Name 3DFX_multisample Name Strings GL_3DFX_multisample GLX_3DFX_multisample WGL_3DFX_multisample Contact Paula Womack, 3dfx Interactive (paulaw 'at' 3dfx.com) Status Complete. Version Date: April 18, 2000; Version 1.1 Number 207 Dependencies OpenGL 1.1 is required. GLX 1.2 is required. WGL_ARB_extensions_string is required. WGL_ARB_pixel_format is required. GLX_EXT_fbconfig and GLX 1.3 affect the definition of this extension. This spec is written against OpenGL 1.2 and GLX 1.3 Overview This extension provides a mechanism to antialias all GL primitives: points, lines, polygons, bitmaps, and images. The technique is to sample all primitives multiple times at each pixel. The color sample values are resolved to a single, displayable color each time a pixel is updated, so the antialiasing appears to be automatic at the application level. Because each sample includes depth and stencil information, the depth and stencil functions perform equivalently to the single-sample mode. An additional buffer, called the multisample buffer, is added to the framebuffer. Pixel sample values, including color, depth, and stencil values, are stored in this buffer. When the framebuffer includes a multisample buffer, it does not also include separate depth or stencil buffers, even if the multisample buffer does not store depth or stencil values. Color buffers (left/right, front/back, and aux) do coexist with the multisample buffer, however. Multisample antialiasing is most valuable for rendering polygons, because it requires no sorting for hidden surface elimination, and it correctly handles adjacent polygons, object silhouettes, and even intersecting polygons. If only points or lines are being rendered, the "smooth" antialiasing mechanism provided by the base GL may result in a higher quality image. This extension is a subset of SGIS_multisample. It differs in these key ways: * Fragment alpha values are not affected by the fragment sample mask * The sample locations may or may not be fixed. Thus, there is no support for rendering the scene multiple times with different sample points. * Fragment masks are not computed for images or for bitmasks. Because of these differences a new extension was created. However, it is not expected that this extension will co-exist with SGIS_multisample. Because of this and the fact that there are only 32 push/pop bits the MULTISAMPLE_BIT_SGIS state value is the same as MUTLISAMPLE_BIT_3DFX. IP Status No issues. Issues Should tbuffer be kept as a separate extension? Yes. The ability to define a write mask for the fragment mask should be kept separate. This feature is orthogonal to SGIS_multisample, while 3DFX_multisample is not. 3DFX_multisample is a strict subset of SGIS_multisample. Should the multisample buffer be defined as a separate buffer? Yes. It does not need to be implemented this way though. Since GL rendering is done off screen and then blitted (to handle window clipping) the multisample buffer can actually be the same as the offscreen front and back buffers. The blit engine handles reads and writes to/from AA buffers so ReadPixels will work correctly. Should we allow the depth values for the different samples to differ when AA is off? No. This will be a bug on hardware that operates this way. In practice it should not be a big issue. New Procedures and Functions None New Tokens Accepted by the parameter of glXChooseFBConfig and glXChooseVisual and by the parameter of glXGetFBConfigAttrib and glXGetConfig: GLX_SAMPLE_BUFFERS_3DFX 0x8050 GLX_SAMPLES_3DFX 0x8051 Accepted in the parameter array of wglGetPixelFormatAttribivARB, and wglGetPixelFormatAttribfvARB, and in the and parameter arrays of wglChoosePixelFormatARB: WGL_SAMPLE_BUFFERS_3DFX 0x2060 WGL_SAMPLES_3DFX 0x2061 Accepted by the parameter of Enable, Disable, and IsEnabled, and by the parameter of GetBooleanv, GetIntegerv, GetFloatv, and GetDoublev: MULTISAMPLE_3DFX 0x86B2 Accepted by the parameter of GetBooleanv, GetDoublev, GetIntegerv, and GetFloatv: SAMPLE_BUFFERS_3DFX 0x86B3 SAMPLES_3DFX 0x86B4 Accepted by the parameter of PushAttrib: MULTISAMPLE_BIT_3DFX 0x20000000 Additions to Chapter 2 of the 1.2 Specification (OpenGL Operation) None Additions to Chapter 3 of the 1.2 Specification (Rasterization) If SAMPLE_BUFFERS_3DFX is one, the rasterization of all GL primitives is changed, and is referred to as multisample rasterization. Otherwise primitive rasterization operates as it is described in the GL specification, and is referred to as single-sample rasterization. The