I'm looking for some pointers on how to sample an image using OpenGL at a list of specified locations. Any links to tutorial or examples similar to the problem below?

At the moment we have a code that calculates the 'output intensity' at a list of specified locations *`x1,y1, x2,y2, ..., xn,yn`* applying a Lanczos2 filter to an input image. The amount of locations at the moment is 20 (which is actually the list of phosphene locations in a visual prosthesis) but it will eventually increase up to 256 and GPU processing will certainly accelerate things. The list of locations can't be hardcoded.

So far I have seen how to implement a median filter and alike, but in my case there is no need to compute the convolution with the filter kernel at every image pixel, just at the locations specified.