Web Design in a Nutshell, 3rd Edition

Spatial Versus Temporal Compression

Spatial (or intraframe) compression takes place on each individual frame of the video, compressing the pixel information as though it were a still image.

Temporal (or interframe) compression happens over a series of frames and takes advantage of areas of the image that remain unchanged from frame to frame, throwing out data for repeated pixels.

Temporal compression relies on the placement of key frames interspersed throughout the frames sequence. The key frames are used as masters against which the following frames (called delta frames) are compared. It is recommended that a key frame be placed once every 3 to 10 seconds. Videos without a lot of motion, such as talking head clips, take the best advantage of temporal compression . Videos with pans and other motion are compressed less efficiently.