How to create ios blur effect with gles

Question

My app cross android/ios, it based on GLES, I want to create a blurring overlay view behide some messagebox, any way implement ios style blur with gles?

Answer 1

Runtime blurring requires sampling input with a kernel, and averaging the samples. This is expensive, and grows more and more expensive with the size of the blur kernel. Gaussian filter with OpenGL Shaders explains how to implement this in OpenGL ES.

However, for efficiency, blurred shadows for things such as message boxes are done offline, because their shape does not change dynamically (even though their scale may). So, you would create an image (offline) that contains the blurred message box, and render a primitive underneath your message box geometry, with blurred image mapped on it.