Pixel value grabbing advice please

Hi there,

OpenGL can be a bit restricting, which also makes it difficult to research good working practices. I know that this question gets asked often on different threads, but i want to ensure I’m exploring in the right direction for iOS.
I am working on a project where we want to display an image which will have been manipulated, then use touch positions to return pixel values. This will be a small part of the project, so need to find a very efficient approach, it doesn’t need to be exact, or even be pixel perfect (i.e. the sampled image could be much lower res than what is displayed)

Thanks in advance!
Miles