frame differencing.
Temporal (time) compression looks for ways to compact the description of the changes during a sequence of frames. It does this by looking
for patterns and repetition over time. For example, in a video clip of a person speaking in front of a static background, temporal compression will notice that the only pixels that change from frame to frame are those forming the face of the speaker. All the other pixels don't change (when the camera is motionless). Instead of describing every pixel in every frame, temporal compression describes all the pixels in the first frame, and then for each frame that
follows, describes only the pixels that are different from the
previous frame.
When most of the pixels in a frame are different from the previous frame, it's preferable to describe the entire frame again. Each whole
frame is called a keyframe, which sets a new starting point for frame differencing. Many codecs can create keyframes at an interval you specify. Some codecs automatically create a keyframe for a frame that
is visually very different from the previous frame. As you specify fewer keyframes, the data rate and file size decreases, and so does the picture quality. The degree of temporal compression is usually controlled by a codec's Quality option and by keyframe
settings, lowering the value for these options increases temporal
compression.
Always use keyframe setting as well as data rate limitations.
Keyframes every 75 at 15 frames per second is good for Sorenson, 15
every 15 fps is a good setting for Cinepak.
| VERSION: N/A | AND ALSO... |
| SOURCE: Bob Caligiuri |