Blur is another image processing operation that can be computed with a 2D filter.  These filters are lowpass filters, since they pass the low frequencies (signal that changes slowly) but diminish the higher frequencies (edges).

The idea is to mix the colour of a pixel with that of its neighbours using a weighted sum.

There are many types of lowpass filter kernels.  Popular ones include:

• Moving average, also called a box filter.  This just averages the pixels in a neighbourhood around a pixel.
• Gaussian filter.  This applies a weighting based on a 2D Gaussian function.  The neighbours are weighted so that ones farther away contribute less.
• (others)

How to create blur (Greg's recipe)

• Pass 1:  Render the scene to texture using an FBO
• Pass 2:  Render a full-screen quad using the texture from pass 1 to the default framebuffer.  In the fragment shader, set the fragment colour to the average of the pixels in an NxN window around the current fragment.

High