Vulkan: Change sampleCoverage calculation

When emulating the sample coverage to a mask, truncate the value. Test: dEQP-GLES3.functional.multisample.fbo*sample_coverage* Bug: angleproject:4568 Change-Id: Ie0f9fee7dc7cf08e2289ff24f0fa69f9929c4ae3 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2433069 Commit-Queue: Brandon Schade <b.schade@samsung.com> Commit-Queue: Mohan Maiya <m.maiya@samsung.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>

Vulkan: Change sampleCoverage calculation
22d7db6e · Brandon Schade · Commit Bot · 6136cbcb · 22d7db6e
Commit 22d7db6e authored Sep 25, 2020 by Brandon Schade Committed by Commit Bot Oct 01, 2020
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Showing with 12 additions and 2 deletions

ContextVk.cpp src/libANGLE/renderer/vulkan/ContextVk.cpp +12 -2

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--- a/src/libANGLE/renderer/vulkan/ContextVk.cpp
+++ b/src/libANGLE/renderer/vulkan/ContextVk.cpp
@@ -165,8 +165,18 @@ uint32_t GetCoverageSampleCount(const gl::State &glState, FramebufferVk *drawFra
    }
    // Get a fraction of the samples based on the coverage parameters.
-    return static_cast<uint32_t>(
+    // There are multiple ways to obtain an integer value from a float -
-        std::round(glState.getSampleCoverageValue() * drawFramebuffer->getSamples()));
+    //     truncation, ceil and round
+    //
+    // round() provides a more even distribution of values but doesn't seem to play well
+    // with all vendors (AMD). A way to work around this is to increase the comparison threshold
+    // of deqp tests. Though this takes care of deqp tests other apps would still have issues.
+    //
+    // Truncation provides an uneven distribution near the edges of the interval but seems to
+    // play well with all vendors.
+    //
+    // We are going with truncation for expediency.
+    return static_cast<uint32_t>(glState.getSampleCoverageValue() * drawFramebuffer->getSamples());
 }
 void ApplySampleCoverage(const gl::State &glState,