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by jerf 4618 days ago
In addition to what was already there, I'd suggest looking at trying to operate on the whole image at a time rather than pixel-by-pixel. To take a simple blur example, a blur that samples the local pixel at 50%, and each of the top, bottom, left, and right by 12.5% is equivalent to drawing the entire image half darkened, then drawing it in each of one pixel to the left, right, up, and down at 12.5% of the original brightness, merging additively. I don't know if this would be faster or not, you'd have to profile it, but I'd consider it worth a try.
1 comments
xenonite 4618 days ago
On the contrary. Consider that the memory is the bottleneck when performing the blur. I understand you would create five instances of the images (50%, 12.5% Left, R, Top, B). This would worsen the bottleneck even more.

Additionally, the advantage of computing pixel by pixel is that the shader can operate massively parallel.

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jerf 4617 days ago
I think I wasn't clear. I mean, send 5 textured polygons to the 3D hardware, with various darkenings and offsets on one texture (which if nothing else can be done via lighting, but there's probably other easier ways), and let it do the blending en masse. Instead of using shaders and blinding it to what you're doing, it may be able to render the polygons much faster, on an optimized path. And it may not. But it's worth a try.
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sliverstorm 4618 days ago
I would think it is going to depend on your cache size. Piecewise will be better if the image can't all fit in the cache, but if the image is small enough you can fit everything in the cache.

Or, do you mean that memory is the bottleneck as in, shipping the image to the GPU's memory space?

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xenonite 4617 days ago
Yes, it really depends on your cache size.

And the problem with that is, you can't guess the cache size. You can help yourself with profiling, but this leads to a local optimization for only some GPUs.

If you wish to run your code optimized for any GPU, the pixel-by-pixel approach usually works best. Then, the GPU scheduler can run as many neighboring threads as possible in subprocessors. Note that every subprocessor has another local cache which is really quick.

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