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Old 29th January 2011, 19:53   #1  |  Link
drcldrcl
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Join Date: Dec 2010
Posts: 6
yuv colour gradient problem

Hi everyone, glad to see doom9 is still here.

A colleague at work asked me if i could transfer a PAL VHS recording of a retirement event from about 15 years ago to DVD.

No problem i thought.

The quality of the recording is average and quite dark with the usual red tint you get with indoor lighting on old vhs cams and luma dropouts.

so after correcting the colour as much as i could with avisynth i still wasnt happy

the left side of the frame seems to have a very slightly different hue to the right and its most noticeable on pans when a red colour (a woman's dress ) is visible as it clearly seems to go orange as it moves across the screen.

I had been correcting using RGB as i assumed the red lighting tint on the original is caused by the light sensors on the camera which use RGB.

But i have the feeling this is a YUV issue related to the tape head electronics.

I examined each channel and the U channel has what i can only describe as drop outs on the left side in the yellow shades - ie areas where there seems to be little or no chroma signal.

so i thought i can solve this - just create a colour yellow to grey gradient, import with imagesource() and use overlay() or mergechroma().

But i could not get it to work - there would still be a visible colour shift on that dress. I spent many hours tweaking it and was never happy. The colleague was pleased enough with it but i will always know that it has a colour problem!

I'm just wondering if anyone else has come across this and how they possibly overcame it.
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Old 29th January 2011, 21:47   #2  |  Link
IanB
Avisynth Developer
 
Join Date: Jan 2003
Location: Melbourne, Australia
Posts: 3,172
Yes I have seen these horizontal chroma distortions before.

For your case with the drooping U channel, probably masktools LutXY with the options of U channel processing only. Examine the V channel more carefully it also probably has a slight droop as well.

A formula something along the lines of (((X-128) * (Y + 500)) / 600) + 128 == > "X 128 - Y 500 + * 600 / 128 +"

Choosing the values of 500 and 600 allows fine control of the U gain by the mask clips U channel, ranging from (500+0)/600=0.8 to (500+255)/600=1.25. You should choose values suited to the gain range you need to apply.
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chroma, dropout, transfer, vhs, yuv

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