| Name |
| |
| NV_multisample_coverage |
| |
| Name Strings |
| |
| GL_NV_multisample_coverage |
| GLX_NV_multisample_coverage |
| WGL_NV_multisample_coverage |
| |
| Contact |
| |
| Mike Strauss (mstrauss 'at' nvidia.com) |
| |
| Status |
| |
| Shipping in NVIDIA release 169.xx drivers (May 2008) for GeForce |
| 8 and up |
| |
| Version |
| |
| Revision: 4 |
| Last Modified Date: March 7, 2013 |
| |
| Number |
| |
| 393 |
| |
| Dependencies |
| |
| ARB_multisample is required. |
| Written using the language of the OpenGL 2.1 specification. |
| Written using the language of the GLX 1.4 specification. |
| |
| Overview |
| |
| The ARB_multisample extension provides a mechanism for antialiasing |
| primitives. This mechanism allows an application to request an |
| additional buffer, the multisample buffer, that is added to the |
| framebuffer. An application can request the number of samples per |
| fragment that are stored in the multisample buffer. Rendering |
| proceeds by writing color, depth, and stencil values for each |
| sample to the multisample buffer. The results are automatically |
| resolved to a single displayable color each time a pixel is |
| updated. |
| |
| Coverage Sample Anti-Aliasing (CSAA) is an extension to multisample |
| antialiasing. The technique separates "samples" into two types of |
| samples. "Color samples" are samples with color, depth, and |
| stencil information stored in the multisample buffer. "Coverage |
| samples" include both color samples and additional samples that only |
| provide pixel coverage information. |
| |
| This extension follows the example of the |
| NV_framebuffer_multisample_coverage extension, which adds CSAA |
| support for framebuffer objects. The base description of |
| multisample rendering is written in terms of coverage samples and |
| color samples. The windows system notion of "samples" |
| (SAMPLES_ARB) is layered on top of coverage and color samples. |
| |
| IP Status |
| |
| None. |
| |
| New Procedures and Functions |
| |
| None. |
| |
| New Tokens |
| |
| Accepted by the <attribList> parameter of glXChooseVisual, and by |
| the <attrib> parameter of glXGetConfig, the <attrib_list> parameter |
| of glXChooseFBConfig, and the <attribute> parameter of |
| glXGetFBConfigAttrib: |
| |
| GLX_COVERAGE_SAMPLES_NV 100001 |
| GLX_COLOR_SAMPLES_NV 0x20B3 |
| |
| Accepted by the <piAttributes> parameter of |
| wglGetPixelFormatAttribivEXT, wglGetPixelFormatAttribfvEXT, and |
| the <piAttribIList> and <pfAttribIList> of wglChoosePixelFormatEXT: |
| |
| WGL_COVERAGE_SAMPLES_NV 0x2042 |
| WGL_COLOR_SAMPLES_NV 0x20B9 |
| |
| Accepted by the <pname> parameter of GetBooleanv, GetDoublev, |
| GetIntegerv, and GetFloatv: |
| |
| COLOR_SAMPLES_NV 0x8E20 |
| |
| (Note: Earlier versions of this extension included a token |
| COVERAGE_SAMPLES_NV that was an alias for SAMPLES/SAMPLES_ARB. This was |
| removed to avoid a name collision with a similar COVERAGE_SAMPLES_NV |
| token from the NV_coverage_sample extension to OpenGL ES.) |
| |
| Additions to Chapter 2 of the OpenGL 2.0 Specification (OpenGL Operation) |
| |
| None. |
| |
| Additions to Chapter 3 of the OpenGL 2.0 Specification (Rasterization) |
| |
| Modify Section 3.2.1, Multisampling (p. 93). |
| |
| (replace third and fourth paragraphs, p. 94) |
| |
| During multisample rendering the contents of a pixel fragment are |
| changed in three ways. First, each fragment includes a coverage |
| value with SAMPLES_ARB bits. The value of SAMPLES_ARB is an |
| implementation-dependent constant, and is queried by calling |
| GetIntegerv with <pname> set to SAMPLES_ARB. |
| |
| Second, each fragment includes SAMPLES_ARB coverage samples. The |
| number of coverage samples includes both samples that indicate only |
