Temporal Compression
ÆInterframe
ÆCan use other frames as references
ÆA Frame tends to be similar to the frame preceding it
ÆSo TC encodes only the changes in one frame to another
ÆWith the exception of key frames
FRAME TYPES AND COMPRESSION
* A keyframe = a complete image compressed using intraframe compression
Ænot based on any other frames…always the first frame
Æimportant for random points as they the only frames that can be decoded all by themselves
Æmost authoring codecs, like Motion JPEG and DV use only keyframes – makes editing easier
*Delta frame = compressed using interframe compression + contains only portions of the adjacent frame that are different
ÆFor typical content, a delta frame might be only 10‐20% the size of the same frame at the same quality as a keyframe
Æ2 main types: P‐frames (predictive – based on previous frame) & B‐frames (bi‐directional – based on previous and subsequent frames)
Note:
Codecs vary in how well they encode complex frame transitions. For example, fades up and down are very difficult for some codecs, but some newer codecs have code built in to handle fades.
COLOUR MODES
‐Way codecs describe colour = important
Æ3 basic types of colour in today’s codecs: