Sample distortion mip chain with different safe guard to avoid NaNs (#4105)

* Fix

* Actual fix

* changelog

Co-authored-by: sebastienlagarde <[email protected]>

Author: FrancescoC-unity

com.unity.render-pipelines.high-definition/CHANGELOG.md

@@ -147,6 +147,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed issue with history buffers when using multiple AOVs (case 1323684).
 - Fixed camera preview with multi selection (case 1324126).
 - Fixed a NaN generating in Area light code.
+- Fix potential NaN on apply distortion pass.
 
 ### Changed
 - Changed Window/Render Pipeline/HD Render Pipeline Wizard to Window/Rendering/HDRP Wizard

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/RenderPass/Distortion/ApplyDistortion.shader

@@ -92,9 +92,15 @@ Shader "Hidden/HDRP/ApplyDistortion"
             float mip = (_ColorPyramidLodCount - 1) * saturate(distortionBlur) * _RoughDistortion;
 
             uint mipCeiled = ceil(mip);
-            float texelsToClamp = (1u << mipCeiled);
 
-            float2 uv = ClampAndScaleUV(distordedUV, _Size.zw, texelsToClamp);
+            int2 mipSize = int2(_Size.xy) >> mipCeiled;
+            // Clamp to the max size that is safe on the lowest mip. Note we recompute the full size this way to account for the
+            // rounding that can happen to sizes (and that _RTHandleScale won't represent correctly as we descend the mip chain)
+            float2 maxCoord = (mipSize << mipCeiled) * _Size.zw;
+            // Take of the half pixel for bilinear
+            maxCoord -= 0.5 * rcp(mipSize);
+
+            float2 uv = min(distordedUV, maxCoord) * _RTHandleScale.xy;
             float4 sampled = SAMPLE_TEXTURE2D_X_LOD(_ColorPyramidTexture, s_trilinear_clamp_sampler, uv, mip);
             return sampled;
         }