| coverage information, as well as samples that include coverage, |
| color, depth, and texture coordinate information. The location of |
| the coverage information only samples may be anywhere within the |
| pixel. These samples may be used with an implementation-dependent |
| algorithm as additional weights when resolving the color values to a |
| single pixel color. |
| |
| Third, each fragment includes COLOR_SAMPLES_NV depth values, |
| color values, and sets of texture coordinates instead of the |
| single depth value, color value, and set of texture coordinates |
| that are maintained in single-sample rendering mode. These samples |
| are referred to as color samples, although they contain more than |
| color information. COLOR_SAMPLES_NV is an implementation-dependent |
| constant, and is queried by calling GetIntegerv with <pname> set to |
| COLOR_SAMPLES_NV. An implementation may choose to assign the same |
| color value and the same set of texture coordinates to more than |
| one sample. The location for evaluating the color value and the |
| set of texture coordinates can be anywhere within the pixel |
| including the fragment center or any of the color sample locations. |
| The color value and the set of texture coordinates need not be |
| evaluated at the same location. Each pixel fragment thus |
| consists of integer x and y grid coordinates, SAMPLES_ARB |
| samples, COLOR_SAMPLES_NV of which contain color and depth values, |
| COLOR_SAMPLES_NV sets of texture coordinates, and a coverage value |
| with a maximum of SAMPLES_ARB bits. |
| |
| (modify seventh paragraph, p. 94) |
| |
| If MULTISAMPLE is enabled, multisample rasterization of all |
| primitives differs substantially from single-sample rasterization. |
| It is understood that each pixel in the framebuffer has SAMPLES_ARB |
| locations associated with it. Of these locations, COLOR_SAMPLES_NV |
| have associated color, depth, and texture coordinates. The |
| remaining locations only supply additional pixel coverage |
| information. These locations are exact positions, rather than |
| regions or areas, and each is referred to as a sample point. The |
| sample points... |
| |
| Additions to Chapter 4 of the OpenGL 2.0 Specification (Per-Fragment |
| Operations and the Frame Buffer) |
| |
| Modify Section 4.1.11, Additional Multisample Fragment |
| Operations (p. 213). |
| |
| (modify second paragraph, p. 213) |
| |
| If MULTISAMPLE is enabled, and the value of SAMPLE_BUFFERS is one, |
| the alpha test, stencil test, depth test, blending, and dithering |
| operations are performed for each pixel color sample, rather than |
| just once for each fragment... |
| |
| (modify third paragraph, p 215) |
| |
| After all operations have been completed on the multisample... |
| The method of combination is not specified, though a simple average |
| computed independently for each color component is recommended when |
| the number of coverage samples and color samples are equal. |
| |
| |
| Additions to Chapter 5 of the OpenGL 2.0 Specification (Special Functions) |
| |
| None. |
| |
| Additions to Chapter 6 of the OpenGL 2.0 Specification (State and |
| State Requests) |
| |
| |
| |
| Additions to Appendix A of the OpenGL 2.0 Specification (Invariance) |
| |
| None. |
| |
| Additions to the GLX Specification |
| |
| Modify Section 3.3.3 Configuration Management (p. 12) |
| |
| (modify fourth paragraph, p. 12) |
| |
| The attribute GLX_SAMPLE_BUFFERS indicates the number of multisample |
| buffers, which must be zero or one. The attribute |
| GLX_COVERAGE_SAMPLES_NV gives the number of coverage samples per |
| pixel, and the attribute GLX_COLOR_SAMPLES_NV gives the number of |
| color samples per pixel; if GLX_SAMPLE_BUFFERS is zero, then |
