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TACC / pvOSPRay

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Volume rendering is wrong #37

Open GregAbram opened 8 years ago

GregAbram commented 8 years ago

If you do a wavelet and use an all-white colormap against a white background, you should get a all-white image - a polar bear in snow picture, because opacity * white + (1 - opacity) * white is white. You do in a RenderView, but not in an OSPRay window - places where the opacity is low are dark. Maybe the data value is multiplied by the opacity twice - once as part of the accumulation along the array and later against the background?

carsonbrownlee commented 8 years ago

I’ve noticed that the colormaps in general for volumes and geometry seem to be a bit off. Will forward to the intel folks, your guess sounds likely for volumes, not sure what’s going on with geometry coloring.
Carson

On Feb 11, 2016, at 12:30 PM, Greg Abram notifications@github.com wrote:

If you do a wavelet and use an all-white colormap against a white background, you should get a all-white image - a polar bear in snow picture, because opacity * white + (1 - opacity) * white is white. You do in a RenderView, but not in an OSPRay window - places where the opacity is low are dark. Maybe the data value is multiplied by the opacity twice - once as part of the accumulation along the array and later against the background?

— Reply to this email directly or view it on GitHub https://github.com/TACC/pvOSPRay/issues/37.

GregAbram commented 8 years ago

Yes, its in OSPRay. RaycastVolumeRenderer_computeVolumeSample is attenuated first by the samples's w:

color = clamp(sampleOpacity / volume->samplingRate) * make_vec4f(sampleColor.x, sampleColor.y, sampleColor.z, 1.0f);

which is accumulated along the ray, where color is the accumulation and volumeColor is the sample:

color = color + (1.0f - color.w) * volumeColor;

Suppose the current color is (0,0,0,0) and we only do one sample, resulting in a volume color of (0.5,0.5,0.5,0.5) - that is, white attenuated by opacity/samplingRate = 0.5. The result of accumulation is (0.5,0.5,0.5,0.5) Now if background is enabled - say (1,1,1,1), it gets applied:

color = color.w * color + (1.0f - color.w) * background;

and the result is 0.5(0.5,0.5,0.5,0.5) + (1-0.5)(1,1,1,1) = (0.75,0.75,0.75,0.75)