are uniformly  
        capitalized.  
  
    2)  Should alpha be sRGB encoded?  
  
        RESOLVED:  No.  Alpha remains linear.  
  
        A rationale for this resolution is found in Alvy Ray's "Should  
        Alpha Be Nonlinear If RGB Is?" Tech Memo 17 (December 14, 1998).  
        See: ftp://ftp.alvyray.com/Acrobat/17_Nonln.pdf  
  
    3)  Should the ability to support sRGB framebuffer update and blending  
        be an attribute of the framebuffer?  
  
        RESOLVED:  Yes.  It should be a capability of some pixel formats  
        (mostly likely just RGB8 and RGBA8) that says sRGB blending can  
        be enabled.  
  
        This allows an implementation to simply mark the existing RGB8  
        and RGBA8 pixel formats as supporting sRGB blending and then  
        just provide the functionality for sRGB update and blending for  
        such formats.  
  
        sRGB support for floating-point formats makes little sense  
        (because floating-point already provide a non-linear distribution  
        of precision and typically have considerably more precision  
        than 8-bit fixed-point framebuffer components allow) and would  
        be expensive to support.  
  
        Requiring sRGB support for all fixed-point buffers means that  
        support for 16-bit components or very small 5-bit or 6-bit  
        components would require special sRGB conversion hardware.  
        Typically sRGB is well-suited for 8-bit fixed-point components  
        so we do not want this extension to require expensive tables  
        for other component sizes that are unlikely to ever be used.  
        Implementations could support sRGB conversion for any color  
        framebuffer format but implementations are not required to  
        (honestly nor are implementations like to support sRGB on anything  
        but 8-bit fixed-point color formats).  
  
    4)  Should there be an enable for sRGB update and blending?  
  
        RESOLVED:  Yes, and it is disabled by default.  The enable only  
        applies if the framebuffer's underlying pixel format is capable  
        of sRGB update and blending.  Otherwise, the enable is silently  
        ignored (similar to how the multisample enables are ignored when  
        the pixel format lacks multisample supports).  
  
    5)  How is sRGB blending done?  
  
        RESOLVED:  Blending is a linear operation so should be performed  
        on values in linear spaces.  sRGB-encoded values are in a  
        non-linear space so sRGB blending should convert sRGB-encoded  
        values from the framebuffer to linear values, blend, and then  
        sRGB-encode the result to store it in the framebuffer.  
  
        The destination color RGB components are each converted  
        from sRGB to a linear value.  Blending is then performed.  
        The source color and constant color are simply assumed to be  
        treated as linear color components.  Then the result of blending  
        is converted to an sRGB encoding and stored in the framebuffer.  
  
    6)  What happens if GL_FRAMEBUFFER_SRGB_EXT is enabled (and  
        GL_FRAMEBUFFER_SRGB_CAPABLE_EXT is true for the drawable) but  
        GL_BLEND is not enabled?  
  
        RESOLVED:  The color result from fragment coloring (the source  
        color) is converted to an sRGB encoding and stored in the  
        framebuffer.  
  
    7)  How are multiple render targets handled?  
  
        RESOLVED:  Render targets that are not  
        GL_FRAMEBUFFER_SRGB_CAPABLE_EXT ignore the state of the  
        GL_FRAMEBUFFER_SRGB_EXT enable for sRGB update and blending.  
        So only the render targets that are sRGB-capable perform sRGB  
        blending and update when GL_FRAMEBUFFER_SRGB_EXT is enabled.  
  
    8)  Should sRGB framebuffer support affect the pixel path?  
  
        RESOLVED:  No.  
  
        sRGB conversion only applies to color reads for blending and  
        color writes.  Color reads for glReadPixels, glCopyPixels,  
        or glAccum have no sRGB conversion applied.  
  
        For pixel path operations, an application could use pixel maps  
        or color tables to perform an sRGB-to-linear conversion with  
        these lookup tables.  
  
    9)  Can luminance (single color component) framebuffer formats  
        support sRGB blending?  
  
        RESOLVED:  Yes, if an implementation chooses to advertise such  
        a format and set the sRGB attribute for the format too.  
  
        Implementations are not obliged to provide such formats.  
  
    10) Should all component sizes be supported for sRGB components or  
        just 8-bit?  
  
        RESOLVED:  This is at the implementation's discretion since  
        the implementation decides what pixel formats such support sRGB  
        update and blending.  
  
        It likely implementations will only provide sRGB-capable  
        framebuffer configurations for configurations with 8-bit  
        components.  
  
    11) What must be specified as far as how do you convert to and from  
        sRGB and linear RGB color spaces?  
  
        RESOLVED:  The specification language needs to only supply the  
        linear RGB to sRGB conversion (see section 4.9.X above).  
  
        The sRGB to linear RGB conversion is documented in the  
        EXT_texture_sRGB specification.  
  
        For completeness, the accepted linear RGB to sRGB conversion  
        (the inverse of the function specified in section 3.8.x) is as  
        follows:  
  
        Given a linear RGB component, cl, convert it to an sRGB component,  
        cs, in the range [0,1], with this pseudo-code:  
  
            if (isnan(cl)) {  
                /* Map IEEE-754 Not-a-number to zero. */  
                cs = 0.0;  
            } else if (cl > 1.0) {  
                cs = 1.0;  
            } else if (cl < 0.0) {  
                cs = 0.0;  
            } else if (cl < 0.0031308) {  
                cs = 12.92 * cl;  
            } else {  
                cs = 1.055 * pow(cl, 0.41666) - 0.055;  
            }  
  
         The NaN behavior in the pseudo-code is recommended but not  
         specified in the actual specification language.  
  
         sRGB components are typically stored as unsigned 8-bit  
         fixed-point values.  If cs is computed with the above  
         pseudo-code, cs can be converted to a [0,255] integer with this  
         formula:  
  
            csi = floor(255.0 * cs + 0.5)  
  
    12) Does this extension guarantee images rendered with sRGB textures  
        will "look good" when output to a device supporting an sRGB  
        color space?  
  
