Motion blur is also a screen space technique.

It works by determining, for a current fragment p, where it was located on a previous frame (let’s call this q).  A blur vector is formed between p and q, and colours are averaged.



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,

• Retrieve the distance to the point from the depth buffer, forming p
• Unproject to local coordinates using the inverse projection matrix and inverse modelview matrix, forming p3d.
• Project p3d using the previous frame’s modelview / projection matrix and shift to the range [0, 1], forming q.
• Blur the image along the vector between p and q.

High

Tutorial: john-champan-graphics