value of SAMPLE_BUFFERS_3DFX is an implementation dependent constant, and is queried by calling GetIntegerv with pname set to SAMPLE_BUFFERS_3DFX. During multisample rasterization the contents of a pixel fragment are changed in two ways. First, each fragment includes a coverage mask with SAMPLES_3DFX bits. The value of SAMPLES_3DFX is an implementation dependent constant, and is queried by calling GetIntegerv with pname set to SAMPLES_3DFX. Second, each fragment includes SAMPLES_3DFX depth values, instead of the single depth value that is maintained in single-sample rasterization mode. Each pixel fragment thus consists of integer x and y grid coordinates, a color, SAMPLES_3DFX depth values, texture coordinates, a coverage value, and the SAMPLES_3DFX-bit mask. The behavior of multisample rasterization is a function of MULTISAMPLE_3DFX, which is enabled and disabled by calling Enable or Disable, with cap set to MULTISAMPLE_3DFX. Its value is queried using IsEnabled, with cap set to MULTISAMPLE_3DFX. If MULTISAMPLE_3DFX is disabled, multisample rasterization of all primitives is equivalent to single-sample rasterization, except that the fragment coverage mask is set to all ones. The depth values may all be set to the single value that would have been assigned by single-sample rasterization, or they may be assigned as described below for MULTISAMPLE_3DFX-enabled multisample rasterization. If MULTISAMPLE_3DFX is enabled, multisample rasterization of all primitives differs substantially from single-sample rasterization. It is understood that each pixel in the framebuffer has SAMPLES_3DFX locations associated with it. These locations are exact positions, rather than regions or areas, and each is referred to as a sample point. The sample points associated with a pixel may be located inside or outside of the unit square that is considered to bound the pixel. Furthermore, the pattern (relative location) of sample points may be identical for each pixel in the framebuffer, or it may differ. The locations of the sample points are fixed by the implementation and it is not possible to query the actual sample locations of a pixel. If the sample patterns differ per pixel, they should be aligned to window, not screen, boundaries. Otherwise rendering results will be window-position specific. The invariance requirement described in section 3.1 is relaxed for all enabled multisample rasterization, because the sample patterns may be a function of pixel location. 3.3.2 Point Multisample Rasterization If MULTISAMPLE_3DFX is enabled, and SAMPLE_BUFFERS_3DFX is one, then points are rasterized using the following algorithm, regardless of whether point antialiasing (POINT_SMOOTH) is enabled or disabled. Point rasterization produces a fragment for each framebuffer pixel with one or more sample points that intersect the region lying within the circle having diameter equal to the current point width and centered at the point's (Xw,Yw). The coverage value for each fragment is 1. Mask bits that correspond to sample points that intersect the circular region are 1, other mask bits are 0. All depth values of the fragment are assigned the depth value of the point being rasterized. The data associated with each fragment are otherwise the data associated with the point being rasterized. Point size range and number of gradations are equivalent to those supported for antialiased points. 3.4.4 Line Multisample Rasterization If MULTISAMPLE_3DFX is enabled, and SAMPLE_BUFFERS_3DFX is one, then lines are rasterized using the following algorithm, regardless of whether line antialiasing (LINE_SMOOTH) is enabled or disabled. Line rasterization produces a fragment for each framebuffer pixel with one or more sample points that intersect the rectangular region that is described in the Antialiasing section of 3.4.2 (Other Line Segment Features). If line stippling is enabled, the rectangular region is subdivided into adjacent unit-length rectangles, with some rectangles eliminated according to the procedure given under Line Stipple, where "fragment" is replaced by "rectangle". The coverage value for each fragment is 1. Mask bits that correspond to sample points that intersect a retained rectangle are 1, other mask bits are 0. Each depth value is produced by substituting the corresponding sample location into equation 3.1, then using the result to evaluate equation 3.3. The data associated with each fragment are otherwise computed by evaluating equation 3.1 at the fragment center, then substituting into equation 3.2. Line width range and number of gradations are equivalent to those supported for antialiased lines. 