| GLX_COVERAGE_SAMPLES_NV and GLX_COLOR_SAMPLES_NV will also be zero. |
| If GLX_SAMPLE_BUFFERS is one, then the number of color, depth, and |
| stencil bits for each sample in the multisample buffer are |
| specified by the GLX_*_SIZE attributes. GLX_SAMPLES is an alias for |
| GLX_COVERAGE_SAMPLES_NV. |
| |
| (modify GLX_SAMPLES entry in Table 3.1: GLXFBConfig attributes, |
| p. 12) |
| |
| Attribute Type Notes |
| ----------------------- ------- ------------------------------ |
| GLX_COVERAGE_SAMPLES_NV integer no. coverage samples per pixel |
| |
| (add a new entry to Table 3.1: GLXFBConfig attributes, p.12, |
| following the GLX_COVERAGE_SAMPLES_NV entry) |
| |
| Attribute Type Notes |
| ----------------------- ------- ------------------------------ |
| GLX_COLOR_SAMPLES_NV integer no. color samples per pixel |
| |
| |
| (add a new following the discussion of GLX_TRASPARENT_TYPE, |
| p 18.) |
| |
| If GLX_COVERAGE_SAMPLES_NV is in <attrib_list>, but |
| GLX_COLOR_SAMPLES_NV is not in <attrib_list> the selected |
| GLXFBConfigs must have the same number of color samples as |
| coverage samples. |
| |
| (modify the GLX_SAMPLES entry in Table 3.4: Default values and |
| match criteria for GLXFBConfig attributes, p. 19) |
| |
| Selection |
| and Sorting Sort |
| Attribute Default Criteria Priority |
| ----------------------- ------- ----------- -------- |
| GLX_COVERAGE_SAMPLES_NV 0 SMALLER 7 |
| |
| |
| (add a new entry to table 3.4: Default values and match criteria |
| for GLXFBConfig attributes, p. 19, following the |
| GLX_COVERAGE_SAMPLES_NV entry. |
| |
| Selection |
| and Sorting Sort |
| Attribute Default Criteria Priority |
| ----------------------- ------- ----------- -------- |
| GLX_COLOR_SAMPLES_NV 0 SMALLER 8 |
| |
| |
| (modify the GLX_DEPTH_SIZE, GLX_STENCIL_SIZE, GLX_ACCUM_*_SIZE, |
| and GLX_X_VISUAL_TYPE entries in table 3.4: Default values and |
| match criteria for GLXFBConfig attributes, p. 19, by incrementing |
| the value of Sort Priority for each) |
| |
| (modify sixth paragraph, p. 20) |
| |
| 7. Smaller GLX_COVERAGE_SAMPLES_NV. |
| |
| (add a new paragraph following the sixth paragraph, p. 20) |
| |
| 8. Smaller GLX_COLOR_SAMPLES_NV. |
| |
| (modify paragraphs seven through ten, p. 20, by incrementing |
| the list number for each) |
| |
| Modify Section 3.4.1 Using Visuals for Configuration Management |
| |
| (modify the GLX_SAMPLES entry in Table 3.7: GLX attributes for |
| Visuals, p. 37) |
| |
| Attribute Type Notes |
| ------------------------- -------- -------------------------- |
| GLX_COVERAGE_SAMPLES_NV integer number of coverage samples |
| per pixel. |
| |
| (add a new entry to table 3.7: GLX attributes for Visuals, p.37, |
| following the GLX_COVERAGE_SAMPLES_NV entry) |
| |
| Attribute Type Notes |
| ------------------------- -------- -------------------------- |
| GLX_COLOR_SAMPLES_NV integer number of color samples |
| per pixel. |
| |
| |
| (modify the GLX_SAMPLES entry in table 3.8: Defaults and selection |
| criteria used by glXChooseVisual, p.38) |
| |
| Attribute Default Selection Criteria |
| ------------------------- ------- ------------------ |
| GLX_COVERAGE_SAMPLES_NV 0 Smaller |
| |
| (add a new entry to table 3.8: Defaults and selection criteria used |
| by glXChooseVisual, p.38, following the GLX_COVERAGE_SAMPLES_NV |
| entry) |
| |
| Attribute Default Selection Criteria |
| ------------------------- ------- ------------------ |
| GLX_COLOR_SAMPLES_NV 0 Smaller |
| |
| |
| GLX Protocol |
| |
| None. |
| |
| Additions to the WGL Specification |
| |
| Modify additions from ARB_multisample |
| |
| (modify first and second paragraphs) |
| |
| The parameter WGL_SAMPLE_BUFFERS_ARB is added to |
| wglGetPixelFormatAttrib*v. When queried, by calling |
