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Q & A: Color aliasing

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Most recent answer: 10/22/2007
Why does a black and white houndstooth pattern show up as red, pink and green on a NTSC color television?
- Lois (age 27)
Dallas, TX
I think you're seeing a phenomenon called "color aliasing" in the video biz.

It may not be the color TV, but the camera used to take the video picture of the black and white pattern that's at fault here. The problem is that modern digital video cameras have little square pixels on their light-detecting elements, and these aren't infinitely small. They are made up of little silicon devices etched on a single wafer of silicon. The light-sensitive parts cannot tell the difference between one color and another -- they just measure how much light is hitting each one. So each pixel has a colored filter over it to pick out only one color at a time. These are nearly invariably red, green, and blue, and each pixel is responsible for just one color.

If your black-and-white pattern has small features or even just sharp edges, then what will happen is that a pixel in the detector may be illuminated by a white part of the pattern, record its own particular color, while its neighbor may not have any light from it from the pattern, because it corresponds to a black part of the pattern. If the input pattern is repetitious like the pattern of the pixels in the camera, you can get bizarrely colored pictures -- say, all the red pixels will have light on them or all the green ones, say. Even if the patterns don't line up exactly or have the same size of pixels and features, you can still get oddly-colored stripes or curvy regions. This is a real problem in the video industry, and professionals have to be careful not to include objects with a high density of regular patterns in their TV images or they may look strange to viewers. (Or you can get a really good camera which breaks the image among three light-sensitive detectors, and so each spot in the image is covered by all color-sensitive pixels).

Now it could be that the TV really is what's doing this to you. For example, instead of having taken a picture of the offending houndstooth pattern with a camera, suppose you made it with a computer and piped the result into your TV (lots of computers will do this). You can still get color aliasing because of the way the TV's color is generated -- again with small dots on the screen and a dark mask behind it. Three electron guns shoot electrons at a spot on the screen, but since the guns are in different places, the electrons hit the same spot on the screen from different angles. If they all pass through the same small hole in a sheet with lots of these holes in it, then they will make three spots on the other side. On these spots you'll find red, green, and blue phosphors. Now it should be that for any set of three dots on the screen, a white TV signal should hit all three the same amount. Then you'd only see the colors if you looked really really closely. There's also the added feature that there's a limit to how sharp an edge can be when shown on a TV screen -- engineers call this a "bandwidth restriction" -- TV signals have to live in the radio frequencies and not interfere with each other, and this limits the sharpness of edges. If your houndstooth pattern has lots of sharp edges, you'll be hitting the bandwidth restriction on the TV.

TV sets aren't perfect, however. The red, green, and blue electron guns may not be pointed all exactly in the same place and they may have different bandwidths. That way you'll see colored fuzzy areas near the edges of what should be a white region on a black background. Ordinary broadcast TV signals don't do this much (except for writing text, and sometimes it's hard to read text off the TV for exactly this reason). But if you've hooked a computer up which doesn't know about what looks nice or not on a TV, you can get something rather ugly.


(published on 10/22/2007)

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