What is postprocessing?

What is postprocessing? In Crysis there is that option to select it as high, medium or low. Is this more CPU intensive or GPU intensive?

 

From the tweakguides Crysis guide: http://www.tweakguides.com/Crysis_7.html

Looks like something your GPU will mostly be performing.

 

Post Processing seems to be the new GFX eye candy fad in games.

It the effects you see in games like:

Depth of Field - Blur landscapes in the distance or close up like Camera Out of Focus.

Motion Blur - Blurs screen movement to give games a more "movie feel" during fast motion.

HDR - High Dynamic Range Lighting - Lighting of environment changes according to were the camera view is (Inside/Outside)

Colour Correction and Clamping - Seems to limit the range of colours to give a more realistic feel or washed out feel.

Looks to be all GPU intensive (Shaders) and can stress graphics cards a lot more at higher resolutions. Coli McRae DiRT is a good example of Post Proccesing intensive game which can chug on even the newest cards at extreme resolutions.

 

The GPU is basically applying things to a finished image. It's like applying a filter in Photoshop. Harleyquin and bunbuns have shared some nice knowledge.

 

Technically, here's what it is:
(And yay, for once they've actually come up with a descriptive name, as you'll see)

First, you render your game the normal way, except for one thing. Instead of rendering to the screen (after which you can't do anything with it), you render it and save it as a texture.

Then you load up that texture, run a hell of a lot of shaders on that, and render the result to the screen.
Post-processing. Processing done *after* rendering.

The advantage is that you can then work on the final image, whereas in the first pass, you work on each object in isolation. You don't know which background the object is rendered against, what kind of colors are used in the entire scene, the average lighting levels and so on.

All that can be computed as a postprocessing pass. Then you can make bloom lighting (simply find bright areas and smear them out a bit), depth of field or a million other effects. Post-processing is basically just image manipulation, similar to what you can do in photoshop. ( edit: Damn, Chaz beat me to that one)

It's also slow, because it requires a second render pass. And obviously, it's 100% GPU-dependant. (more specifically, 100% pixel shader-dependant. Vertex shaders are useless for the second pass, since all you really render is a screen-sized rectangle (4 vertices), with a texture on it)
running a vertex shader on 4 vertices is unlikely to stress any GPU. It's when all the pixel shaders have to be run a second time that the performance hit occurs