Merge pull request #359 from EvgeniiG/tweak_lod_pattern

Implement a higher quality uniform noise function

Assets/ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/LitData.hlsl

 void GetSurfaceAndBuiltinData(FragInputs input, float3 V, inout PositionInputs posInput, out SurfaceData surfaceData, out BuiltinData builtinData)
 {
 #ifdef LOD_FADE_CROSSFADE // enable dithering LOD transition if user select CrossFade transition in LOD group
-    LODDitheringTransition(posInput.unPositionSS, unity_LODFade.x);
+    LODDitheringTransition(posInput.unPositionSS, unity_LODFade.x, _Time.x);
 #endif
 
     ApplyDoubleSidedFlipOrMirror(input); // Apply double sided flip on the vertex normal
 void GetSurfaceAndBuiltinData(FragInputs input, float3 V, inout PositionInputs posInput, out SurfaceData surfaceData, out BuiltinData builtinData)
 {
 #ifdef LOD_FADE_CROSSFADE // enable dithering LOD transition if user select CrossFade transition in LOD group
-    LODDitheringTransition(posInput.unPositionSS, unity_LODFade.x);
+    LODDitheringTransition(posInput.unPositionSS, unity_LODFade.x, _Time.x);
 #endif
 
     ApplyDoubleSidedFlipOrMirror(input); // Apply double sided flip on the vertex normal

Assets/ScriptableRenderPipeline/ShaderLibrary/Common.hlsl

 #endif
 #include "API/Validate.hlsl"
 
+#include "Noise.hlsl"
+
 // Some shader compiler don't support to do multiple ## for concatenation inside the same macro, it require an indirection.
 // This is the purpose of this macro
 #define MERGE_NAME(X, Y) X##Y
 // LOD dithering transition helper
 // LOD0 must use this function with ditherFactor 1..0
 // LOD1 must use this function with ditherFactor 0..1
-void LODDitheringTransition(uint2 unPositionSS, float ditherFactor)
+void LODDitheringTransition(uint2 unPositionSS, float ditherFactor, float time)
-    // Generate a fixed pattern
-    float p = cos(dot(unPositionSS, float2(443.8975, 397.2973)));
-    p = frac(p * 491.1871);
+    // Generate a spatio-temporally varying pattern.
+    float p = GenerateHashedRandomFloat(uint3(unPositionSS, asuint(time)));
 
     // We want to have a symmetry between 0..0.5 ditherFactor and 0.5..1 so no pixels are transparent during the transition
     // this is handled by this test which reverse the pattern

Assets/ScriptableRenderPipeline/ShaderLibrary/Noise.hlsl

+#ifndef UNITY_NOISE_INCLUDED
+#define UNITY_NOISE_INCLUDED
+
+// A single iteration of Bob Jenkins' One-At-A-Time hashing algorithm.
+uint JenkinsHash(uint x)
+{
+    x += (x << 10u);
+    x ^= (x >>  6u);
+    x += (x <<  3u);
+    x ^= (x >> 11u);
+    x += (x << 15u);
+    return x;
+}
+
+// Compound versions of the hashing algorithm.
+uint JenkinsHash(uint2 v)
+{
+    return JenkinsHash(v.x ^ JenkinsHash(v.y));
+}
+
+uint JenkinsHash(uint3 v)
+{
+    return JenkinsHash(v.x ^ JenkinsHash(v.y) ^ JenkinsHash(v.z));
+}
+
+uint JenkinsHash(uint4 v)
+{
+    return JenkinsHash(v.x ^ JenkinsHash(v.y) ^ JenkinsHash(v.z) ^ JenkinsHash(v.w));
+}
+
+// Construct a float with half-open range [0:1] using low 23 bits.
+// All zeros yields 0, all ones yields the next smallest representable value below 1.
+float ConstructFloat(uint m) {
+    const uint ieeeMantissa = 0x007FFFFFu; // Binary FP32 mantissa bitmask
+    const uint ieeeOne      = 0x3F800000u; // 1.0 in FP32 IEEE
+
+    m &= ieeeMantissa;                     // Keep only mantissa bits (fractional part)
+    m |= ieeeOne;                          // Add fractional part to 1.0
+
+    float  f = asfloat(m);                 // Range [1:2]
+    return f - 1;                          // Range [0:1]
+}
+
+// Pseudo-random value in half-open range [0:1]. The distribution is reasonably uniform.
+// Ref: https://stackoverflow.com/a/17479300
+float GenerateHashedRandomFloat(uint x)
+{
+    return ConstructFloat(JenkinsHash(x));
+}
+
+float GenerateHashedRandomFloat(uint2 v)
+{
+    return ConstructFloat(JenkinsHash(v));
+}
+
+float GenerateHashedRandomFloat(uint3 v)
+{
+    return ConstructFloat(JenkinsHash(v));
+}
+
+float GenerateHashedRandomFloat(uint4 v)
+{
+    return ConstructFloat(JenkinsHash(v));
+}
+
+#endif // UNITY_NOISE_INCLUDED

Assets/ScriptableRenderPipeline/ShaderLibrary/Noise.hlsl.meta

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