[Mesa-dev] Clarifications w.r.t MSAA
Hi all, I'm getting a little lost in all the interactions between the different parts of the GL standards and what I understand of the expectations when it comes to MSAA. It would be nice if I could have some clarifications. I'll start with what I think I understand (and please correct me when I'm wrong) and add a number of questions. I'll also ignore the resolve part, which isn't an issue (at least for me :-). MSAA is a variant on the supersampling theme where the coverage is supersampled but depth, stencil and color may or may not be. The destination buffer has enough space to store the full results of a complete supersampling, but some of the values may be duplicated. The variable MIN_SAMPLE_SHADING_VALUE allows the application to control the minimum number of values that have to be computed. It can say for instance that in a 16xMSAA case at least 4 samples per pixel are required. So let's take a case of 16xMSAA (say with the DX11 pattern) and let's look at the pipeline. First the coverage is sampled for the 16 fixed positions, leaving C active samples. Then there should be early depth testing then shading, or the other way around, depending on the shaders. First question: how many depths should be computed, and for which coordinates? Which of these values is associated with which sample? Second question: how many samples should be shaded, and for which coordinates? What is the impact of depth testing failure? Third question: what happens when a variable has a sample qualifier in the fragment shader? Or centroid? Fourth question: how does gl_SampleMask interact with all that when more than one sample is evaluated. And what does gl_SampleMaskIn look like in the same case? I hope you people can help me clarify all that stuff :-) Best, OG. ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
Re: [Mesa-dev] Clarifications w.r.t MSAA
On 06/12/2012 12:27 PM, Olivier Galibert wrote: Hi all, I'm getting a little lost in all the interactions between the different parts of the GL standards and what I understand of the expectations when it comes to MSAA. It would be nice if I could have some clarifications. I'll start with what I think I understand (and please correct me when I'm wrong) and add a number of questions. I'll also ignore the resolve part, which isn't an issue (at least for me :-). MSAA is a variant on the supersampling theme where the coverage is supersampled but depth, stencil and color may or may not be. The destination buffer has enough space to store the full results of a complete supersampling, but some of the values may be duplicated. The variable MIN_SAMPLE_SHADING_VALUE allows the application to control the minimum number of values that have to be computed. It can say for instance that in a 16xMSAA case at least 4 samples per pixel are required. So let's take a case of 16xMSAA (say with the DX11 pattern) and let's look at the pipeline. First the coverage is sampled for the 16 fixed positions, leaving C active samples. Then there should be early depth testing then shading, or the other way around, depending on the shaders. First question: how many depths should be computed, and for which coordinates? Which of these values is associated with which sample? One for each sample point. The depth buffer will be multisampled as well. Coverage sampling (CSAA) where you have extra coverage samples that do NOT (necessarily) correspond to color sample locations are not covered by the GL spec, it's vendor-specific. Second question: how many samples should be shaded, and for which coordinates? What is the impact of depth testing failure? As many as the user requested via glMinSampleShading, and the sample locations to choose seem to be up to the implementation. If the depth test fails for all the samples associated with a fragment shader invocation, you can probably choose not to run it, which should not be noticeable. Third question: what happens when a variable has a sample qualifier in the fragment shader? Or centroid? From GL spec 4.1/3.9 Fragment Shaders: (Sorry if quoting the spec doesn't help, since you may have read it already, but now that I looked up the relevant parts for the questions I probably wouldn't formulate it much differently myself.) When interpolating variables declared using centroid in, the variable is sampled at a location within the pixel covered by the primitive generating the fragment. When interpolating variables declared using sample in when MULTISAMPLE is enabled, the fragment shader will be invoked separately for each (!) covered sample and the variable will be sampled at the corresponding sample point. Fourth question: how does gl_SampleMask interact with all that when more than one sample is evaluated. And what does gl_SampleMaskIn look like in the same case? If the fragment shader is being evaluated at any frequency other than per-fragment, bits of the sample mask not corresponding to the current fragment shader invocation are ignored. When per-sample shading is active due to the use of a fragment input qualified by sample, only the bit for the current sample is set in gl_SampleMaskIn. When GL state specifies multiple fragment shader invocations for a given fragment, the sample mask for any single fragment shader invocation may specify a subset of the covered samples for the fragment. In this case, the bit corresponding to each covered sample will be set in exactly one fragment shader invocation. I hope you people can help me clarify all that stuff :-) Best, OG. ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
Re: [Mesa-dev] Clarifications w.r.t MSAA
