I am rendering an image using OpenGL on C++, and want to access the resulting image to do some more processing on it. (I’m rendering an image, have an actual image it’s supposed to look like, and want to compute the pixel difference between the two.)
So far I have only been rendering images to the screen, though, and I can’t figure out how to render an image and then later get access at the direct pixels which were drawn. I don’t especially care if I can see the image on the screen or not, all I want is that the image gets rendered to some region of memory which I can access from the CPU. How do you do this?
Alternatively, would it be possible to send the image it’s supposed to look like to OpenGL and compute the pixel difference on the GPU? Either option is fine with me, but the faster I can make it the better. (Right now, I can render about 100 frames per second, but still haven’t figured out how to do the comparisons.)
Yes, you could do it on the GPU. Put the 2 images in textures. Draw a frame-filling quad multi-textured with the two textures, and be sure to provide texture coordinates. Write a fragment shader to compute the difference. (When a commenter asked if you wanted to use a programmable pipeline, this is one reason it matters. If you only use the fixed-function pipeline, you wouldn’t have the option of writing a fragment shader.)