Convolving an image with OpenGL ES on iPhone: possible?

Question

I've googled around a few times, but I have not gotten a straight answer. I have a matrix that I would like to convolve with a discrete filter (e.g. the Sobel operator for edge detection). Is it possible to do this in an accelerated way with OpenGL ES on the iPhone?

If it is, how how? If it is not, are there other high-performance tricks I can use to speed up the operation? Wizardly ARM assembly operations that can do it fast? Ultimately I want to perform as fast of a convolution as possible on an iPhone's ARM processor.

Answer 1

You should be able to do this using programmable shaders under OpenGL ES 2.0. I describe OpenGL ES 2.0 shaders in more detail in the video for my class on iTunes U.

Although I've not done image convolution myself, I describe some GPU-accelerated image processing for Mac and iOS here. I present a sample application that uses GLSL shaders (based on Core Image filters developed by Apple) that does realtime color tracking from the iPhone's camera feed.

Since I wrote this, I've created an open source framework based on the above example which has built-in image convolution filters, ranging from Sobel edge detection to custom 3x3 convolution kernels. These can run up to 100X faster than CPU-bound implementations.

However, if you were to do this on the CPU, you might be able to use the Accelerate framework to run some of the operations on the iPhone's NEON SIMD unit. In particular, FFT operations (which are usually a key component in image convolution filters, or so I've heard) can get a ~4-5X speedup by using the routines Apple provides here.