@MrBunny:
IIRC JPEG uses flooring, but MPEG uses rounding after quantization. Mainly because rounding to nearest integer results in a smaller quantization error.
Quote:
...is that you should be really careful with putting really high coefficients in a QM, since large coeffs will result in large error
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But high values in the matrix will drive more coefficients to zero. Because quantization matrix values are 8-bit values, I believe the highest you can put in there is 256. If you want a certain coefficient to become zero, why shouldn't you just put 256 in that position?