Re: Saturate / Grayscale

If I desaturate (shft-ctrl-u) an image and compare (using difference
blend mode) it to the same image that I have done a mode>grayscale
convert to, I get small differences. Is this normal? Should there be
differences in a grayscale image compared to a desaturated image?


Yes indeed. Not all monochromes are created equal, and this has been true
from the earliest days of Photography, when blue was about the only light
capable of registering on a photographic plate. A Hue/Sat adjustment is
based on the HSB color space, which is based on a geometric conversion of
RGB to a six sided pyramid, aka hexcone, with white at the center of the
base, and black at the point. The brightness of a color value is equal to
the distance along the central axis of the hexcone. For purposes of
converting to grayscale, the important characteristic of this hexcone is
that it gives equal brightness ratings to Red, Green, and Blue.

Photoshop's conversion to grayscale is somewhat more sophisticated. It
gives different weights to red, gree, and blue. Green is the brightest of
the three, and blue is the darkest.

There are better ways to convert to grayscale. My favorite is one I learned
from Dan Margulis's books, and is based on using the channel mixer to
manually adjust the weights of the different primaries. Another method,
promoted by Russell Brown uses two HSB adjustment layers, one that
desaturates, and the other which selectively changes hue.

Here's a good article that says more:
http://www.swpp.co.uk/professional_imagemaker/monochrome.htm
---
Mike Russell - www.curvemeister.com


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Relevant Pages

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