Autodesk 64006-051108-9001 User Guide - Page 191

Encode Tab 183 Some codecs let you specify different options for keyframes. The possible keyframe options are: •None (Natural Only) - This turns off all keyframes other than the first frame and naturally occurring keyframes. •Every - The number you enter in the Every field with this option is like a "timer" that counts the number of frames between keyframes. Every time there is a keyframe, automatic or natural, the timer resets. Natural keyframes may also occur depending on the codec. •All - This option makes every frame in the movie a keyframe. This feature is useful in encoding movies for editing purposes. Depending on the codec, the frequency at which you need to insert keyframes can vary. As a start, set keyframe frequency rates according to the following table. Codec Cinepak Sorenson H.264 Keyframe Frequency 15 - 30 100 - 300 30 With modern codecs, having more automatic keyframes gives the viewer more random access at the expense of quality at a given data rate. The "keyframe strobing" effect is an issue with low data rates. It can occur when adding more keyframes and result in lower image quality for your movie. When using streaming codecs, such as the Sorenson Video codec, a keyframe every five to ten seconds is usually a better starting point because these codecs depend heavily on temporal encoding to achieve high quality at low data rates and do not need very many keyframes. Temporal encoding targets compression of information between frames. In fact, increasing the keyframe rate often degrades the final image quality of movies made with these codecs. For example, if the source is 15 fps, start at a keyframe frequency of 150. The number you enter in the keyframes field with the Every pop-up menu selected is like a "timer" that counts the number of frames between keyframes. Every time there is a keyframe, automatic or natural, the timer is reset. For example, a keyframe rate of 15 does not mean a user should expect keyframes on frames 15, 30 and 45, but rather every 15 frames after the "timer" resets. Additional keyframes may be inserted by the codec as needed for scene changes and other "high-change" segments. Also, if the natural keyframe rate is higher than the automatic keyframe rate, there are no automatic keyframes. Higher keyframe frequency is important for movies that viewers randomly access because each time a movie is accessed, the current frame must be calculated from the nearest keyframe. If the nearest keyframe is many frames away, it can take a substantial amount of time to generate the current frame.This applies to progressive-streaming or CD-ROM content, but generally not to realtime-streaming content.