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I initially thought that the word "post-processing" referred specifically to the set of techniques aimed at improving a rendered image (like antialiasing, depth of field, bloom and others) while "multi-pass rendering" or "compositing" both referred to the entire process used a creating an image (including any post-processing effects).

But, according to Wikipedia, the word "post-processing" also includes the techniques used to build a rendered image like shadow mapping. Which makes me wonder if all those words actually refer to the same thing.

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  • \$\begingroup\$ "Post-processing" is less specific. Post meaning it happens after something, and what you are processing is important for context too. You could "post-process" several different things at several different moments while rendering a frame. It's not logically sound to say all of those different forms of "post-processing" are the same thing, so they still deserve their own names that are more specific. Like "Multi-Pass Rendering", "Compositing", and "Screen Space Shading". Those are all different things. \$\endgroup\$
    – Romen
    Sep 15 at 14:14
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    \$\begingroup\$ I'm surprised to see shadow mapping included in that Wikipedia list of "post processing" effects. It might have been an over-eager edit, or it may be referring to somewhat unusual variants on shadowing, where shadows are painted into the scene after everything else, rather than as part of a lighting pass (which would normally be considered to occur "during" not "post" rendering). \$\endgroup\$
    – DMGregory
    Sep 19 at 3:10

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Multi pass rendering, although it is used in other scenarios, nowadays is typically used to describe a way to render stereoscopic images. Multi pass means that depth rendering and other initial calculations for rendering to each eye are done separately for each eye (more than one pass= multi). Some newer methods are more performant by doing a single pass on these calculations for both eyes at once.

One single pass is typically more performant than multi pass. However, this optimization comes with some hits to flexibility. It’s more difficult to do different rendering to each eye.

Post processing typically refers to anything you do to the rendered image after rendering occurs. So you could do post-processing on either multipass rendered images or single pass images. Some typical post processing effects include exposure, bloom, tinting, split toning etc. although there are a huuuuge array of different things you can do in post processing.

Compositing usually involves combining multiple rendered image to form a single result. For example you may overlay one rendered image on top of another. Portal effects typically use compositing. The area of the primary view image that is taken up by the portal is replaced by the image rendered from a secondary viewpoint that is viewing the scene at a different location. These two viewpoints are composited to create the resulting effect.

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  • \$\begingroup\$ So how you would you classify Shadow mapping? Also, are you sure Multi-pass rendering is specific to stereoscopic rendering? What if, in order to produce a non-stereoscopic image I need to render a single scene multiple times from different points of view? For instance, from the POV of the light for shadow maps, or from under a water surface to render water reflection. Both these techniques are arguably more common than stereoscopic projection (at least in the world of video game) so why wouldn't this be called multi-pass rendering? \$\endgroup\$
    – neeh
    Sep 19 at 16:18
  • \$\begingroup\$ I’m not sure of the reasons. But these days multipass tends to speak mostly about stereo rendering in VR. I’m sure other things qualify as multiple passes. For example some materials are rendered via multiple passes that increase detail and accuracy. But in my experience multi pass is mostly used for VR. Shadow mapping is often multipass, with one pass for each light. But I suspect it’s so common that it’s just called shadow mapping and isn’t lumped with other things. Again this is based on my own experience and conventions that I’ve learned. Others might have different opinions. \$\endgroup\$
    – Adam B
    Sep 20 at 2:05

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