        RESOLVED:  No.  
  
        Whether the displayed framebuffer is displayed to a monitor that  
        faithfully reproduces the sRGB color space is beyond the scope  
        of this extension.  This involves the gamma correction and color  
        calibration of the physical display device.  
  
    13) How does this extension interact with EXT_framebuffer_object?  
  
        RESOLVED:  When rendering to a color texture, an application  
        can query GL_FRAMEBUFFER_SRGB_CAPABLE_EXT to determine if the  
        color texture image is capable of sRGB rendering.  
  
        This boolean should be false for all texture internal formats  
        except may be true (but are not required to be true) for the sRGB  
        internal formats introduced by EXT_texture_sRGB.  The expectation  
        is that implementations of this extension will be able to support  
        sRGB update and blending of sRGB textures.  
  
    14) How is the constant blend color handled for sRGB framebuffers?  
  
        RESOLVED:  The constant blend color is specified as four  
        floating-point values.  Given that the texture border color can  
        be specified at such high precision, it is always treated as a  
        linear RGBA value.  
  
    15) How does glCopyTex[Sub]Image work with sRGB?  Suppose we're  
        rendering to a floating point pbuffer or framebuffer object and  
        do CopyTexImage.  Are the linear framebuffer values converted  
        to sRGB during the copy?  
  
        RESOLVED:  No, linear framebuffer values will NOT be automatically  
        converted to the sRGB encoding during the copy.  If such a  
        conversion is desired, as explained in issue 12, the red, green,  
        and blue pixel map functionality can be used to implement a  
        linear-to-sRGB encoding translation.  
  
    16) Should this extension explicitly specify the particular  
        sRGB-to-linear and linear-to-sRGB conversions it uses?  
  
        RESOLVED:  The conversions are explicitly specified but  
        allowance for approximations is provided.  The expectation is  
        that the implementation is likely to use a table to implement the  
        conversions the conversion is necessarily then an approximation.  
  
    17) How does this extension interact with multisampling?  
  
        RESOLVED:  There are no explicit interactions.  However, arguably  
        if the color samples for multisampling are sRGB encoded, the  
        samples should be linearized before being "resolved" for display  
        and then recoverted to sRGB if the output device expects sRGB  
        encoded color components.  
  
        This is really a video scan-out issue and beyond the scope  
        of this extension which is focused on the rendering issues.  
        However some implementation advice is provided:  
  
        The implementation sufficiently aware of the gamma correction  
        configured for the display device could decide to perform an  
        sRGB-correct multisample resolve.  Whether this occurs or not  
        could be determined by a control panel setting or inferred by  
        the application's use of this extension.  
  
    18) Why is the sRGB framebuffer GL_FRAMEBUFFER_SRGB_EXT enable  
        disabled by default?  
  
        RESOLVED:  This extension could have a boolean  
        sRGB-versus-non-sRGB pixel format configuration mode that  
        determined whether or not sRGB framebuffer update and blending  
        occurs.  The problem with this approach is 1) it creates may more  
        pixel formation configurations because sRGB and non-sRGB versions  
        of lots of existing configurations must be advertised, and 2)  
        applicaitons unaware of sRGB might unknowingly select an sRGB  
        configuration and then generate over-bright rendering.  
  
        It seems more appropriate to have a capability for sRGB  
        framebuffer update and blending that is disabled by default.  
        This allows existing RGB8 and RGBA8 framebuffer configurations  
        to be marked as sRGB capable (so no additional configurations  
        need be enumerated).  Applications that desire sRGB rendering  
        should identify an sRGB-capable framebuffer configuration and  
        then enable sRGB rendering.  
  
        This is different from how EXT_texture_sRGB handles sRGB support  
        for texture formats.  In the EXT_texture_sRGB extension, textures  
        are either sRGB or non-sRGB and there is no texture parameter  
        to switch textures between the two modes.  This makes sense for  
        EXT_texture_sRGB because it allows implementations to fake sRGB  
        textures with higher-precision linear textures that simply convert  
        sRGB-encoded texels to sufficiently precise linear RGB values.  
  
        Texture formats also don't have the problem enumerated pixel  
        format descriptions have where a naive application could stumble  
        upon an sRGB-capable pixel format.  sRGB textures require  
        explicit use of one of the new EXT_texture_sRGB-introduced  
        internal formats.  
  
    19) How does sRGB and this extension interact with digital video  
        output standards, in particular DVI?  
  
        RESOLVED:  The DVI 1.0 specification recommends "as a default  
        position that digital moniotrs of all types support a color  
        transfer function similar to analog CRT monitors (gamma=2.2)  
        which makes up the majority of the compute display market." This  
        means DVI output devices should benefit from blending in the  
        sRGB color space just like analog monitors.  
  
homeprevnext Revision History
  
        Rev.    Date    Author    Changes  
        ----  --------  --------  -------------------------------------  
           1  10/21/06  barthold  Added revision history           
         0.4  10/20/06  mjk       Added issue 19  
         0.3            mjk       Internal spec development.  
  
  
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