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 <[email protected]>
Commit-Queue: Mohan Maiya <[email protected]>
Reviewed-by: Jamie Madill <[email protected]>
Reviewed-by: Shahbaz Youssefi <[email protected]>

Author: Brandon Schade

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>(
-        std::round(glState.getSampleCoverageValue() * drawFramebuffer->getSamples()));
+    // There are multiple ways to obtain an integer value from a float -
+    //     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,