PBR Renderer: render opacity instead of transparency to alpha channel

Author: TheMostDiligent

Hydrogent/interface/Tasks/HnBeginFrameTask.hpp

@@ -73,7 +73,7 @@ struct HnBeginFrameTaskParams
     };
     RenderTargetFormats Formats;
 
-    float4 ClearColor = {0, 0, 0, 0};
+    float3 ClearColor = {0, 0, 0};
 
     bool UseReverseDepth = false;
 
@@ -89,8 +89,8 @@ struct HnBeginFrameTaskParams
         float4 UnshadedColor = {1, 1, 1, 1};
         float  PointSize     = 1;
 
-        float4 LoadingAnimationColor0             = {0.1f, 0.100f, 0.10f, 1.0f};
-        float4 LoadingAnimationColor1             = {1.0f, 0.675f, 0.25f, 1.0f};
+        float3 LoadingAnimationColor0             = {0.1f, 0.100f, 0.10f};
+        float3 LoadingAnimationColor1             = {1.0f, 0.675f, 0.25f};
         float  LoadingAnimationWorldScale         = 1.0f;
         float  LoadingAnimationSpeed              = 0.25f;
         float  LoadingAnimationTransitionDuration = 0.5f;

Hydrogent/shaders/HnPostProcess.psh

@@ -142,7 +142,7 @@ void main(in  FullScreenTriangleVSOutput VSOut,
     }
 #endif
 
-    float Opacity = 1.0 - Color.a;
+    float Opacity = Color.a;
 
     float SSRScale = g_Attribs.SSRScale * Opacity;
     if (SSRScale > 0.0)

Hydrogent/src/Tasks/HnBeginFrameTask.cpp

@@ -258,16 +258,14 @@ void HnBeginFrameTask::PrepareRenderTargets(pxr::HdRenderIndex* RenderIndex,
     {
         if (i == HnFrameRenderTargets::GBUFFER_TARGET_SCENE_COLOR)
         {
-            if (RenderParam->GetViewMode() != HN_VIEW_MODE_SCENE_DEPTH)
-            {
-                // NB: we should clear alpha to one as it accumulates the total transmittance
-                ClearValues[i] = float4{m_Params.ClearColor.r, m_Params.ClearColor.g, m_Params.ClearColor.b, 1.0};
-            }
-            else
+            float3 ClearColor = m_Params.ClearColor;
+            if (RenderParam->GetViewMode() == HN_VIEW_MODE_SCENE_DEPTH)
             {
                 // Clear background to white in scene depth debug view mode
-                ClearValues[i] = float4{1};
+                ClearColor = float3{1.0};
             }
+            // NB: we should clear alpha to zero as it accumulates the total opacity
+            ClearValues[i] = float4{ClearColor, 0.0};
         }
         else
         {
@@ -672,8 +670,8 @@ void HnBeginFrameTask::UpdateFrameConstants(IDeviceContext* pCtx,
                 }
 
                 RendererParams.LoadingAnimation.Factor     = LoadingAnimationFactor;
-                RendererParams.LoadingAnimation.Color0     = m_Params.Renderer.LoadingAnimationColor0;
-                RendererParams.LoadingAnimation.Color1     = m_Params.Renderer.LoadingAnimationColor1;
+                RendererParams.LoadingAnimation.Color0     = float4{m_Params.Renderer.LoadingAnimationColor0, 0.0};
+                RendererParams.LoadingAnimation.Color1     = float4{m_Params.Renderer.LoadingAnimationColor1, 0.0};
                 RendererParams.LoadingAnimation.WorldScale = m_Params.Renderer.LoadingAnimationWorldScale;
                 RendererParams.LoadingAnimation.Speed      = m_Params.Renderer.LoadingAnimationSpeed;
 

Hydrogent/src/Tasks/HnRenderBoundBoxTask.cpp

@@ -87,8 +87,8 @@ void main(in BoundBoxVSOutput VSOut,
 {
     BoundBoxOutput BoundBox = GetBoundBoxOutput(VSOut);
 
-    // Write 1.0 alpha as if bound box was fully transparent.
-    Color = float4(BoundBox.Color.rgb, 1.0);
+    // Write 0.0 alpha as if bound box was fully transparent.
+    Color = float4(BoundBox.Color.rgb, 0.0);
 
     MotionVec = float4(BoundBox.MotionVector, 0.0, 1.0);
 }

Hydrogent/src/Tasks/HnRenderEnvMapTask.cpp

@@ -176,7 +176,7 @@ void HnRenderEnvMapTask::Prepare(pxr::HdTaskContext* TaskCtx,
     EnvMapAttribs.pEnvMap       = pEnvMapSRV;
     EnvMapAttribs.AverageLogLum = 0.3f;
     EnvMapAttribs.MipLevel      = 1;
-    EnvMapAttribs.Alpha         = 1;
+    EnvMapAttribs.Alpha         = 0; // Fully transparent
     EnvMapAttribs.Options       = EnvMapRenderer::OPTION_FLAG_COMPUTE_MOTION_VECTORS;
 
     {

PBR/src/PBR_Renderer.cpp

@@ -2057,9 +2057,14 @@ void PBR_Renderer::CreatePSO(PsoHashMapType&             PsoHashMap,
                 RT0.BlendOp   = BLEND_OPERATION_ADD;
             }
 
-            // Compute total transmittance, e.g. (1.0 - A0) * (1.0 - A1) ... * (1.0 - An),
+            // Compute total opacity, e.g. 1.0 - (1.0 - A0) * (1.0 - A1) ... * (1.0 - An),
             // in alpha channel.
-            RT0.SrcBlendAlpha  = BLEND_FACTOR_ZERO;          // SrcA * 0
+            // Dst[n]   = 1.0 - (1.0 - A[0]) * ... * (1.0 - A[n])
+            // Dst[n+1] = 1.0 - (1.0 - A[0]) * ... * (1.0 - A[n]) * (1.0 - A[n+1])
+            // Dst[n+1] = 1.0 - (1.0 - Dst[n]) * (1.0 - A[n+1]) = Dst[n] + A[n+1] - Dst[n] * A[n+1]
+            // Dst[n+1] = A[n+1] * 1.0 + Dst[n] * (1.0 - A[n+1])
+            //            SrcA           DstA            SrcA
+            RT0.SrcBlendAlpha  = BLEND_FACTOR_ONE;           // SrcA * 1.0
             RT0.DestBlendAlpha = BLEND_FACTOR_INV_SRC_ALPHA; // DstA * (1.0 - SrcA)
             RT0.BlendOpAlpha   = BLEND_OPERATION_ADD;
 

Shaders/PBR/private/RenderPBR.psh

@@ -624,16 +624,16 @@ PSOutput main(in VSOutput VSOut,
         if (Factor > 0.0)
         {
             float4 LoadingAnimationColor = GetLoadingAnimationColor(Shading.Pos, Factor);
-            OutColor = lerp(OutColor, float4(LoadingAnimationColor.rgb, 1.0), LoadingAnimationColor.a);
+            OutColor = lerp(OutColor, float4(LoadingAnimationColor.rgb, 0.0), LoadingAnimationColor.a);
         }
     }
 #endif
 
     OutColor.rgb *= Transmittance;
     if (BasicAttribs.AlphaMode != PBR_ALPHA_MODE_BLEND)
     {
-        // Write 0 alpha to indicate full opacity
-        OutColor.a = 0.0;
+        // Write 1.0 alpha to indicate full opacity
+        OutColor.a = 1.0;
     }
 
 #if CONVERT_OUTPUT_TO_SRGB