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Re: HD workflow???

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Walter SoykaRe: HD workflow???
by on Apr 23, 2012 at 10:18:21 pm

After Effects, since it is bult to do image manipulation, always uses uncompressed frames for its internal rendering. AE thinks in uncompressed. To use any kind of compressed media, AE must decompress it into RAM for processing, then recompress it for output.

This is not unique to AE. Any time a frame of video needs to be modified, it must be decompressed for processing first. NLEs can only avoid recompression of media in certain specific situations.

H.264 uses inter-frame compression. That means that in order to reduce file size or data rate, not every frame stores complete information. Videos are broken down into a series of groups of pictures (called GOPs). These groups are made up of complete frames (called keyframes or I-frames) and incomplete frames (B- and P-frames). The incomplete frames refer back to common pixels of similar blocks from nearby frames. In order to compute a single incomplete frame, the computer may need to consider several frames, both complete and incomplete. This raises the computational power necessary for decode, and complicates cuts. If the cut takes place within a GOP and breaks the references sn incomplete frame relies on, that frame and any other similarly dependent frame must be recalculated. This means that even cuts from an inter-frame compressed clip may actually require a generation of decompression and recompression.

Codecs like uncompressed, ProRes, and DNxHD all use intra-frame compression, which means that every single frame is completely self-contained. An NLE can copy compressed frames for a cuts-only sequence, and computational requirements for decode are vastly lower.

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
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