question about understanding channel mixer

DF
Posted By
David_Freed
Jun 7, 2007
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207
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1
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Closed
When I go into channel mixer, I see sliders for each of the color channels (RGB or CMYK); these sliders are ruled from -200% to +200% for each color.

Can someone give me some info. about why the scale is to 200% both plus and minus–this doesn’t make immediate sense to me: how can you have 200% of a color and how can you have negative pixels of a color?

I’m trying to use the channel mixer to make etching-type plates using photopolymer film. I’d like to create two plates: one based on black channel info. and the other some combination of the color info. (similar to a duotone, but not exactly).

Thanks, David.
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George_Austin
Jun 8, 2007
The slider percentages are percentages of the existing values applied to the OUTPUT CHANNEL you designate.

For example, say you chose red as the output channel and the existing RGB values at a given pixel location were 200/150/100. By default, the red channel slider sets itself to 100% and the green and blue channels are set to zero. That means the existing value in the red channel is as yet unchanged—It is 100% of the existing red value (200) and there are no additions or subtractions using green and blue values.

Now, if you set the red, green, and blue sliders to, say, 110%, 30% and -20%, respectively, the new RED channel value will be 110% of 200 = 220 plus 30% of 150 = 45 minus 20% of 100 = -20, equalling 245 for the output red channel. The blue and green channels are themselves unchanged. Only the designated OUTPUT CHANNEL is affected.

Whenever the manipulation results in a final value greater than 255, 255 is used, and whenever it is negative, 0 is used. But note that the percentage of a channel can exceed 255 or be negative in the calculation. It is only the final result that must lie between 0 and 255.

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