3.5.7 Polygon Multisample Rasterization If MULTISAMPLE_3DFX is enabled, and SAMPLE_BUFFERS_3DFX is one, then polygons are rasterized using the following algorithm, regardless of whether polygon antialiasing (POLYGON_SMOOTH) is enabled or disabled. Polygon rasterization produces a fragment for each framebuffer pixel with one or more sample points that satisfy the point sampling criteria described in section 3.5.1, including the special treatment for sample points that lie on a polygon boundary edge. If a polygon is culled, based on its orientation and the CullFace mode, then no fragments are produced during rasterization. Fragments are culled by the polygon stipple just as they are for aliased and antialiased polygons. The coverage value for each fragment is 1. Mask bits that correspond to sample points that satisfy the point sampling criteria are 1, other mask bits are 0. Each depth value is produced by substituting the corresponding sample location into the barycentric equations described in section 3.5.1, using the approximation to equation 3.4 that omits w components. The data associated with each fragment are otherwise computed by barycentric evaluation using the fragment's center point. If POLYGON_OFFSET_FILL is enabled then the offset value computed in equation 3.7 is added to all sample depth values. The rasterization described above applies only to the FILL state of PolygonMode. For POINT and LINE, the rasterizations described in 3.3.2 (Point Multisample Rasterization) and 3.4.4 (Line Multisample Rasterization) apply with the additional requirement that the sample depth values must be offset by the value computed in equation 3.7 when POLYGON_OFFSET_POINT or POLYGON_OFFSET_LINE is enabled. 3.6.5 Multisample Rasterization of Pixel Rectangles If MULTISAMPLE_3DFX is enabled, and SAMPLE_BUFFERS_3DFX is one, then pixel rectangles are rasterized using the following algorithm. Let (Xrp,Yrp) be the current raster position. (If the current raster position is invalid, then DrawPixels is ignored.) If a particular group (index or components) is the nth in a row and belongs to the mth row, consider the region in window coordinates bounded by the rectangle with corners (Xrp + Zx*n, Yrp + Zy*m) and (Xrp + Zx*(n+1), Yrp + Zy*(m+1)) where Zx and Zy are the pixel zoom factors specified by PixelZoom, and may each be either positive or negative. A fragment representing group n,m is produced for each framebuffer pixel with one or more sample points that lie inside, or on the bottom or left boundary, of this rectangle. Each fragment so produced takes its associated data from the group and from the current raster position, in a manner consistent with SGIX_pixel_texture (if it is implemented) or in a manner consistent with the discussion in the Conversion to Fragments subsection of section 3.6.4 of the GL specification. All depth sample values are assigned the same value, taken either from the group (if it is a depth component group) or from the current raster position (if it is not). A single pixel rectangle will generate multiple, perhaps very many fragments for the same framebuffer pixel, depending on the pixel zoom factors. 3.7.1 Bitmap Multisample Rasterization If MULTISAMPLE_3DFX is enabled, and SAMPLE_BUFFERS_3DFX is one, then bitmaps are rasterized using the following algorithm. If the current raster position is invalid, the bitmap is ignored. Otherwise, a screen-aligned array of pixel-size rectangles is constructed, with its lower-left corner at (Xrp,Yrp), and its upper right corner a (Xrp+w,Yrp+h), where w and h are the width and height of the bitmap. Rectangles in this array are eliminated if the corresponding bit in the bitmap is zero, and are retained otherwise. Bitmap rasterization produces a fragment for each framebuffer pixel with one or more sample points either inside or on the bottom or left edge of a retained rectangle. Additions to Chapter 4 of the 1.2 Specification (Per-Fragment Operations and the Frame Buffer) 4.1.9 Multisample Fragment Operations If the DrawBuffers mode is NONE, no change is made to any multisample or color buffer. Otherwise, fragment processing is as described below. If MULTISAMPLE_3DFX is enabled, and SAMPLE_BUFFERS_3DFX is one, the stencil test, depth test, blending, and dithering operations described in sections 4.1.4, 4.1.5, 4.1.6, and 4.1.7 are performed for each pixel sample, rather than just once for