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Old 25th May 2003, 19:42   #13  |  Link
crusty
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Join Date: May 2002
Location: Holland
Posts: 309
So higher QM values will result in more variation thrown away in the first place, but will work with a bigger margin of error when it doesn't thrown away variation.
But as long as you use the higher values only for finer detail, it really doesn't matter that much now does it?
I don't know for sure, but I assume that higher rounding errors are less of a problem for finer detail.
The end effect would be that finer detail, if allowed through by the QM value, would simply have a bigger luminance error.
Is the human eye less sensitive for the luminance of fine detail?

The only type of scenes that I can think of that would be affected, off the top of my head, would be dark skies with stars in them or 'proper' space scenes.
This is because a real night sky with stars in them has high contrast fine detail.
The stars are basically points, but there is a difference in luminance that is very easily spotted. They vary easily from the average value, which would be the mean value of the sky (very dark blue), so they would easily get above even a high treshold, but they would then have a pretty large margin of error in luminance.
If you wanted to portray this type of scene realistically, you would have to find out the DCT frequency of the stars and then lower the QM value to allow for a smaller margin of error.
Otherwise the stars will look washed out and you might loose the 'twinkling' effect that a real-life night sky has.

Mind you, DVD's use mpeg2 compression which also uses DCT and macroblocks, so unless they specifically used a smaller QM value for the conversion from movie to DVD, this type of scene will look washed out on DVD anyway.
Also, most 'artificially' created (Special Effects) space scenes don't come close to realistically portraying stars. When astronauts first went up into space, they found the stars to be very bright and to have none of the 'twinkling' effect whatsoever, which offcourse is created by the atmosphere.
But in space the stars are so bright compared to the darkness of space that no TV or monitor can get close to portaying this kind of contrast.

Another thing;
I think that especially for anime movies many of the QM values could be higher, because contrast is usually much higher. I don't know for sure because I have never done an anime movie before, but I'll do Akira probably somewhere this month and I'll try it with different custom matrices.
I suspect a QM with lower values to low frequencies and higher to high frequencies will give a better result. And because anime has in general much sharper contrast than real-life footage, all the values could probably be higher to start with.
It might also be possible to find a 'sweet spot' for each different anime movie or serie because one movie could use finer lines than the other.
To all: Please try custom matrices based on this and report the effect.


Question to developers:
Is mosquito noise around sharp edges created by using too high frequencies for these edges?
I can imagine a very bright thin line getting a higher frequency than a very bright thick line, ergo the macroblock with a thin line getting additional thin lines in decoding because of the higher frequency used.
Is this correct or is this just plain nonsense?
If it's correct, then I assume that some form of postprocessing filter could be made to detect this type of noise and remove it, not just on Xvid, but on all Mpeg4.
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