AVI blothchy

[vhsreject]

ZippyP.wrote

If you're converting a 700MB sized avi to DVD then I suggest 1/2 DVD resolution (352x480) as well, as that is likely the closest. Upsizing is generally not recommended. It should reduce some of the blockiness you see. As well you can turn on the "soften block noise" filter in TMPGEnc under the quantize matrix tab.

im making this thread because i believe it to be specific to avi.the picture in general looks great but any night scene or black in the shot or clouds ect break up into blotchy blocky pixels.you get that effect where a film of black is over the black and wherever it breaks up you get the blotchy pixels.i dont know what its called.one real good example is,if you turn the brightness down on the tv(old set)the black blends but then the rest of the dvd suffers.ive seen a lot of pointers to TMPEGnc and tweaking the avi post production.im going to try to find a way to process the avi with Virtualdub filters ect before burning the DVD.so if anyone knows how to "get out the blotchiness in black" please let me know.thanks

[jagabo]

im making this thread because i believe it to be specific to avi.the picture in general looks great but any night scene or black in the shot or clouds ect break up into blotchy blocky pixels.

Because when you over compress video that's the first place you lose detail.

if anyone knows how to "get out the blotchiness in black"

Basically, you don't. About the best you'll be able to do is drop the black level (darken the video) so that the blotches disappear in the black.

[guns1inger]

There are two main causes.

Jagabo has address one of them - bitrate. If the bitrate is too low, you get blocky artifacts. The main areas you will see this are action scenes and transitions (because the entire screen is changing and a high bitrate is needed to accommodate this).

The other place you will see this is areas of lower colour range or contrast, and this can occur even when the bitrate is relatively high. The way the codec works is to try to use areas like sky or deep shadow to squeeze a little extra bitrate by reducing the gradients between colours even further than normal. The result is banding and blocking in images that have little or no action. I have seen this is video that has enough bitrate to handle action in scenes with busy backgrounds and lots going on. If a surafce is flat coloured, this algorithm comes into play. If a surface is patterened or detailed, it is skipped.

[vhsreject]

actually the simple contrast/brightness filter in VirtualDub-MPEG2 seems to do the trick.the overall picture still suffers though.ahh the wonders of virtualdub.perhaps someone knows a filter that specifically deals with this issue.thank you for the responses

[redwudz]

The Deblocking filter? http://compression.ru/video/deblocking/smartdeblocking_en.html

[guns1inger]

There are a few different filters for attempting to clear up blocking artifacts. All of them ultimately do soften the image, so you might want to carefully apply an unsharpen mask afterwards, or perhaps Donald Graft's msharpen filter.

The final hurdle however is to avoid recompressing the video in such a way as put all the artifacts back again.

[vhsreject]

ugh

[jagabo]

One other issue: be sure the black level and gamma are set correctly on your display device. If the smears are black and so-nearly-black-you-can't-tell-the-difference, but you have your black level turned up, you will be more likely to see them.

[vhelp]

Morning all.

Another point not addressed has to do with the *source* attributes. By that, I mean to say, what are the errors and by how much. The errors I'm refering to are the compression artifacts like pixelation and other common compression errors brought up already in this discussion.

The point is, if the source has artifacts then your re-encoding will bring them out though usually more pronounced. That is the way things are in this video world or at least with divx/xvid or heavily compressed videos.

And, the only way to reduce it is through a good set of filtering and skill. The deblocking might actually help but only to some degree. You have to experiment. These things don't work themselves out over one try. They usually take many! And, when you apply your refined set of methods they will undoubltly not work for other videos without some adjustments to the methods. Thats the way it is because not all videos are alike. They are all different.. diff light levels; bitrate levels; compression errors; pixelations and their placements; etc etc etc.

-vhelp 4877

[vhsreject]

the deblockers dont seem to work by their selves.turning down the brightness dose work to a certain point but will not fully blend all the blotch.i can only use xvid for compression on this avi and the quality is to to high,one pass.i did notice that the blotch effect dose seem to reproduce its self or at least carry over.im going to experiment with gamma filters and brightness levels tomorrow.thanks for the help so far.

[jagabo]

All the things you can do to reduce the blotches will cause other problems. Some of the deblocking filters add noise to dark areas -- less blotches, more noise. Temporal filters just change the nature of the blotches. Decreasing black level sinks them into the black but you lose black detail. Once you have them there's no getting rid of them.

[poisondeathray]

jagabo is right on. Much of the information has already been lost. XviD and DivX are very poor for the situation you describe, dark backgrounds at the light/dark interface. The problem is with XviD, and intensified by using a low bitrate

Using a higher bitrate for the .avi will help, but it's a crude method. Even at ~2-3000kbps (~2-3CD size) you can often see pixellation in this scenario. A better choice would be to use x264, there would be no blocking assuming your original source is decent quality, even at the ~1CD size. (unfortunately you said you are constrained to XviD and avi for some reason)

There are XviD builds that have the Adaptive Quantization ported from x264, which also helps in a bit this scenario http://forum.doom9.org/showthread.php?t=135093

Just to clarify, are you converting a DVD to .avi to put on a DVD so you can play on your standalone player?

[vhsreject]

im going avi to dvd-r.but that process is fine.im looking more at techniques to pre-process the avi to take some of the glitch out.

[guns1inger]

You can't get rid of it. You can reduce it to a degree by using the methods noted above, but that is about the best you can expect. The damage is already done.

[vhsreject]

time to think expirement and work thanks guys