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Old 26th May 2003, 18:38   #19  |  Link
crusty
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Join Date: May 2002
Location: Holland
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MrBunny:
Quote:
When some of these components get zeroed, the edge gets eroded (less sharp, less defined and unsettles the surrounding area) resulting in the mosquito noise we all hate.
So, if higher QM values for higher frequencies results in less accurate edge compression and more mosquito noise, I suggest the following to the developers:
Insert some form of 'edge detection algorithm' between DCT and Quantization, at least for the higher frequencies. That way, if normal picture content is below the threshold, it will not be encoded, but if there is an edge in the picture, the resulting DCT value would get an additional bonus, so it would go over the treshold easier, therefore reducing noise.
It would probably hurt compressability a bit, but it would result in less mosquito noise. It would be part of the encoding process only, so it would not break mpeg4 compatability.
Call it 'DCT edge detection' or something and allow the user to adjust it's 'DCT edge bonus'.
The question is offcourse whether the improvement in quality would counter the cost in bitrate, but there's only one way to find out....program it and let us test it.
Nic, SysKin, Koepi and others....any thoughts on this?

Didée:
Quote:
Zeroing by DCTfilter will directly remove image detail, whereas zeroing by encoder's matrix will remove the differences that are left over by ME. That are two different pairs of shoes!
This leads me to conclude that, since Qpel works with smaller motions than Hpel, Qpel will be less efficient when using high QM values, because the effect of Qpel will be nullified by the high thresholds of the QM. The minimum use of bits by Qpel will remain the same tho.
Am I right in this?

Quote:
What gets DCT'ed is the image after the motion compensation, and therefore it is not possible to directly draw the conclusion "fine detail" -> "high frequency", or the other way round. For example, it is perfectly possible that you have a block with pretty fine detail, but that block gets very nicely catched by ME, so the result that gets DCT'ed consists only out of low frequencies!
Isn't it so that when a pictures content is catched by ME, the corresponding macroblock doesn't contain ANY texture bits, but just motion vectors?

Kilg0r3:
Quote:
After all there are matrices that distinguish between these two. Other, however, have the same value for I-and P-cells. Any general suggestion what might be a wiser way to go?
If DCT in P- and B-frames generally results in low frequencies, low settings for high frequencies seem to me to have little or no influence to the end result. Maybe a higher threshold for these could lead to higher compressability without too much quality loss.
Also, if high QM values for high frequencies seem to counter the effect of Qpel, I would suggest not using Qpel with high compression matrices, since the amount of bits requiered by the Qpel technique may not be countered by the beneficial effect of Qpel.
The higher rounding errors created by high QM values would then also make Qpel less interesting.

So, to sum up:
1-higher QM values will lead to bigger rounding errors, nullifying subtle luminance variations in Keyframes.
2-Qpel 'might' be useless with high QM values for high frequencies. Don't use Qpel with high compression QM's, at least not with high compression P/B-frame QM's.
3-Using high values for high frequencies in B/P-frame QM could improve compressability. Lowering the high frequency QM values in I-frames could be used to add bits to fine detail in keyframes, possibly creating better values for the B/P-frame process to work with.
4-using the same QM for both Inter- and Intra-frames is less efficient than using different QM's, because of the difference in method used.
Any discussion on these points would be greatly appreciated.

Also, does anybody have the QM's for DivX 3 Low-motion and Fast-motion?
I wonder if they are any different.
It would also be very interesting to see the effects of different QM's on low and high-motion content.

Too bad I'm trying to flush the content of my harddrive because I'm installing a new motherboard this week....otherwise I would be experimenting with this stuff all day long.
Only 30 movies to go.....
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