add option to enable pre-multiplying alpha for the background

Author: jeffamstutz

devices/rtx/device/gpu/gpu_objects.h

@@ -693,6 +693,7 @@ struct RendererGPUData
   float inverseVolumeSamplingRate;
   float occlusionDistance;
   bool cullTriangleBF;
+  bool premultiplyBackground;
   bool tonemap; // enable internal tonemapping during sample accumulation
 };
 

devices/rtx/device/renderer/AmbientOcclusion_ptx.cu

@@ -198,8 +198,10 @@ VISRTX_GLOBAL void __raygen__()
         color *= opacity;
 
         const auto bg = getBackground(frameData, ss.screen, ray.dir);
-        accumulateValue(color, vec3(bg) * bg.a, opacity);
-        accumulateValue(opacity, bg.w, opacity);
+        const bool premultiplyBg = rendererParams.premultiplyBackground;
+        accumulateValue(
+            color, premultiplyBg ? vec3(bg) * bg.a : vec3(bg), opacity);
+        accumulateValue(opacity, bg.a, opacity);
         accumulateValue(outputColor, color, outputOpacity);
         accumulateValue(outputOpacity, opacity, outputOpacity);
         break;

devices/rtx/device/renderer/DirectLight_ptx.cu

@@ -406,8 +406,10 @@ VISRTX_GLOBAL void __raygen__()
         color *= opacity;
 
         const auto bg = getBackground(frameData, ss.screen, ray.dir);
-        accumulateValue(color, vec3(bg) * bg.a, opacity);
-        accumulateValue(opacity, bg.w, opacity);
+        const bool premultiplyBg = rendererParams.premultiplyBackground;
+        accumulateValue(
+            color, premultiplyBg ? vec3(bg) * bg.a : vec3(bg), opacity);
+        accumulateValue(opacity, bg.a, opacity);
         accumulateValue(outputColor, color, outputOpacity);
         accumulateValue(outputOpacity, opacity, outputOpacity);
         break;

devices/rtx/device/renderer/Raycast_ptx.cu

@@ -159,7 +159,9 @@ VISRTX_GLOBAL void __raygen__()
       color *= opacity;
 
       const auto bg = getBackground(frameData, ss.screen, ray.dir);
-      accumulateValue(color, vec3(bg) * bg.a, opacity);
+        const bool premultiplyBg = rendererParams.premultiplyBackground;
+        accumulateValue(
+            color, premultiplyBg ? vec3(bg) * bg.a : vec3(bg), opacity);
       accumulateValue(opacity, bg.w, opacity);
       accumulateValue(outputColor, color, outputOpacity);
       accumulateValue(outputOpacity, opacity, outputOpacity);

devices/rtx/device/renderer/Renderer.cpp

@@ -167,6 +167,7 @@ void Renderer::commitParameters()
   m_cullTriangleBF = getParam<bool>("cullTriangleBackfaces", false);
   m_volumeSamplingRate =
       std::clamp(getParam<float>("volumeSamplingRate", 0.125f), 1e-3f, 10.f);
+  m_premultiplyBackground = getParam<bool>("premultiplyBackground", false);
   if (m_checkerboard)
     m_spp = 1;
 }
@@ -207,6 +208,7 @@ void Renderer::populateFrameData(FrameGPUData &fd) const
   fd.renderer.inverseVolumeSamplingRate = 1.f / m_volumeSamplingRate;
   fd.renderer.numIterations = std::max(m_spp, 1);
   fd.renderer.maxRayDepth = m_maxRayDepth;
+  fd.renderer.premultiplyBackground = m_premultiplyBackground;
 }
 
 OptixPipeline Renderer::pipeline()

devices/rtx/device/renderer/Renderer.h

@@ -90,6 +90,7 @@ struct Renderer : public Object
   bool m_tonemap{true}; // enable internal tonemapping during sample accumulation
   int m_sampleLimit{0};
   bool m_cullTriangleBF{false};
+  bool m_premultiplyBackground{false};
   float m_volumeSamplingRate{1.f};
 
   helium::ChangeObserverPtr<Array2D> m_backgroundImage;

devices/rtx/device/visrtx_device.json

@@ -83,6 +83,13 @@
           "default": true,
           "description": "enable internal tonemapping during sample accumulation"
         },
+        {
+          "name": "premultiplyBackground",
+          "types": ["ANARI_BOOL"],
+          "tags": [],
+          "default": false,
+          "description": "pre-multiply alpha channel with background color"
+        },
         {
           "name": "cullTriangleBackfaces",
           "types": ["ANARI_BOOL"],
@@ -184,6 +191,13 @@
           "default": true,
           "description": "enable internal tonemapping during sample accumulation"
         },
+        {
+          "name": "premultiplyBackground",
+          "types": ["ANARI_BOOL"],
+          "tags": [],
+          "default": false,
+          "description": "pre-multiply alpha channel with background color"
+        },
         {
           "name": "cullTriangleBackfaces",
           "types": ["ANARI_BOOL"],
@@ -301,6 +315,13 @@
           "default": true,
           "description": "enable internal tonemapping during sample accumulation"
         },
+        {
+          "name": "premultiplyBackground",
+          "types": ["ANARI_BOOL"],
+          "tags": [],
+          "default": false,
+          "description": "pre-multiply alpha channel with background color"
+        },
         {
           "name": "volumeSamplingRate",
           "types": ["ANARI_FLOAT32"],