each fragment. Failure of the stencil or depth test results in termination of the processing of that sample, rather than discarding of the fragment. All operations are performed on the color, depth, and stencil values stored in the multisample buffer (to be described in a following section). The contents of the color buffers are not modified at this point. Stencil, depth, blending, and dithering operations are performed for a pixel sample only if that sample's fragment mask bit is 1. If the corresponding mask bit is 0, no operations are performed for that sample. Depth operations use the fragment depth value that is specific to each sample. The single fragment color value is used for all sample operations, however, as is the current stencil value. If MULTISAMPLE_3DFX is disabled, and SAMPLE_BUFFERS_3DFX is one, the fragment may be treated exactly as described above, with optimization possible because the fragment mask must be all 1's. Further optimization is allowed, however. An implementation may choose to identify a centermost sample, and to perform stencil and depth tests on only that sample. Regardless of the outcome of the stencil test, all multisample buffer stencil sample values are set to the appropriate new stencil value. If the depth test passes, all multisample buffer depth sample values are set to the depth of the fragment's centermost sample's depth value, and all multisample buffer color sample values are set to the color value of the incoming fragment. Otherwise, no change is made to any multisample buffer color or depth value. After all operations have been completed on the multisample buffer, the color sample values are combined to produce a single color value, and that value is written into each color buffer that is currently enabled, based on the DrawBuffers mode. The method of combination is not specified, though a simple average computed independently for each color component is recommended. 4.2.2.5 Fine Control of Multisample Buffer Updates When SAMPLE_BUFFERS_3DFX is one, ColorMask, DepthMask, and StencilMask control the modification of values in the multisample buffer. The color mask has no effect on modifications to the color buffers. If the color mask is entirely disabled, the color sample values must still be combined (as described above) and the result used to replace the color values of the buffers enabled by DrawBuffers. 4.2.3.5 Clearing the Multisample Buffer The color samples of the multisample buffer are cleared when one or more color buffers are cleared, as specified by the Clear mask bit COLOR_BUFFER_BIT and the DrawBuffers mode. If the DrawBuffers mode is NONE, the color samples of the multisample buffer cannot be cleared. Clear mask bits DEPTH_BUFFER_BIT and STENCIL_BUFFER_BIT indicate that the depth and stencil samples of the multisample buffer are to be cleared. If Clear mask bit DEPTH_BUFFER_BIT is specified, and if the DrawBuffers mode is not NONE, then the multisample depth buffer samples are cleared. Likewise, if Clear mask bit STENCIL_BUFFER_BIT is specified, and if the DrawBuffers mode is not NONE, then the multisample stencil buffer is cleared. 4.3.2 Reading Pixels [These changes are made to the text in section 4.3.2, following the subheading Obtaining Pixels from the Framebuffer.] Follow the sentence "If there is no depth buffer, the error INVALID_OPERATION occurs." with: If there is a multisample buffer (SAMPLE_BUFFERS_3DFX is 1) then values are obtained from the depth samples in this buffer. It is recommended that the depth value of the centermost sample be used, though implementations may choose any function of the depth sample values at each pixel. Follow the sentence "if there is no stencil buffer, the error INVALID_OPERATION occurs." with: If there is a multisample buffer, then values are obtained from the stencil samples in this buffer. It is recommended that the stencil value of the centermost sample be used, though implementations may choose any function of the stencil sample values at each pixel. This extension makes no change to the way that color values are obtained from the framebuffer. Additions to Chapter 5 of the 1.2 Specification (Special Functions) None Additions to Chapter 6 of the 1.2 Specification (State and State Requests) An additional group of state variables, MULTISAMPLE_BIT_3DFX, is defined by this extension. When PushAttrib is called with bit MULTISAMPLE_BIT_3DFX set, the multisample group of state variables is pushed onto the attribute stack. When PopAttrib is called, these