To create a composite from multiple images, you can make parts of one or more of the images transparent so that other images can show through. You can make portions of a layer transparent using any of several features in After Effects, including the following:
Roto Brush and Refine Edge tools in CC ( See Roto Brush and Refine Edge)
mattes (See Track mattes and traveling mattes.)
painting on the alpha channel (See Paint with the Brush tool.)
the Preserve Underlying Transparency layer option (See Preserve underlying transparency during compositing.)
keying effects (See Keying.)
To make an entire layer uniformly transparent or semi-transparent, modify its Opacity property.
Layers can also be composited together without modifying the transparency of the layers themselves. For example, you can use blending modes or some of the Channel effects to blend image data from multiple layers into a composite. (See Blending modes and layer styles and Channel effects.)
After Effects also includes the mocha shape for After Effects (mocha shape AE) plug-in, which converts paths from mocha-AE into mattes in After Effects. (See Resources for Imagineer mocha shape for After Effects.)
Adobe Photoshop is an excellent application for performing many compositing tasks, including defining areas of transparency using its selection and painting tools. You can use Adobe Photoshop together with After Effects for optimum efficiency and best results when doing compositing work.
Online resources for compositing
Aharon Rabinowitz provides an introduction to compositing, “What is Compositing?”—part of the Multimedia 101 series on the Creative COW website.
Mark Christiansen provides a detailed overview of compositing—covering masks, mattes, blending modes, and alpha channels—in a chapter from his After Effects Studio Techniques book posted on the Adobe Press website.
Chris and Trish Meyer provide a tutorial on the Artbeats website that demonstrates how to create a light wrap, so that a foreground element blends in more convincingly with a background.
Rich Young collects tutorials and resources for creating light wraps on the After Effects Portal website.
Rich Young collects resources and tutorials for various methods of creating vignettes.
Rich Young collects resources for sky replacement on the ProVideo Coalition website.
Chris Zwar provides tips on color keying and compositing on his website.
Jeff Foster provides free sample chapters from his book The Green Screen Handbook: Real World Production Techniques. The sample chapters cover basic compositing, color keying, garbage mattes, hold-out mattes, and how to avoid common problems with greenscreen shots. For more information, see Todd Kopriva's blog.
Compositing fire, explosions, muzzle flashes
Mark Christiansen provides tips and detailed techniques for creating and compositing fire, explosions, muzzle flashes, bullet hits, and energy blasts in the “Pyrotechnics: Creating Fire, Explosions, and Energy Phenomena in After Effects” chapter of After Effects Studio Techniques on the Peachpit Press website.
Compositing fog, smoke, and clouds
Mark Christiansen provides tips and detailed techniques for creating and compositing fog, smoke, mist, rain, and snow in the “Climate: Air, Water, Smoke, Clouds in After Effects” chapter of After Effects Studio Techniques on the Peachpit Press website.
Daniel Broadway provides tips for compositing fog or mist into a scene on his website.