Problem with image edge when resizing in virtualdub

[brassplyer]

I notice that when using the resize filter to set a 4:3 image inside a 16:9 image with pillarboxing, there's an issue with the right side of the image. The color is missing from the part right up against the border.

Anyone familiar with this and how it can be cured?

Thanks.

[manono]

Was the chroma shifted earlier in the process, as is often done with VHS caps?

[jagabo]

Use mod4 (or larger) frame sizes.

[sanlyn]

Why not do this in Avisynth? If you want 160-pixel borders at each side:
AddBorders(160,0,160,0)
The default border color in Avisynth is RGB 000 (black).

jagabo is correct, in YUV your frame dimension should be at least mod-4 (dimension is evenly divisible by 4) for better colorspace conversions. There are some plugins that require mod-8 dimensions in processing, usually noted in the plugin's documenation. You're likely going thru at least one YUV colorspace step anyway, just to get your capture into VirtualDub to begin with, because your capture is either YV12 (4:2:0) or YUY2 (4:2:2) and you're using QTGMC. So to properly move your Avisynth processing for work in VirtualDub:

AddBorders(160,0,160,0)
ConvertToRGB32(matrix="rec601",interlaced=true) 
# <- or interlaced=false, whichever applies)

Also, you might consider using the Rec709 HD matrix if you're going HD. There's not that much difference, it's mostly in saturation levels. Opening a YUV video directly in VirtualDub without specifying the conversion will use Vdub's rec601 matrix by default.

[brassplyer]

Use mod4 (or larger) frame sizes.

Translation?

[sanlyn]

Use mod4 (or larger) frame sizes.

Translation?

You have the answer 2 posts earlier:

...frame dimension should be at least mod-4 (dimension is evenly divisible by 4)...

The number 640 is evenly divisible by 4 -> : 640 / 4 = 160
The number 790 is not evenly divisible by 4 (decimal remaining) -> : 790 / 4 = 197.5

[jagabo]

Keep at least mod4 sizes at every operation (crop, resize, addborders).

And of course, make sure it wasn't in your source or caused by any other filtering you did.

[brassplyer]

Was the chroma shifted earlier in the process, as is often done with VHS caps?

Not earlier in the chain but after - the chroma shift appears to be what's causing it. It appears every other frame.

It doesn't appear I can just crop it out either, when I crop that edge, the problem area just moves over.

[jagabo]

Was the chroma shifted earlier in the process, as is often done with VHS caps?

Not earlier in the chain but after - the chroma shift appears to be what's causing it. It appears every other frame.

Well then of course you're getting the discoloration. You're shifting gray chroma from the black borders onto the picture. Perform the chroma shift before adding the borders.

[brassplyer]

Was the chroma shifted earlier in the process, as is often done with VHS caps?

Not earlier in the chain but after - the chroma shift appears to be what's causing it. It appears every other frame.

Well then of course you're getting the discoloration. You're shifting gray chroma from the black borders onto the picture. Perform the chroma shift before adding the borders.

Isn't chroma the color component? I think it's more that the color is being shifted to the left exposing the luma but yeah.

[jagabo]

If you shift "black" chroma over onto the color picture you will be left with a greyscale picture.

[brassplyer]

If you shift "black" chroma over onto the color picture you will be left with a greyscale picture.

The filter addresses chroma I and Q, which is the black chroma?

[jagabo]

Both I and Q from the black borders will both have "gray" chroma values (128). When you perform a chroma shift you are moving the colors (to the left in this case) but leaving the luma (greyscale) picture in place.

[sanlyn]

I keep looking at the image in post #1. Why did it "gray-out" only a portion of the lower part of the right border, but had far less effect on other parts of the same border?

[brassplyer]

I keep looking at the image in post #1. Why did it "gray-out" only a portion of the lower part of the right border, but had far less effect on other parts of the same border?

Dunno. If I kill the chroma shift filter the issue goes away. I may just live with it without the filter since the chroma issue isn't terrible particularly with the color noise filter that's also in the chain. And I think I may not even be accurately identifying the issue, maybe not shift so much as bleed.

[sanlyn]

I had a similar problem -- not with a chroma shift filter, but with a strong anti-rainbow plugin used on 20 pixels of discoloration along a border. Different story, but if you refer to FlaXen's VirtualDub VHS filter, make sure the chroma denoising switches are turned off. I find the "VHS" shifter to be an OK finishing touch for very small 1 or 2 pixel shifts, but stronger shift/bleeding is more effectively handled in YUV.

Chroma shift:
- Try the ChromaShift() plugin: http://avisynth.org.ru/docs/english/externalfilters/chromashift.htm

Bleeding, and some chroma shift as well:
- Try a light dose (or two) of this popular technique: mergechroma(aWarpSharp(depth=20.0, thresh=0.75, blurlevel=1, cm=1)). Tweak "depth" and "blurlevel" for stronger effect, but don't go insane with the numbers, Often works best by running it 2 times in a row with the lower values shown. For stronger de-coloring try depth up to 30 or blurlevel up to 3.

There's no total fix for chroma bleed or shift. The cleanest technique is a few rather gentle filter passes instead of a sledgehammer.