| Name |
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| NV_multisample_filter_hint |
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| Name Strings |
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| GL_NV_multisample_filter_hint |
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| Contact |
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| Mark J. Kilgard, NVIDIA Corporation (mjk 'at' nvidia.com) |
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| Notice |
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| Copyright NVIDIA Corporation, 2001. |
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| IP Status |
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| NVIDIA Proprietary. |
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| Status |
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| Shipping, May 2001 |
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| Version |
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| NVIDIA Date: May 16, 2001 |
| Version: 1 |
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| Number |
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| 259 |
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| Dependencies |
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| Written based on the OpenGL 1.2.1 specification. |
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| Requires ARB_multisample. |
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| Overview |
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| OpenGL multisampling typically assumes that the samples of a given |
| pixel are weighted uniformly and averaged to compute the pixel's |
| resolved color. This extension provides a hint that permits |
| implementations to provide an alternative method of resolving the |
| color of multisampled pixels. |
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| As an example of such an alternative method, NVIDIA's GeForce3 GPU |
| provides a technique known as Quincunx filtering. This technique |
| is used in two-sample multisampling, but it blends the pixel's two |
| samples and three additional samples from adjacent pixels. The sample |
| pattern is analogous to the 5 pattern on a die. The quality of this |
| technique is widely regarded as comparable to 4 sample multisampling. |
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| Issues |
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| Is the glHint mechanism the right mechanism to expose this functionality? |
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| RESOLUTION: Yes. Multisample filtering quality is subject to |
| the kinds of variations that the glHint was intended to control. |
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| Arguably, the glHint mechanism only provides two definite settings: |
| GL_FASTEST and GL_NICEST while there may be many different |
| techniques for controlling multisample filtering quality. |
| We expect hardware to support only one or two techniques rather |
| than a multitude of nearly indistinguishable sampling techniques. |
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| When does changing the multisampling filter hint take effect? |
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| RESOLUTION: It may not be until the next swap buffers or glClear |
| operation that the multisample hint actually takes effect. |
| This may be implementation dependent. |
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| What is the meaning of GL_DONT_CARE for the multisample hint? |
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| RESOLUTION: By default, NVIDIA expects to treat GL_DONT_CARE |
| the same as GL_FASTEST. However, the meaning of GL_DONT_CARE |
| for this hint may be subject to a registry (or environment) setting, |
| possibly settable through a control panel. |
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| Does GL_NICEST require Quincunx filtering? |
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| RESOLUTION: No. NVIDIA's GeForce3 Quincunx filtering is one |
| possible technique that may be used to implement the GL_NICEST |
| setting, but future GPUs may use other techniques. |
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| Can the meaning of the multisample hint vary depending on the number |
| of samples of the drawable? |
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| RESOLUTION: Yes. |
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| The following describes how GeForce3 uses the multisample hint: |
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| When using 2-sample multisampling with GeForce3, the multisample |
| filter hint affects multisample filtering as follows: GL_NICEST uses |
| 5-tap Quincunx multisample filtering while GL_FASTEST uses standard |
| even-weighted 2-tap multisample filtering of the pixel's 2 samples. |
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| When using 4-sample multisampling with GeForce3, the multisample |
| filter hint affects multisample filtering as follows: GL_NICEST |
| uses 9-tap 3x3 multisample filtering while GL_FASTEST uses standard |
| even-weighted 4-tap multisample filtering of the pixel's 4 samples. |
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| What is the difference between a "tap" and a "sample"? |
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| In the context of multisample filtering, a sample is |
| a subpixel frame buffer sample containing color, depth, and |
| stencil information. A tap is a source of data for filtering. |
| Typically, samples are filtered by evenly weighting all the samples |
| belonging to a pixel. In this case, the number of taps for the |
| filter is equal to the number of samples for the pixel. In other |
