GPUImage Taking sum of columns of image

Question

Im using GPUImage in my project and I need an efficient way of taking the column sums. Naive way would obviously be retrieving the raw data and adding values of every column. Can anybody suggest a faster way for that?

Answer 1

One way to do this would be to use the approach I take with the GPUImageAverageColor class (as described in this answer), only instead of reducing the total size of each frame at each step, only do this for one dimension of the image.

The average color filter determines the average color of the overall image by stepping down in a factor of four in both X and Y, averaging 16 pixels into one at each step. If operating in a single direction, you should be able to use hardware interpolation to get an 18X reduction in a single direction per step with good performance. Your final step might either require a quick CPU-based iteration on the much smaller image or a tweaked version of this shader that pulls the last few pixels in a column together into the final result pixel for that column.

You notice that I've been talking about averaging here, because the output values for any OpenGL ES operation will need to be in terms of colors, which only have a 0-255 range per channel. A sum will easily overflow this, but you could use an average as an approximation of your sum, with a more limited dynamic range.

If you only care about one color channel, you could possibly encode a larger value into the RGBA channels and maintain a 32-bit sum that way.

Beyond what I describe above, you could look at performing this sum with the help of the Accelerate framework. While probably not quite as fast as doing a shader-based reduction, it might be good enough for your needs.