Can I prevent Posterization after applying AviSynth scripts?

[grannyGeek]

I see a number of threads that diagnosed the user's problem as being "posterization"
< and you know, us poor newbies sometimes have reason to be confused ---
the VirtualDubMod help file shows a nice clear screen-shot of this phenomenon, and captioned it "Banding".
Most of you guys seem to refer to straight horizontal color streaks when *you* talk about banding. No wonder I don't know what to ask sometimes >

Anyway, I have a (film-source) vhs capture to avi using Huffyuv YUV2. Color is a bit washed out, and I want to intensify the color a bit.
However, each aviSynth filter that I have tried, when I encode to avi (again using Huff to YUV2) scenes with smoke / fog or skyscapes (that big blue gradient) it comes out with quite a lot of posterization.
My script uses Decomb / Telecide and Decimate, Convolution3d with MovieLQ setting, and DeSpot to kill the dirt and hair.
I have tried ColorYUV(Levels =TV>PC) and Tweak -- different saturations and brightness.


????
Should I try re-capturing with different settings in the ProcAmp tab in VDubMod?

????
Or, does anyone have a suggestion for a filter or script that might work better than my attempts?



thanks for any and all input ---
granny

[guns1inger]

Before you go nuts being concerned about the script, check your virtualdub settings. I find that from time to time virtualdub reverts to 16bit display mode. I know when it has does this because I see the banding you are describing.

The following is for virtualdubmod, but I suspect it holds for most variants

Go to Options -> Preferences
Look for a setting for Output Colour Depth or similar. Make sure this is set to either 24bit, or Match Display Depth if you are running your dsiplay at 24 or 32 bits.
Click on Save and preview again.

If you are still having banding issues, then it may be the filters, but for the most part I find the settings have to pretty extreme to produce this. 9/10, Vdub's display depth is the problem on my machine.

[grannyGeek]

guns1inger, thanks for the advice, I will check it out tonight and post back in the morning.

Keeping my fingers crossed ---
granny

[lordsmurf]

This is posterization:
http://www.cambridgeincolour.com/tutorials/posterization.htm

Banding is entirely different.

[FulciLives]

I do get a posterization effect when using VirtualDubMod in preview mode. I never thought to check the setting that guns1inger mentioned.

I'm just used to the way it looks and I know it is just the VirtualDubMod window and that the video doesn't really look like that.

- John "FulciLives" Coleman

[manono]

Yeah, I don't pay any attention to what it looks like in VDubMod either.

[lordsmurf]

Colorspace conversions on a crappy quality source video can lead to noticeable posterization. It's not just the preview.

[Guest34343]

Colorspace conversions on a crappy quality source video can lead to noticeable posterization. It's not just the preview.

Please provide an example stream that illustrates your claim. And what kind of conversion are you talking about? What do you mean by "crappy quality source"?

Chroma downampling can cause this but I don't see what that has to do with "crappy quality source".

[grannyGeek]

Hey guns1inger, your suggestion got me most of the way there .
Output setting had indeed reverted to 16 bit, changed it to 24, and got considerable improvement.

Still some minor artifacting, but that only seems to appear if I mess with the color and levels. My settings were awfully conservative though --
Tweak (cont= 0.9, bright= -2).ColorYUV(levels=TV->PC.
If I use only Convolution3d and DeSpot to clean up, it looks really smooth.

I’m attaching some small cropped screenshots, but of course, they don’t look so obvious as it does in motion during playback. In motion, the colors appear to crawl instead of blending smoothly. (and not nearly as badly as my first efforts, so I might just run with this new output)

Also in the attachment is extract from VdubMpeg help file, and extract from LordSmurf’s link.
????
I am hoping someone is willing to educate me, because I don’t quite understand the difference between those two examples. They seem like the same effect ??

????
is what I’m seeing in my own clips ( you can see the artifacts by zooming in a good bit)
posterization, banding, or garden-variety pixelization ????



Hoping for more good advice ---

Greedy granny


EDIT ---
Had some attacment problems, re-loaded as zip file.
sorry about that


bandingpix.zip

[grannyGeek]

Hi, I'm back to beg for more advice about my artifacts.

I'm attaching a zip file with two 15-frame clips.
First is the capture with only Convolution3D and RemoveDirt applied.
second has added ColorYUV levels=tv>pc and Tweak settings.
I encoded with Panasonic DV for smaller file size, and even though the color washed back out, the artifacts look the same as when I used Huff to YUV2.


Does anyone have a suggestion for me?

Maybe I will simply have to leave out the color tweaks and be happy that the clip is clean.

Thanks for any input ---
granny

compareartifacts.zip

[lordsmurf]

Colorspace conversions on a crappy quality source video can lead to noticeable posterization. It's not just the preview.

Please provide an example stream that illustrates your claim. And what kind of conversion are you talking about? What do you mean by "crappy quality source"? Chroma downampling can cause this but I don't see what that has to do with "crappy quality source".

I'll have to dig around, I don't know that I have anything on hand anymore. The case that sticks out most in my mind was some old VHS tapes run through a Panasonic DVD recorder, which does some degree of posterizing (posterization?) when it tries to clean video. The tapes were so damaged that the analog ghosting was turned into this posterized mess. There was also some motion blurring adding by the encoding, not present on the source tapes.

In fact, I have some DVDs, again from a Panasonic, directly from a clean satellite source, recorded in SP mode of all things, that shows posterizing. It's the encoder choking, just not bad enough to barf out too many visible macroblocks.

Lesser quality source, lesser quality encoder chipsets, they lead to various flaws. Typically these will be blocks, but posterizing is one of them too.

The nature of MPEG compression, as well as colorspace compression, can lead to this.

This sort of thing can be re-created in VirtualDub, TMPGEnc and several others. I've done it a few times when I chained a half dozen filters. In the end, the output looks far better than the source. But the posterization was there too, added by the digital processing.

This effect is usually mistaken for visible macroblocking.

[grannyGeek]

well, kind of a "never mind"
I simply re-captured my tape, and adjusted the color settings in the ProcAmp tab in the VirtualDub "Capture Filter".
I can re-create the artifacts at will by "over-filtering" -- run 2 instances of Convolution3d, or add a couple of extra de-noisers to the mix. So I guess I just stretched the video a bit beyond its reach.


I do wish LordSmurf would provide some details, because I sure would like to know HOW banding is "entirely different". All I have been able to figure out is that both can be caused by using a too-low bit setting.
LordSmurf wrote:

This is posterization:
http://www.cambridgeincolour.com/tutorials/posterization.htm

Banding is entirely different

Anyway, thanks for the help and everything.

[lordsmurf]

Posterization vs banding:
http://www.graphics.com/modules.php?name=Sections&op=viewarticle&artid=371

Although, I have to say, that the line in that article "such as switching to a new color mode" is a bit confusing. Technically, that is posterization.

They are adjacent concepts, dealt with similarly, but not always identically.

People also use "banding" (non-technical) to describe color shifts or chroma noise that appears to be in stripes or bands across the screen, but only across a portion of the image. In some cases, interferences from electrical or FM noise, like herringbone or similar.

[grannyGeek]

LordSmurf, thank you for the additional information.
That helped a bit to get a better handle on things.

I only wish they didn't appear so similar, it would all be easier if they were different and distinctive in appearance
I think it makes it hard to find out what to do when I'm not even sure which phenomenon I'm dealing with.

thanks again