Name ARB_framebuffer_sRGB Name Strings GL_ARB_framebuffer_sRGB GLX_ARB_framebuffer_sRGB WGL_ARB_framebuffer_sRGB Contributors Herb (Charles) Kuta, Quantum3D 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 Jon Leech Adam Jackson, Red Hat Contact Mark J. Kilgard, NVIDIA Corporation (mjk 'at' nvidia.com) Notice Copyright (c) 2008-2013 The Khronos Group Inc. Copyright terms at http://www.khronos.org/registry/speccopyright.html Specification Update Policy Khronos-approved extension specifications are updated in response to issues and bugs prioritized by the Khronos OpenGL Working Group. For extensions which have been promoted to a core Specification, fixes will first appear in the latest version of that core Specification, and will eventually be backported to the extension document. This policy is described in more detail at https://www.khronos.org/registry/OpenGL/docs/update_policy.php Status Approved by the ARB on July 11, 2008 Version Date: September 19, 2019 Revision: 1.3 Number ARB Extension #46 Dependencies OpenGL 1.1 is 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. ARB_framebuffer_object is required. EXT_texture_sRGB interacts with this extension. ARB_draw_buffers interacts with this extension. 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. New Procedures and Functions None New Tokens Accepted by the parameter of glXChooseVisual and glXChooseFBConfig, and by the parameter of glXGetConfig and glXGetFBConfigAttrib: GLX_FRAMEBUFFER_SRGB_CAPABLE_ARB 0x20B2 Accepted by the parameter of wglGetPixelFormatAttribivEXT, wglGetPixelFormatAttribfvEXT, and the and of wglChoosePixelFormatEXT: WGL_FRAMEBUFFER_SRGB_CAPABLE_ARB 0x20A9 Accepted by the parameter of Enable, Disable, and IsEnabled, and by the parameter of GetBooleanv, GetIntegerv, GetFloatv, and GetDoublev: FRAMEBUFFER_SRGB 0x8DB9 Additions to Chapter 2 of the 2.0 Specification (OpenGL Operation) None Additions to Chapter 3 of the 2.0 Specification (Rasterization) None 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 is enabled and the value of FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING for the framebuffer attachment corresponding to the destination buffer is SRGB (see section 6.1.3), 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 is disabled, or the value of FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING is not SRGB, no linearization is performed. The resulting linearized R, G, and B and unmodified A values are recombined as the destination color used in blending computations. 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 is enabled and the value of FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING for the framebuffer attachment corresponding to the destination buffer is SRGB (see section 6.1.3), 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). If FRAMEBUFFER_SRGB is disabled or the value of FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING is not SRGB, then cs = cl The resulting cs values for R, G, and B and the unmodified A 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." Additions to Chapter 5 of the 2.0 Specification (Special Functions) None Additions to Chapter 6 of the 2.0 Specification (State and State Requests) None Additions to the OpenGL Shading Language specification None Additions to the GLX Specification If GLX_FRAMEBUFFER_SRGB_CAPABLE_ARB is specified for glXChooseVisual or glXChooseFBConfig, it is treated as an exact match. If it is not specified, it is treated as GLX_DONT_CARE. Sorting of visuals and fbconfigs by GLX_FRAMEBUFFER_SRGB_CAPABLE_ARB is undefined. Dependencies on ARB_color_buffer_float If ARB_color_buffer_float is not supported, ignore the edits to ARB_color_buffer_float language. Dependencies on EXT_texture_sRGB and ARB_framebuffer_object If EXT_texture_sRGB and ARB_framebuffer_object are both supported, the implementation should set the value of FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING to LINEAR 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 the value of FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING to SRGB for the EXT_texture_sRGB introduced internal formats. Implementations are encouraged to allow sRGB update and blending when rendering to sRGB textures using ARB_framebuffer_object but this is not required. In any case, FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING should indicate whether or not sRGB update and blending is supported. Dependencies on ARB_draw_buffers, EXT_texture_sRGB, and ARB_framebuffer_object If ARB_draw_buffers, EXT_texture_sRGB, and ARB_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 SRGB capable, sRGB update and blending only apply to the color buffers that are actually sRGB-capable. GLX Protocol None. Errors Relaxation of INVALID_ENUM errors --------------------------------- Enable, Disable, IsEnabled, GetBooleanv, GetIntegerv, GetFloatv, and GetDoublev now accept FRAMEBUFFER_SRGB as allowed in the "New Tokens" section. New State Add to table 6.20 (Pixel Operations) Get Value Type Get Command Initial Value Description Sec. Attribute ---------------- ---- ----------- ------------- --------------- ----- ------------------- FRAMEBUFFER_SRGB B IsEnabled False sRGB update and 4.1.X color-buffer/enable blending enable New Implementation Dependent State None Issues 1) What should this extension be called? RESOLVED: ARB_framebuffer_sRGB. The "ARB_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 a destination buffer's 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 is enabled (and GL_FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING is GL_SRGB), 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 sRGB-capable ignore the state of the GL_FRAMEBUFFER_SRGB enable for sRGB update and blending. Note that unlike EXT_framebuffer_sRGB, the sRGB-capable query may be performed on a per-color-attachment basis. 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 ARB_framebuffer_object? RESOLVED: ARB_framebuffer_object is required to provide the GL_FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING query used to determine if the color texture image is capable of sRGB rendering. This query should be LINEAR for all texture internal formats other than sRGB internal formats such as those introduced by EXT_texture_sRGB. Implementations of this extension must 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 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. 20) Why don't the new tokens and entry points in this extension have "ARB" suffixes like other ARB extensions? RESOLVED: Unlike most ARB extensions, this is a strict subset of functionality already approved in OpenGL 3.0. This extension exists only to support that functionality on older hardware that cannot implement a full OpenGL 3.0 driver. Since there are no possible behavior changes between the ARB extension and core features, source code compatibility is improved by not using suffixes on the extension. This does not apply to the GLX and WGL elements of this extension, since those elements are not part of a new core GLX or WGL release yet. 21) Where's the specification language for the GLX and WGL pixel format selection interface? TO BE DONE. The {GLX,WGL}_FRAMEBUFFER_SRGB_CAPABLE_ARB pixel format attributes are used to select default framebuffers which are sRGB-capable in the fairly obvious way, but this language was missing in the original EXT_framebuffer_sRGB and needs to be added here. Revision History Rev. Date Author Changes ---- -------- -------- ------------------------------------- 1.3 09/19/19 ajax Clarify GLX interaction. 1.2 08/11/08 jleech Use per-FBO-attachment state for sRGB-capable queries, rather then the EXT's single boolean query for the entire framebuffer. 1.1 08/08/08 jleech Remove ARB suffixes. 1 10/21/06 barthold Added revision history 0.4 10/20/06 mjk Added issue 19 0.3 mjk Internal spec development.