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an arithmetic problem about uncompressed 4:2:2
I used Compressor to make uncompressed 8-bit and 10-bit 4:2:2 versions of a 30 second silent PAL clip. I’m trying to test my understanding of these formats by studying the file sizes. They’re 622082094 and 829422238 bytes respectively.
4:2:2 means that there’s Y data at every pixel with U data and V data each at just half the pixels. So for 8-bit, the average pixel has 16 bits while for 10-bit, the average pixel has 20 bits. So the bit count for the film will be 30*25*720*576*(16 or 20), and the byte count is 1/8 of this. The calculated answers are 622080000 for 8-bit and 777600000 for 10-bit. So the first answer agrees beautifully with the experimental file size. 2094 bytes for header info is reasonable. The second answer is way off. Apparently 10-bit uncompressed 4:2:2 can’t be understood the way I understood 8-bit uncompressed 4:2:2. What is going on here?I repeated the experiment using a 4 bit 4:4:4 30 second silent PAL clip and got exactly the same two file sizes.
It might be a clue that file size in the 10-bit case is very nearly 1/16 more than the calculated answer.
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3 Replies
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I’ve learned the answer to the arithmetic problem. Not interesting; it has to do with computers, not video. The uncompressed 10-bit 4:2:2 video is being stored using 32-bit data units to hold 30 bits of information. Thus 1/16 of the file size is waste. The pixels with just Y data must be grouped in threes.
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And, knowing that bit of trivia will help exactly how?
David Roth Weiss
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Los AngelesPOST-PRODUCTION WITHOUT THE USUAL INSANITY ™
A forum host of Creative COW’s Apple Final Cut Pro, Business & Marketing, and Indie Film & Documentary forums.
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DRW, as I wrote, the answer turned out not to be interesting. Please try to appreciate how an outsider — experienced in image technology but not in video — sees the swarm of competing codecs and their messy incomplete descriptions.
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