Originally posted by Barry_Green
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The P-frame is not targeted to match the I-frame, it's targeted to match the raw uncompressed
frame that's fed to the coder.
frame that's fed to the coder.
A P-frame looks at the changes between the previous frame, and the target frame. If the target frame is incredibly complex, and the previous frame is mushy, the P-frame will use its bandwidth to encode the changes between the mushy frame and the detailed reference frame.
much away from the I-frame, the keyframe looks poor, but the following frames in the GOP will also look poor if they do not have enough bitrate. If you use too much for the I-frame , the video deteriorate as you proceed in the GOP, as the P-frames do not have enough bitrate
Each frame in the GoP does not care what the prior frame looks like, it instead cares what the
target frame (the uncompressed image) looks like, and makes its best effort to look like that target
frame. So yes, cyclic codec pulsing is very possible if the I-frame does a poor job on the first frame
in the GoP.
target frame (the uncompressed image) looks like, and makes its best effort to look like that target
frame. So yes, cyclic codec pulsing is very possible if the I-frame does a poor job on the first frame
in the GoP.
In the clip I will post, you can watch it happen. The keyframe (I-frame) looks horrible because it's
got nowhere near enough bandwidth to represent the image properly. Then the P-frames bring it
successively up to snuff, adding more and more detail, until - smack, 15 frames later, the cycle
starts all over again with a lousy I-frame.
Whereas if the I-frame was a good-quality image in the first place, then the P-frames would have
very little work to do to maintain that quality image throughout the GoP.
got nowhere near enough bandwidth to represent the image properly. Then the P-frames bring it
successively up to snuff, adding more and more detail, until - smack, 15 frames later, the cycle
starts all over again with a lousy I-frame.
Whereas if the I-frame was a good-quality image in the first place, then the P-frames would have
very little work to do to maintain that quality image throughout the GoP.
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