GLSL - one-pass gaussian blur

Question

It is possible to implement fragment shader to do one-pass gaussian blur? I have found lot of implementation of two-pass blur (gaussian and box blur):

• http://callumhay.blogspot.com/2010/09/gaussian-blur-shader-glsl.html
• http://www.gamerendering.com/2008/10/11/gaussian-blur-filter-shader/
• http://www.geeks3d.com/20100909/shader-library-gaussian-blur-post-processing-filter-in-glsl/

and so on.

I have been thinking of implementing gaussian blur as convolution (in fact, it is the convolution, the examples above are just aproximations):

http://en.wikipedia.org/wiki/Gaussian_blur

Answer 1

Yes, you can implement Gaussian blur in one pass, by sampling all n^2 pixels in the kernel (for kernel width n). It's usually faster to run it on the rows and columns in two passes, since then you have O(n) pixels to sample rather than O(n^2). This is not an approximation, since Gaussian blur is mathematically separable.