| wglGetPixelFormatAttrib*v with <piAttributes> set to |
| WGL_SAMPLE_BUFFERS_ARB, it returns the number of multisample |
| buffers included in the pixel format. For a normal pixel format, |
| the return value is zero. A return value of one indicates that a |
| single multisample buffer is available. The number of coverage |
| samples per pixel is queried by calling wglGetPixelFormatAttrib*v |
| with <piAttributes> set to WGL_COVERAGE_SAMPLES_NV. The number of |
| color samples per pixel is queried by calling |
| wglGetPixelFormatAttrib*v with <piAttributes> set to |
| WGL_COLOR_SAMPLES_NV. It is understood that the number of color, |
| depth, and stencil bits per sample in the multisample buffer are as |
| specified by the WGL_*_SIZE parameters. It is also understood that |
| there are no single-sample depth or stencil buffers associated with |
| this pixel format -- the only depth and stencil buffers are those |
| in the multisample buffer. WGL_COVERAGE_SAMPLES_NV and |
| WGL_COLOR_SAMPLES_NV are zero if WGL_SAMPLES_BUFFERS_ARB is zero. |
| WGL_SAMPLES_ARB is an alias for WGL_COVERAGE_SAMPLES_NV. |
| |
| wglChoosePixelFormatARB accepts WGL_SAMPLE_BUFFERS_ARB in |
| <piAttribIList> and <pfAttribIList> with the corresponding value |
| set to the minimum number of multisample buffers that can be |
| accepted. Pixel formats with the smallest number of multisample |
| buffers that meets or exceeds the specified minimum number are |
| preferred. Currently operations with more than one multisample |
| buffer are undefined, so the returned value will be either zero or |
| one. |
| |
| (add a new paragraph, missing from ARB_multisample, following the |
| second paragraph) |
| |
| wglChoosePixelFormatARB accepts WGL_COVERAGE_SAMPLES_NV in |
| <piAttribIList> and <pfAttibIList> with the corresponding value |
| set to the requested number of coverage samples for the multisample |
| buffer. wglChoosePixelFormatARB accepts WGL_COLOR_SAMPLES_NV in |
| <piAttribIList> and <pfAttribIList> with the corresponding value |
| set to the requested number of color samples for the multisample |
| buffer. A pixel format with differing numbers of coverage and |
| color samples may only be chosen when WGL_COLOR_SAMPLES_NV is |
| specified. |
| |
| |
| Errors |
| |
| None. |
| |
| New State |
| |
| |
| New Implementation Dependent State |
| |
| Minimum |
| Get Value Type Get Command Value Description Sec. |
| -------------------- ---- ----------- ------- -------------------------- ----- |
| SAMPLES_ARB Z+ GetIntegerv 0 Number of coverage samples 3.2.1 |
| (state already in ARB_multisample, |
| just defined to be coverage samples) |
| COLOR_SAMPLES_NV Z+ GetIntegerv 0 Number of color samples 3.2.1 |
| |
| Issues |
| |
| 1) How do we provide backward compatibility for glXChooseFBConfig |
| and wglChoosePixelFormatARB if COVERAGE_SAMPLES_NV aliases to |
| SAMPLES_ARB? |
| |
| RESOLVED: Traditional multisample antialiasing can be |
| be described as CSAA where the number of coverage and color |
| samples are the same (all samples provide coverage, color, |
| depth, and stencil information). Existing applications and |
| applications that do not wish to use this extension will use |
| SAMPLES_ARB when choosing a multisample GLXFBConfig or pixel |
| format. In this case, glXChooseFBConfig and |
| wglChoosePixelFormatARB should never return a CSAA visual or |
| pixel format. This is enforced in the spec by requiring the |
| presence of COLOR_SAMPLES_NV in the attribute list in order to |
| return a CSAA GLXFBConfig or pixel format. |
| |
| (NOTE: The token COVERAGE_SAMPLES_NV was removed in version 4 |
| of the extension, but its value was always indistinguishable |
| from SAMPLES_ARB.) |
| |
| 2) How do we provide backward compatibility for |
