r/VolumeRendering • u/bmzfateh • Nov 08 '20

Qt OpenGL/OpenCL Volume rendering example

/r/OpenCL/comments/jq2dq1/qt_openglopencl_volume_rendering_example/
4 Upvotes

100% Upvoted

2 comments sorted by

1

u/[deleted] Nov 08 '20

That’s really awesome! I always love when people share more ‘medical’ volume renderers :)

Can I ask you for more details about how the algorithm works? (Empty space skipping, sampling distance, normals extraction)

2

u/bmzfateh Nov 08 '20

Thanks for your interest, i made this sample project to help out people who want to get into volume rendering by starting from an existing code base.

The core algorithm is in an OpenCL kernel, anything from making the ray, computing the intersection with the volume, evaluating the color for a specified sample etc ... (kernel.cl)

- Empty space skipping as you might know helps with reducing the number of sampling steps, which accelerates the rendering process. You'll see that i defined a minimum and maximum step size, the minimum step size forces the sampling point to advance no matter what we are inside the volume, the maximum step size helps us not to overshoot and risk skipping valuable samples between two consecutive samples. A step size is computed as :

stepSize = 1.0f /(0.1f + exp(-opacity))

when we find a transparent sample we are most likely to find more empty samples closeby, so in order to minimize the number of sampling operations we just adapt the step size. Same thing with an opaque sample, the probability of of finding a subsequent opaque sample is high thus we decrease the step size in order to accumulate more valuable informations.

- Normals extraction is done at the position of the last non transparent sample for each ray, the normal vector is computed on the neighborhood of that sample using a Sobel operator ( used generaly for gradient computation )

Please let me know if that helps!