iphone - Blur Effect (Wet in Wet effect) in Paint Application Using OpenGL-ES

Question

I am developing Paint application using OpenGL-ES for iPhone and i want to implement Gaussian blur effect(Wet in Wet) for painting. Please have look at the image describing my requirement for Blur effect :

I tried to search how for OpenGL function but did not get anything. Can anyone guide me to a right direction in this problem.. Any kind of help or suggestion will be highly appreciated.. Thanks..

Answer 1

You should be able to render the same brush stroke many times pixels apart to get the effect you want. If you jitter the renders with a Gaussian distribution you will get a Gaussian blur.

This would be similar to jitter antialiasing with an accumulation buffer, but instead of using subpixel offsets you would use multi-pixel offsets as big as you want the blur effect. You'd would want to probably render around 16 times to make it look smooth. http://www.opengl.org/resources/code/samples/advanced/advanced97/notes/node63.html

This is also similar(or really the same thing) as jittering to create motion blur. http://glprogramming.com/red/chapter10.html

You wouldn't even NEED to use a separate accumulation buffer here, just render each pass with alpha that adds up to solid. One thing to remember, you want to always jitter across the same offsets so that successive frames look the same(i.e. if you are using random offsets then every frame will have slightly different blur effect).