HDRenderPipeline: Parrallax occlusion mapping add lod fading

Assets/ScriptableRenderLoop/HDRenderPipeline/Material/Lit/LitDataInternal.hlsl

     float2 uv = ADD_IDX(layerTexCoord.base).uv;
 
     // Compute lod as we will sample inside a loop (so can't use regular sampling)
-    float lod = CALCULATE_TEXTURE2D_LOD(ADD_IDX(_HeightMap), ADD_ZERO_IDX(sampler_HeightMap), uv);
+    // It appear that CALCULATE_TEXTURE2D_LOD only return interger lod. We want to use float lod to have smoother transition and fading
+    // float lod = CALCULATE_TEXTURE2D_LOD(ADD_IDX(_HeightMap), ADD_ZERO_IDX(sampler_HeightMap), uv);
+    float lod = ComputeTextureLOD(uv, GET_TEXELSIZE_NAME(ADD_IDX(_HeightMap))); 
 
     // Do a first step before the loop to init all value correctly
     float2 texOffsetCurrent = 0;
     float2 offset = texOffsetCurrent - ratio * texOffsetPerStep;
 
 #endif
+
+    // TODO: expose LOD fading
+    //float lodThreshold = 0.0;
+    //offset *= (1.0 - saturate(lod - lodThreshold));
 
     // Apply offset only on base. Details could use another mapping and will not be consistant...
     // Don't know if this will still ok.

Assets/ScriptableRenderLoop/HDRenderPipeline/Material/Lit/LitProperties.hlsl

 
 TEXTURE2D(_HeightMap);
 SAMPLER2D(sampler_HeightMap);
+float4 _HeightMap_TexelSize; // Unity facility. This will provide the size of the heightmap to the shader
 
 float _HeightAmplitude;
 float _HeightCenter;
 PROP_DECL(float, _NormalScale);
 
 PROP_DECL_TEX2D(_HeightMap);
+float4 _HeightMap0_TexelSize;
+float4 _HeightMap1_TexelSize;
+float4 _HeightMap2_TexelSize;
+float4 _HeightMap3_TexelSize;
 
 PROP_DECL_TEX2D(_DetailMask);
 PROP_DECL_TEX2D(_DetailMap);

Assets/ScriptableRenderLoop/ShaderLibrary/Common.hlsl

 }
 
 // ----------------------------------------------------------------------------
+// Texture utilities
+// ----------------------------------------------------------------------------
+
+// texelSize is Unity XXX_TexelSize feature parameters
+// x contains 1.0/width, y contains 1.0 / height, z contains width, w contains height
+float ComputeTextureLOD(float2 uv, float4 texelSize)
+{
+    uv *= texelSize.zw;
+
+    float2 ddx_ = ddx(uv);
+    float2 ddy_ = ddy(uv);
+    float d = max(dot(ddx_, ddx_), dot(ddy_, ddy_));
+
+    return  max(0.5 * log2(d), 0.0);
+}
+
+// ----------------------------------------------------------------------------
 // Texture format sampling
 // ----------------------------------------------------------------------------
 

Assets/ScriptableRenderLoop/ShaderLibrary/CommonMaterial.hlsl

 // Transforms 2D UV by scale/bias property
 #define TRANSFORM_TEX(tex, name) ((tex.xy) * name##_ST.xy + name##_ST.zw)
 
+#define GET_TEXELSIZE_NAME(name) (name##_TexelSize)
+
 #endif // UNITY_COMMON_MATERIAL_INCLUDED