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Monday, July 30, 2007



Bram's comments on the last article got me thinking about halftones. This is probably obvious to most people but it was news to me. Without good color profiles and calibrated monitors, you of course can't get predictable appearance. Things like sRGB will help the first but 99% of monitors are not calibrated. But with halftoning you can (assuming LCDs are well behaved about half tone patterns-- CRTs sure are not) get predictable greyscales. For example here are two images with a 25% and a 50% greyscale. (and RE the last article, not which is more of a middle grey). It just doesn't matter what your gamma is (assuming your black point is dark!) to predict what these greys are.

This basic idea is often used for gamma estimation-- the best example is Greg Ward's great gamma image.

2 comments:

Bramz said...

Actually, you need to use horizontal or vertical lines to mix black and white to correctly get 50% grey on a CRT, as the risetime of your electron beam is a limitation here. It's pretty hard to get from black to white in just one pixel. On a CRT display, horizontal and vertical lines might have a different perception of grey because of that.

On a computer CRT, this means you need vertical lines to correctly get your 50% grey, as the scanlines are vertical IIRC.

LCD screens of course don't have that limitation =)

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