Encode Tab
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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.
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.
Codec
Keyframe Frequency
Cinepak
15 - 30
Sorenson
100 - 300
H.264
30
Содержание 64006-051108-9001 - Discreet Cleaner - Mac
Страница 1: ...from discreet November 2000 ...
Страница 8: ...Contents viii ...
Страница 16: ...Chapter 1 Introduction 8 ...
Страница 24: ...Chapter 2 Capture 16 ...
Страница 54: ...Chapter 4 Projects 46 ...
Страница 68: ...Chapter 5 Settings 60 ...
Страница 148: ...Chapter 7 Formats 140 ...
Страница 160: ...Chapter 8 Encoding 152 ...
Страница 220: ...Chapter 8 Encoding 212 ...
Страница 248: ...Chapter 11 Workflow 240 ...
Страница 254: ...Chapter 12 Troubleshooting 246 ...
Страница 284: ...Glossary 276 ...