Image Processing Reference
In-Depth Information
Cross-Color Artifacts
This happens when some horizontal fine detail occurs at such a high frequency that it
leaks through the filters and the color circuits act as if some chroma information is being
coded. It is luma information, though. This occurs when you use composite connections
and send luma and chroma down the same cable. Component systems that keep luma and
chroma separate don't have this problem. It is difficult to show in printed form because
the effect changes color rather than brightness. It is very like the moiré effect, but there will
be twinkling areas of blue and yellow that shimmer as the image moves.
You might overhear a video engineer referring to someone wearing a 3-MHz shirt.
Newsreaders at the BBC used to wear checkered jackets and houndstooth tweed, and this
caused cross-color artifact problems when the BBC started to broadcast in color during the
1. Use a better PAL, SECAM, or NTSC decoder.
2. Get the presenter to change clothes and then reshoot.
3. De-saturate the color.
Chroma Noise
This is where an area of solid color looks blotchy. It is due to noise's being introduced into
the chroma channel and seems to affect the blue component more than any other. When
compressed, the appearance is very blocky because the codec is working hard to render the
fine detail. The trouble is that the fine detail is noise. It will show up when processing legacy
analog footage and is less likely to cause problems with modern, digitally processed content.
1. Apply film grain removal to the chroma channel only.
2. Blur the chroma channel.
Dot Crawl
This is a strange effect that shows around the edges of objects where the color changes
abruptly. It is caused by the color information leaking into the luma channel and is the worst
when you are connecting equipment together with a composite video signal. Using S-video
connectors to deliver the luma and chroma down separate wires eliminates this problem.
The area shown in gray on either side of the object in the illustration is where the dot-
crawl effect will be worst.
If some video has been transferred and recorded with dot crawl in evidence, it is
there forever and you cannot remove it. Keep your luma and chroma separate. Having
said that, grain removal might help reduce it slightly.
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