| filtering schemes, the number of taps and samples may not be equal |
| (and potentially not evenly weighted as well). For example, |
| GeForce3's quincunx filtering uses 5 taps even though each pixel |
| has only 2 multisample samples. Three of the five taps source |
| samples outside the pixel's footprint of two samples. |
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| Should the multisample filtering technique be determined by the |
| visual/PFD rather than OpenGL rendering context state? |
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| RESOLUTION: No. The number of multisample samples per pixel that |
| a window has is a property of the visual/PFD, but the filtering |
| technique does not have to be defined up-front at when the pixel |
| format is set. |
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| While not quite consistent with the way ARB_multisample is specified, |
| NVIDIA uses the SwapBuffers operation as a trigger for downsampling |
| multisample sample buffers (other operations such as glReadPixels |
| also trigger downsampling). But a SwapBuffers operation can be |
| requested without a current OpenGL rendering context. What happens |
| when a SwapBuffers operation is performed with no current OpenGL |
| rendering context? |
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| RESOLUTION: The multisample filter hint is treated as GL_DONT_CARE |
| in this case. Applications that want the multisample filter hint |
| to apply to their BufferSwap operation should perform the BufferSwap |
| operation while bound to an OpenGL rendering context. |
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| New Procedures and Functions |
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| None |
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| New Tokens |
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| Accepted by the <target> parameter of Hint and by the <pname> |
| parameter of GetBooleanv, GetIntegerv, GetFloatv, and GetDoublev: |
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| MULTISAMPLE_FILTER_HINT_NV 0x8534 |
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| Additions to Chapter 2 of the GL Specification (OpenGL Operation) |
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| None |
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| Additions to Chapter 3 of the GL Specification (Rasterization) |
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| None |
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| Additions to Chapter 4 of the GL Specification (Per-Fragment Operations |
| and the Framebuffer) |
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| None |
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| Additions to Chapter 5 of the GL Specification (Special Functions) |
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| -- Section 5.6 "Hints" |
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| Replace the description of hint targets in the first paragraph with: |
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| "target may be one of PERSPECTIVE_HINT, indicating the desired |
| quality of parameter interpolation; POINT_SMOOTH_HINT, indicating the |
| desired sampling quality of points; LINE_SMOOTH_HINT, indicating the |
| desired sampling quality of lines; POLYGON_SMOOTH_HINT, indicating |
| the desired sampling quality of polygons; FOG_HINT, indicating |
| whether fog calculations are done per pixel or per vertex; and |
| MULTISAMPLE_FILTER_HINT, indicating the desired quality of multisample |
| filtering. The MULTISAMPLE_FILTER_HINT is ignored if the frame buffer |
| has no multisample samples. When NICEST (or possibly DONT_CARE) |
| multisample filtering is requested and the frame buffer supports |
| multisampling, the multisample filter pattern may involve samples |
| outside the pixel's sample set. The exact NICEST (or possibly |
| DONT_CARE) multisample filtering technique used is implementation |
| dependent and may vary with the number of multisample samples |
| supported." |
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| Additions to Chapter 6 of the GL Specification (State and State Requests) |
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| None |
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| Additions to the GLX, WGL, and AGL Specification |
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| Add the following to the description of what happens at SwapBuffers |
| time. |
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| "When a SwapBuffers operation is performed by a thread without |
| a current OpenGL rendering context and the target drawable to be |
| swapped is multisampled, any multisample filtering operation that |
| occurs should be done as if the GL_MULTISAMPLE_FILTER_HINT value is |
| set to GL_DONT_CARE." |
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| GLX Protocol |
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| None |
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| Errors |
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| None |
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| New State |
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| (table 6.23, p213) add the following entry: |
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| Get Value Type Get Command Initial Value Description Sec Attribute |
| ---------------------------- ---- ----------- ------------- ----------- ------ -------------- |
| MULTISAMPLE_FILTER_HINT_NV Z3 GetIntegerv DONT_CARE Multisample filter 5.6 hint |
| quality hint |
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| Revision History |
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| None |