On 06/12/2012 02:25 PM, Olivier Galibert wrote: On Tue, Jun 12, 2012 at 01:50:08PM +0200, Christoph Bumiller wrote: First question: how many depths should be computed, and for which coordinates? Which of these values is associated with which sample? One for each sample point. The depth buffer will be multisampled as well. Coverage sampling (CSAA) where you have extra coverage samples that do NOT (necessarily) correspond to color sample locations are not covered by the GL spec, it's vendor-specific. Ok. So that means that if the shader writes z, you have to do full supersampling then. No, I don't think that's the case. You get per-sample depth values if you use fixed-pipe depth, but shader-computed depth should simply be replicated (to all samples covered by the shader invocation), like color outputs. Second question: how many samples should be shaded, and for which coordinates? What is the impact of depth testing failure? As many as the user requested via glMinSampleShading, and the sample locations to choose seem to be up to the implementation. Do you know what's usually expected? Center of collapsed samples, one of the samples, center of the pixel? Third question: what happens when a variable has a sample qualifier in the fragment shader? Or centroid? When interpolating variables declared using sample in when MULTISAMPLE is enabled, the fragment shader will be invoked separately for each (!) covered sample and the variable will be sampled at the corresponding sample point. So a sample anywhere means full supersampling, ok. Thanks, OG. ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
Re: [Mesa-dev] Clarifications w.r.t MSAA
On Tue, Jun 12, 2012 at 8:39 AM, Christoph Bumiller e0425...@student.tuwien.ac.at wrote: On 06/12/2012 02:25 PM, Olivier Galibert wrote: On Tue, Jun 12, 2012 at 01:50:08PM +0200, Christoph Bumiller wrote: First question: how many depths should be computed, and for which coordinates? Which of these values is associated with which sample? One for each sample point. The depth buffer will be multisampled as well. Coverage sampling (CSAA) where you have extra coverage samples that do NOT (necessarily) correspond to color sample locations are not covered by the GL spec, it's vendor-specific. Ok. So that means that if the shader writes z, you have to do full supersampling then. No, I don't think that's the case. You get per-sample depth values if you use fixed-pipe depth, but shader-computed depth should simply be replicated (to all samples covered by the shader invocation), like color outputs. I don't think thats how it wors, each sample will have its color and depth value no matter if fixed pipeline or not. When resolving the msaa surface, you only use the sample that cover the surface to make the average. Anyway that's my understanding. Cheers, Jerome ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
Re: [Mesa-dev] Clarifications w.r.t MSAA
On 06/12/2012 06:52 PM, Jerome Glisse wrote: On Tue, Jun 12, 2012 at 8:39 AM, Christoph Bumiller e0425...@student.tuwien.ac.at wrote: On 06/12/2012 02:25 PM, Olivier Galibert wrote: On Tue, Jun 12, 2012 at 01:50:08PM +0200, Christoph Bumiller wrote: First question: how many depths should be computed, and for which coordinates? Which of these values is associated with which sample? One for each sample point. The depth buffer will be multisampled as well. Coverage sampling (CSAA) where you have extra coverage samples that do NOT (necessarily) correspond to color sample locations are not covered by the GL spec, it's vendor-specific. Ok. So that means that if the shader writes z, you have to do full supersampling then. No, I don't think that's the case. You get per-sample depth values if you use fixed-pipe depth, but shader-computed depth should simply be replicated (to all samples covered by the shader invocation), like color outputs. I don't think thats how it wors, each sample will have its color and depth value no matter if fixed pipeline or not. When resolving the Sorry, fixed-pipe was misleading, I meant the z-value from the rasterizer (which can be regarded as fixed functionality), not without (custom) shaders. If the shader is only invoked once for each fragment (i.e. MinSampleShading == 1), all the samples that belong to that fragment will share the same color and depth values. msaa surface, you only use the sample that cover the surface to make the average. Anyway that's my understanding. Cheers, Jerome ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
Re: [Mesa-dev] Clarifications w.r.t MSAA
On Tue, Jun 12, 2012 at 1:34 PM, Christoph Bumiller e0425...@student.tuwien.ac.at wrote: On 06/12/2012 06:52 PM, Jerome Glisse wrote: On Tue, Jun 12, 2012 at 8:39 AM, Christoph Bumiller e0425...@student.tuwien.ac.at wrote: On 06/12/2012 02:25 PM, Olivier Galibert wrote: On Tue, Jun 12, 2012 at 01:50:08PM +0200, Christoph Bumiller wrote: First question: how many depths should be computed, and for which coordinates? Which of these values is associated with which sample? One for each sample point. The depth buffer will be multisampled as well. Coverage sampling (CSAA) where you have extra coverage samples that do NOT (necessarily) correspond to color sample locations are not covered by the GL spec, it's vendor-specific. Ok. So that means that if the shader writes z, you have to do full supersampling then. No, I don't think that's the case. You get per-sample depth values if you use fixed-pipe depth, but shader-computed depth should simply be replicated (to all samples covered by the shader invocation), like color outputs. I don't think thats how it wors, each sample will have its color and depth value no matter if fixed pipeline or not. When resolving the Sorry, fixed-pipe was misleading, I meant the z-value from the rasterizer (which can be regarded as fixed functionality), not without (custom) shaders. If the shader is only invoked once for each fragment (i.e. MinSampleShading == 1), all the samples that belong to that fragment will share the same color and depth values. So i think we agree but according to spec MinSampleShading=1 - the fragment shader is run once for each sample. MinSampleShading value is a fraction of x/MIN_SAMPLE_SHADING_VALUE_ARB So if you have 8 sample surface and you set MinSampleShading to 0.5 you will get the fragment shader invoked for 4 sample. Note that according to spec implementation might ignore the fraction and only cover the case MinSampleShading==1 Cheers, Jerome ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
