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,

• Form a vector from the centre of the screen (texture coordinate [0.5, 0.5]T) and the current texture coordinate. 
• Sample the image along this vector, forming a blurred colour.
• Use mix, based on distance, so that pixels farther from the centre get more blurring

Note: texture2D() is deprecated -- use texture() instead.

High

Tutorial: stackoverflow