Alternative to NV Occlusion Query - getting the number of fragments which passed the depth test
- by Etan
In "modern" environments, the "NV Occlusion Query" extension provide a method to get the number of fragments which passed the depth test. However, on the iPad / iPhone using OpenGL ES, the extension is not available.
What is the most performant approach to implement a similar behaviour in the fragment shader?
Some of my ideas:
Render the object completely in white, then count all the colors together using a two-pass shader where first a vertical line is rendered and for each fragment the shader computes the sum over the whole row. Then, a single vertex is rendered whose fragment sums all the partial sums of the first pass. Doesn't seem to be very efficient.
Render the object completely in white over a black background. Downsample recursively, abusing the hardware linear interpolation between textures until being at a reasonably small resolution. This leads to fragments which have a greyscale level depending on the number of white pixels where in their corresponding region. Is this even accurate enough?
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