CMYK grey vs. RGB grey

R
Posted By
replytonewsgrouponlyplease
Nov 8, 2003
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398
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Why is it if I desaturate an image in RGB then convert it to CMYK and desaturate it again it’s a different shade?
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cmyk gray rgb vs

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F
Flycaster
Nov 8, 2003
"Dr. J. Smith" wrote in message
Why is it if I desaturate an image in RGB then convert it to CMYK and desaturate it again it’s a different shade?

Rounding errors would be my guess, given that many mode changes. Maybe someone else has a better explanation.

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S
Steve
Nov 8, 2003
methinks this happens with everything when converted….some just below our ability to detect – but shadows, my god!!! converting from RGB to CMYK, I have to reduce opacity to around 4% to be happy with the output.

just my 2cents worth 🙂

Steve

"Flycaster" wrote in message
"Dr. J. Smith" wrote in message
Why is it if I desaturate an image in RGB then convert it to CMYK and desaturate it again it’s a different shade?

Rounding errors would be my guess, given that many mode changes. Maybe someone else has a better explanation.

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MR
Mike Russell
Nov 8, 2003
Dr. J. Smith wrote:
Why is it if I desaturate an image in RGB then convert it to CMYK and desaturate it again it’s a different shade?

This turns out to be an interesting question, so the answer will be longer than the question by quite a bit 🙂

Desaturate uses mathmatics based on the HSL color space. HSL is defined geometrically in terms of RGB ratios.

For Hue/Sat/Lightness calculations, Photoshop treats a CMYK image as an RGB image, ignoring the K value. By this logic, when a CMYK image is desaturated, gray is generated by creating equal amounts of CMY inks. Since cyan is the weaker of the three inks, a desaturated CMYK image will take on a red cast.

One servicable work-around is to convert to an RGB working space for the desaturate operation, then back to CMYK afterward.

This quirk also affects the layer modes, the sponge tool, Image>Adjust>Variations, and Image>Adjust>Hue/Sat . For example a CMYK layer set to luminance mode will impart a red cast to the underlying layers. I recall reading that some of these operations have been changed in Photoshop CS, perhaps others will comment on this.

—

Mike Russell
http://www.curvemeister.com
http://www.zocalo.net/~mgr
http://geigy.2y.net
B
belowme
Nov 10, 2003
i’m gonna have to agree with you on this answer. the three colors mixed will create a muddy brown so a slight amount of black ink is added where the shadows are. rgb can be made up into millions of colors, cmyk is made up with 4 colors when printing presses print 4 colors they only buy 4 colors of ink that can be mixed in only one way to create black thats why when you convert to cmyk it changes slightly to exactly match what a printing press can recreate.
"Mike Russell" wrote in message
Dr. J. Smith wrote:
Why is it if I desaturate an image in RGB then convert it to CMYK and desaturate it again it’s a different shade?

This turns out to be an interesting question, so the answer will be longer than the question by quite a bit 🙂

Desaturate uses mathmatics based on the HSL color space. HSL is defined geometrically in terms of RGB ratios.

For Hue/Sat/Lightness calculations, Photoshop treats a CMYK image as an
RGB
image, ignoring the K value. By this logic, when a CMYK image is desaturated, gray is generated by creating equal amounts of CMY inks.
Since
cyan is the weaker of the three inks, a desaturated CMYK image will take
on
a red cast.

One servicable work-around is to convert to an RGB working space for the desaturate operation, then back to CMYK afterward.

This quirk also affects the layer modes, the sponge tool, Image>Adjust>Variations, and Image>Adjust>Hue/Sat . For example a CMYK layer set to luminance mode will impart a red cast to the underlying
layers.
I recall reading that some of these operations have been changed in Photoshop CS, perhaps others will comment on this.

—

Mike Russell
http://www.curvemeister.com
http://www.zocalo.net/~mgr
http://geigy.2y.net

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