fix(vtkVolumeFS): fix samples weighting in average blending

[Jo-Byr]

Context

Average blend mode has some weighting errors resulting in a noise texture being applied on the volume.

This is mostly visible if the number of samples on the volume is low, as the last sample is weighted more than it should, which is less visible if it's averaged with more samples.

Results

Said errors are fixed

Changes

Remove weighting of unique samples
Add missing weight to numerator of last sample in average and additive blending

PR and Code Checklist

• semantic-release commit messages
• Run npm run reformat to have correctly formatted code

[sankhesh]

LGTM

[daker]

@Jo-Byr Can you apply the same to WebGPU

diff --git a/Sources/Rendering/WebGPU/VolumePassFSQ/index.js b/Sources/Rendering/WebGPU/VolumePassFSQ/index.js
index 172a346510..0fbda7b240 100644
--- a/Sources/Rendering/WebGPU/VolumePassFSQ/index.js
+++ b/Sources/Rendering/WebGPU/VolumePassFSQ/index.js
@@ -1400,13 +1400,9 @@ fn traverseAverage(vTex: texture_3d<f32>, vNum: i32, rowIdx: i32, rayLengthSC: f
   let firstValue = getTextureValue(vTex, tpos, vNum);
   if (raySpan <= 1.0)
   {
-    // Match the OpenGL mapper's effective coverage for a subsample projection
-    // ray. Its rasterized entry/exit interval is 5/4 of the analytic interval
-    // used by this fullscreen WebGPU pass.
-    let thinRayWeight = raySpan * 1.25;
     traverseVals[vNum] = processVolumeSample(
       vTex, fragPos, vNum, rowIdx,
-      minPosSC + rayStepSC * rayBounds.x, tpos, firstValue * thinRayWeight, tfunRows, false);
+      minPosSC + rayStepSC * rayBounds.x, tpos, firstValue, tfunRows, false);
     return;
   }
 
@@ -1438,7 +1434,7 @@ fn traverseAverage(vTex: texture_3d<f32>, vNum: i32, rowIdx: i32, rayLengthSC: f
   let endValue = getTextureValue(vTex, endTpos, vNum);
   if (valueWithinIPRange(endValue, vNum))
   {
-    sum = sum + endValue;
+    sum = sum + endValue * (rayBounds.y - curDist);
     totalWeight = totalWeight + rayBounds.y - curDist;
   }
 
@@ -1486,7 +1482,7 @@ fn traverseAdditive(vTex: texture_3d<f32>, vNum: i32, rowIdx: i32, rayLengthSC:
   {
     traverseVals[vNum] = processVolumeSample(
       vTex, fragPos, vNum, rowIdx,
-      minPosSC + rayStepSC * rayBounds.x, tpos, firstValue * raySpan, tfunRows, false);
+      minPosSC + rayStepSC * rayBounds.x, tpos, firstValue, tfunRows, false);
     return;
   }
 
@@ -1514,7 +1510,7 @@ fn traverseAdditive(vTex: texture_3d<f32>, vNum: i32, rowIdx: i32, rayLengthSC:
   let endValue = getTextureValue(vTex, endTpos, vNum);
   if (valueWithinIPRange(endValue, vNum))
   {
-    sum = sum + endValue;
+    sum = sum + endValue * (rayBounds.y - curDist);
   }
   traverseVals[vNum] = processVolumeSample(
     vTex, fragPos, vNum, rowIdx,

[daker]

LGTM