CMYK-RGB, Len or any other?

BC
Posted By
Brian_Cushing
Dec 13, 2003
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274
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This a curiousity question only. When I convert an on screen image from RGB to CMYK the difference is very apparent However, if I then convert the same CMYK back to RGB, no change. Is it me or..?

Thanks,
Brian

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RL
Robert_Levine
Dec 13, 2003
Sounds right. The RGB gamut is way bigger than CMYK so it only makes sense that when going from RGB to CMYK you’d see a color shift. The CMYK gamut sits inside the RGB gamut so when going from CMYK to RGB th color remain pretty much unchanged.

Bob
MM
Mac_McDougald
Dec 13, 2003
The RGB has a larger color range.
CMYK cannot represent all that range, so changes some colors to like colors and you see a change.

Going back to RGB, the RGB space can show all the CMYK values, so no change. You’ve already thrown out some of the RGB hues and can’t get them back.

That’s why it’s wise to always keep an RGB version of a file. Especially since unless you have exact CMYK color settings from your print bureau for the specific job, you’ll probably have to re-convert anyway. There is no "one size fits all" CMYK profile.

Mac
A
Adam
Dec 13, 2003
wrote:
This a curiousity question only. When I convert an on screen image from RGB to CMYK the difference is very apparent However, if I then convert the same CMYK back to RGB, no change. Is it me or..?

Thanks,
Brian

You probably know that many colors exist in RGB that are not possible with CMYK, and vice versa.

When you convert an RGB image to CMYK, photoshop "adjusts" the colors which would be impossible to print, and tries to find the closest match possible in CMYK-space. Often, its not very close, which is obvious to your eyes.

What is important to realize, is the image you end up with contains only colors that are *shared* by the two color spaces. So when you convert back to RGB, no adjustment needs to be made to the colors, and you don’t see any difference.

Technically, the colors are slightly adjusted because the colors are calculated differently and there is a finite number of possible colors, usually 16.8 million, if I remember correctly. You may see a slight change due to rounding errors. These rounding errors are cumulative, so you should avoid switching between color spaces many times. Eventually it will start to cause ugly "banding" and other problems. You can reduce the effects by starting with "16 bits per channel".

-Adam
DM
Don_McCahill
Dec 13, 2003
Think of it as having two buckets of different size. If you pour the large bucket into the small one, you are going to spill water. But when you pour back from the small into the large, nothing is lost.
BG
barry_gray
Dec 13, 2003
nice Don.
GT
Gene Trujillo
Dec 13, 2003
I should point out that although RGB gamuts are generally bigger than CYMK gamuts, you can’t necessarily convert CYMK>RGB with total impunity. Although RGB gamuts are generally larger than CYMK gamuts, size of the gamut is not the only factor involved. Even though it is generally smaller, CYMK can produce colors outside of an RGB gamut, e.g. sRGB will clip cyans.

It’s usually not nearly as noticable as the clipping from an RGB to a CYMK, but you should be aware of it.
GT
Gene Trujillo
Dec 13, 2003
Here’s a good visual for it:

Lay a quarter on a table. Place a penny on top of it. Slide the penny so that it hangs over the edge of the quarter a millimeter or two.

The quarter represents sRGB, the penny represents a hypothetical press gamut.

The quarter is bigger so more clipping will usually occur when you convert from RGB to CYMK, but they don’t fully overlap. They penny goes places the quarter can’t, too.

I don’t know how useful this is, but it’s something to know. It might be handy if you start with a really wide gamut RGB, convert to CYMK, then for some reason need to convert again to a smaller gamut RGB.
L
LenHewitt
Dec 13, 2003
Brian,

Just adding to what others have already told you, converting back and forth will have a deleterious (although small) effect upon the image data. The conversion isn’t perfect in either direction and data changes will be additive.
BC
Brian_Cushing
Dec 13, 2003
Thanks all, Makes perfect sense, even to a dim wit like me!

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