| glXGetFBConfigAttrib and wglGetPixelFormatAttrib*v if |
| COVERAGE_SAMPLES_NV aliases to SAMPLES_ARB? |
| |
| RESOLVED: The glXGetFBConfigAttrib and |
| wglGetPixelFormatAttrib*v routines provide a mechanism for |
| applications to query properties of a GLXFBConfig or pixel |
| format. This is useful for obtaining additional information |
| about a GLXFBConfig or pixel format after choosing one with the |
| appropriate routine. An application can also use these |
| routines to apply custom GLXFBConfig/pixel format selection |
| logic when presented with a set of GLXFBConfigs or pixel |
| formats. |
| |
| In the latter case, existing applications and future |
| applications that do not use this extension may accidentally |
| select a CSAA GLXFBConfig or pixel format because they only |
| look at SAMPLES_ARB, which aliases to COVERAGE_SAMPLES_NV. |
| This problem is not unique to CSAA GLXFBConfigs and pixel |
| formats. An application can run into a problem selecting the |
| wrong GLXFBConfig or pixel format any time a new GLXFBConfig or |
| pixel format attribute is added that differs from the old |
| default. The GLX specification provides GLXFBConfig sorting |
| guidelines to aid in working around this problem. For WGL, |
| vendors are encouraged to order their visuals and pixel formats |
| in such a way that gives preference to old formats. With |
| regards to CSAA, it is recommended that vendors list traditional |
| multisample pixel formats first, and CSAA pixel formats later. |
| The assumption being that applications will tend to choose the |
| first pixel format that meets the criteria. |
| |
| (NOTE: The token COVERAGE_SAMPLES_NV was removed in version 4 |
| of the extension, but its value was always indistinguishable |
| from SAMPLES_ARB.) |
| |
| 3) The NV_framebuffer_multisample_coverage extension supplies |
| specific rules with regards to how the requested number of |
| coverage and color samples are rounded to an implementation |
| supported value. Should this extension provide similar rules? |
| |
| RESOLVED. The NV_framebuffer_multisample_coverage extension |
| is built on top of EXT_framebuffer_multisample, which provides |
| explicit rules for rounding the number of requested samples. |
| Specifically, the spec says to round the number of requested |
| samples up to the next implementation supported value. |
| NV_framebuffer_multisample_coverage says that the number of |
| coverage samples is rounded to the next highest implementation |
| supported value. The number of color samples is rounded up to |
| the next highest implementation supported value given the |
| chosen number of coverage samples. |
| |
| Choosing WGL pixel formats has always been a flexible |
| and fuzzy process. On the other hand, the GLX specification |
| provides explicit rules for sorting and choosing GLXFBConfigs. |
| Language has been added to the GLX spec to sort first on |
| coverage samples, and then on color samples. This has a |
| similar effect as the rules given in |
| NV_framebuffer_multisample_coverage as long as both |
| COVERAGE_SAMPLES_NV and COLOR_SAMPLES_NV are specified. For |
| WGL pixel format selection, the implementation is free to |
| sort in whatever way it wishes. Although it is recommended |
| that sorting be done in a way that is compatible with |
| NV_framebuffer_mulitsample_coverage. |
| |
| Revision History |
| |
| Revision 1, 6/22/2007, mstrauss: Initial version |
| Revision 2, 6/23/2007, mstrauss: Removed GLX visual language. |
| Added GLXFBconfig language. |
| Revision 3, 4/20/2009, jajones: Filled in GLX token values. |
| Restored GLX 1.2 compatibility |
| table entries. |
| Revision 4, 3/7/2013, pbrown: Removed the OpenGL API token |
| COVERAGE_SAMPLES_NV, which was |
| just an alias for SAMPLES_ARB. |
| |
| |
