| Name |
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| CHROMIUM_framebuffer_mixed_samples |
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| Name Strings |
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| GL_CHROMIUM_framebuffer_mixed_samples |
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| Version |
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| Last Modifed Date: 7. December, 2015 |
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| Dependencies |
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| OpenGL ES 2.0 is required. |
| |
| This extension interacts with CHROMIUM_framebuffer_multisample. |
| |
| Overview |
| |
| This extension allows multisample rendering with a raster and |
| depth/stencil sample count that is larger than the color sample count. |
| Rasterization and the results of the depth and stencil tests together |
| determine the portion of a pixel that is "covered". It can be useful to |
| evaluate coverage at a higher frequency than color samples are stored. |
| This coverage is then "reduced" to a collection of covered color samples, |
| each having an opacity value corresponding to the fraction of the color |
| sample covered. The opacity can optionally be blended into individual |
| color samples. |
| |
| The key features of this extension are: |
| |
| - It allows a framebuffer object to be considered complete when its depth |
| or stencil samples are a multiple of the number of color samples. |
| |
| - It redefines SAMPLES to be the number of depth/stencil samples (if any); |
| otherwise, it uses the number of color samples. SAMPLE_BUFFERS is one if |
| there are multisample depth/stencil attachments. Multisample |
| rasterization and multisample fragment ops are allowed if SAMPLE_BUFFERS |
| is one. |
| |
| - It replaces several error checks involving SAMPLE_BUFFERS by error |
| checks directly referencing the number of samples in the relevant |
| attachments. |
| |
| - A coverage reduction step is added to Per-Fragment Operations which |
| converts a set of covered raster/depth/stencil samples to a set of |
| covered color samples. The coverage reduction step also includes an |
| optional coverage modulation step, multiplying color values by a |
| fractional opacity corresponding to the number of associated |
| raster/depth/stencil samples covered. |
| |
| New Procedures and Functions |
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| void CoverageModulationCHROMIUM(enum components); |
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| New Tokens |
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| Accepted by the <pname> parameter of GetBooleanv, GetIntegerv and |
| GetFloatv: |
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| COVERAGE_MODULATION_CHROMIUM 0x9332 |
| |
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| Additions to specification text related to framebuffers and framebuffer objects |
| (Framebuffers and Framebuffer Objects) |
| |
| Pending CHROMIUM_framebuffer_multisample specification. |
| Apply relevant rules in spirit of the overview to the specification text. |
| |
| |
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| Additions to Chapter 3 of OpenGL ES 2.0 Specification (Rasterization) |
| |
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| Modify Section 3.2 (Multisampling) |
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| Pending CHROMIUM_framebuffer_multisample specification. |
| Apply relevant rules in spirit of the overview to the specification text. |
| |
| Additions to Chapter 4 of OpenGL ES 2.0 Specification |
| (Per-Fragment Operations and the Framebuffer) |
| |
| Modify Figure 4.1 (Per-fragment operations) |
| |
| Add a new stage called "Coverage Reduction" between "Dept Buffer Test" and |
| "Blending". |
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| Add a new Section 4.1.Y (Coverage Reduction) after 4.1.5. |
| |
| If the value of effective raster samples is greater than the value of |
| color samples, a fragment's coverage is reduced from |
| effective raster samples bits to color samples bits. There is an |
| implementation-dependent association of raster samples to color samples. |
| The reduced "color coverage" is computed such that the coverage bit for |
| each color sample is 1 if any of the associated bits in the fragment's |
| coverage is on, and 0 otherwise. Blending and writes to the framebuffer |
| are not performed on samples whose color coverage bit is zero. |
| |
| For each color sample, the associated bits of the fragment's coverage are |
| counted and divided by the number of associated bits to produce a |
| modulation factor R in the range (0,1] (a value of zero would have been |
| killed due to a color coverage of 0). Specifically: |
| |
| N = value of effective raster samples; |
| M = value of color samples; |
| R = popcount(associated coverage bits) / (N / M); |
| |
| For each draw buffer with a floating point or normalized color format, the |
| fragment's color value is replicated to M values which may each be |
| modulated (multiplied) by that color sample's associated value of R. This |
| modulation is controlled by the function: |
| |
| CoverageModulationCHROMIUM(enum components); |
| |
| <components> may be RGB, RGBA, ALPHA, or NONE. If <components> is RGB or |
| RGBA, the red, green, and blue components are modulated. If components is |
| RGBA or ALPHA, the alpha component is modulated. The initial value of |
| COVERAGE_MODULATION_CHROMIUM is NONE. |
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| New State |
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| Get Value Get Command Type Initial Value Description Sec. Attribute |
| --------- ----------- ---- ------------- ----------- ---- --------- |
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| COVERAGE_MODULATION_CHROMIUM GetIntegerv E NONE Which components are 4.1.Y - |
| multiplied by coverage |
| fraction |
| Issues |
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| (1) How is coverage modulation intended to be used? |
| |
| RESOLVED: Coverage modulation allows the coverage to be converted to |
| "opacity", which can then be blended into the color buffer to accomplish |
| antialiasing. This is similar to the intent of POLYGON_SMOOTH. For example, |
| if non-premultiplied alpha colors are in use (common OpenGL usage): |
| |
| glCoverageModulationCHROMIUM(GL_ALPHA); |
| glEnable(GL_BLEND); |
| glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, |
| GL_ONE, GL_ONE_MINUS_SRC_ALPHA); |
| |
| or if using pre-multiplied alpha colors (common in 2D rendering): |
| |
| glCoverageModulationCHROMIUM(GL_RGBA); |
| glEnable(GL_BLEND); |
| glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA); |