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.
Rich Young collects resources and tutorials for various methods of creating vignettes.
Chris Zwar provides tips on color keying and compositing on his website.
You can export a rendered movie as a sequence of still images, in which case each frame of the movie is output as a separate still-image file. When you render one movie using multiple computers on a network, the movie is always output as a still-image sequence. Many 3D animation programs accept sequences of still images. Sequences of PNG files are often a good choice for transfer of visual elements from After Effects to Flash Professional.
If you are creating a movie for transfer to film, you will need to create a sequence of still images that you can then transfer to film using a film recorder.
Creating a sequence of PSD files is a good way to transfer frames to Photoshop for touchup and editing. You can then import the image sequence back into After Effects.
When specifying the output filename for a still-image sequence, you actually specify a file-naming template. The name that you specify must contain pound signs surrounded by square brackets ([#####]). As each frame is rendered and a filename created for it, After Effects replaces the [#####] portion of the name with a number indicating the order of the frame in the sequence. For example, specifying mymovie_[#####].tga would cause output files to be named mymovie_00001.tga, filmout_00002.tga, and so on.
The maximum number of frames in a still-image sequence is 32,766.
You can export a single frame from a composition, either as an Adobe Photoshop (PSD) file with layers intact or as a rendered image. This is useful for editing files in Adobe Photoshop, creating a proxy, or exporting an image from a movie for posters or storyboards.
The Photoshop Layers command preserves all layers from a single frame of an After Effects composition in the resulting Photoshop file. Nested compositions up to five levels deep are preserved in the PSD file as layer groups. The PSD file inherits the color bit depth from the After Effects project.
In addition, the layered Photoshop file contains an embedded composite (flattened) image of all the layers. This feature ensures that the file is compatible with applications that don’t support Photoshop layers; such applications display the composited image and ignore the layers.
A layered Photoshop file saved from After Effects may look different from the frame viewed in After Effects if the frame uses features that Photoshop doesn’t support. For example, if the frame contains a blending mode that isn’t available in Photoshop, a blending mode that most resembles it is substituted in the layer, but the embedded composite image (viewable only by applications that don’t support Photoshop layers) looks the same. Alternatively, you can render the frame using the Composition > Save Frame As > File command to export a flattened and rendered version of the file to the PSD format.
PSD files generated by Save Frame As > Photoshop Layers have the sRGB IEC61966-2.1 ICC color profile embedded if color management is disabled for the project (the project's working color space is set to None). If color management is enabled for the project (the project's working color space is set to something other than None), then PSD files generated by Save Frame As > Photoshop Layers have the color profile embedded that corresponds to the project's working color space. (See Color management and color profiles.)
To render a single frame, choose Composition > Save Frame As > File. Adjust settings in the Render Queue panel if necessary, and then click Render.
To export a single frame as an Adobe Photoshop file with layers, choose Composition > Save Frame As > Photoshop Layers.
To change the default output settings for the Save Frame As > File command, change the settings for the Frame Default render settings template (See Create, edit, and manage render settings templates.)