state variables are restored to their previous values if they were pushed. Some multisample state is included in the ENABLE_BIT group as well. In order to avoid incompatibility with GL implementations that do not support 3DFX_multisample, ALL_ATTRIB_BITS does not include MULTISAMPLE_BIT_3DFX. Additions to the GLX 1.3 Specification The parameter GLX_SAMPLE_BUFFERS_3DFX is added to glXGetFBConfigAttrib. When queried, by calling glXGetFBConfigAttrib with attribute set to GLX_SAMPLE_BUFFERS_3DFX, it returns the number of multisample buffers included in the visual. For a normal visual, the return value is zero. A return value of one indicates that a single multisample buffer is available. The number of samples per pixel is queried by calling glXGetFBConfigAttrib with attribute set to GLX_SAMPLES_3DFX. It is understood that the number of color, depth, and stencil bits per sample in the multisample buffer are as specified by the GLX_*_SIZE parameters. It is also understood that there are no single-sample depth or stencil buffers associated with this visual -- the only depth and stencil buffers are those in the multisample buffer. GLX_SAMPLES_3DFX is zero if GLX_SAMPLE_BUFFERS_3DFX is zero. glXChooseFBConfig accepts GLX_SAMPLE_BUFFERS_3DFX in attrib_list, followed by the minimum number of multisample buffers that can be accepted. Visuals with the smallest number of multisample buffers that meets or exceeds the specified minimum number are preferred. Currently operation with more than one multisample buffer is undefined, so the returned value will be either zero or one. glXChooseFBConfig accepts GLX_SAMPLES_3DFX in attrib_list, followed by the minimum number of samples that can be accepted in the multisample buffer. Visuals with the smallest number of samples that meets or exceeds the specified minimum number are preferred. The multisample FBConfig attributes are added to table 3.4 as follows: Attribute Default Selection Criteria Sort Priority --------- ------- ------------------ ------------- GLX_SAMPLE_BUFFERS_3DFX 0 Smaller GLX_SAMPLES_3DFX 0 Smaller If the GLX implementation is 1.2 or less, then GLX_SAMPLE_BUFFERS_3DFX and GLX_SAMPLES_3DFX are accepted by glXChooseVisual and glXGetConfig. If the color samples in the multisample buffer store fewer bits than are stored in the color buffers, this fact will not be reported accurately. Presumably a compression scheme is being employed, and is expected to maintain an aggregate resolution equal to that of the color buffers. Additions to WGL The parameters WGL_SAMPLE_BUFFERS_3DFX and WGL_SAMPLES_3DFX are added to wglGetPixelFormatAttribivARB and wglGetPixelFormatAttribfvARB. WGL_SAMPLE_BUFFERS_3DFX indicates 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. WGL_SAMPLES_3DFX gives the number of samples per pixel. It is understood that the number of color, depth, and stencil bits per sample in the multisample buffer are as specified by the WGL_*_BITS 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_SAMPLES_3DFX is zero if WGL_SAMPLE_BUFFERS_3DFX is zero. wglChoosePixelFormatARB accepts WGL_SAMPLE_BUFFERS_3DFX in piAttribIList and/or pfAttribFList, followed by the minimum number of multisample buffers that can be accepted. Pixel formats with the smallest number of multisample buffers that meet or exceed the specified minimum number are preferred. Currently operation with more than one multisample buffer is undefined, so the returned value will be either zero or one. wglChoosePixelFormatARB accepts WGL_SAMPLES_3DFX in piAttribIList and/or pfAttribFList, followed by the minimum number of samples that can be accepted in the multisample buffer. Pixel formats with the smallest number of samples that meet or exceed the specified minimum number are preferred. Errors None New State Get Value Get Command Type Initial Value Attribute --------- ----------- ---- ------------- --------- MULTISAMPLE_3DFX IsEnabled B TRUE multisample/enable New Implementation Dependent State Get Value Get Command Type Minimum Value --------- ----------- ---- ------------- SAMPLE_BUFFERS_3DFX GetIntegerv Z+ 0 SAMPLES_3DFX GetIntegerv Z+ 0 Dependencies on GLX 1.3 and GLX_EXT_fbconfig If GLX 1.3 is not supported and GLX_EXT_fbconfig is not supported then all references to glXGetFBConfigAttrib and glXGetConfig are removed. Revision History Version 1.1 - April 18, 2000 (Jon Leech, SGI) Specified value of MULTISAMPLE_BIT_3DFX. Assigned values to GLX_SAMPLE_BUFFERS_3DFX and GLX_SAMPLES_3DFX. Conversion from HTML -> text and formatting cleanup.