A color space puzzle
A color space puzzle
- Subject: A color space puzzle
- From: "Bruce J. Lindbloom" <email@hidden>
- Date: Wed, 06 Feb 2002 15:33:44 -0600
Here is a color space puzzle I came up with. At first you might find it
surprising. Then you probably won't believe it. But after you confirm it for
yourself, you will be perplexed trying to explain it.
Here is the puzzle:
A popular color space among those on this list is Adobe RGB (1998). A less
popular one is sRGB. If you compare their two definitions, you will find
that they both have the same gamma (2.2), they both have the same white
point (D65), they both have the same red primary (0.64, 0.33) and they both
have the same blue primary (0.15, 0.06). In fact, the ONLY difference
between them is in the green primary: Adobe RGB (1998) is (0.21, 0.71) and
sRGB is (0.30, 0.60). And yet, if you convert an sRGB image to Adobe RGB
(1998) or vice versa, you will find that it is the red and blue channels
that change -- green remains completely unchanged. That doesn't seem right.
How do you explain it?
Note: I said above that the sRGB gamma is 2.2. The standard sRGB working
space in Photoshop (sRGB IEC61966-2.1) uses a definition that has a curve
that is VERY close to, but not exactly the same as gamma 2.2. In order to
see that the green channel does not change AT ALL, you should make a
simplified sRGB working space by using Photoshop's Color Settings, select
"sRGB IEC61966-2.1" for the RGB working space, then immediately select
"Custom" for the RGB working space and you'll see you get the "simplified"
version of sRGB.
--
Bruce J. Lindbloom
www.brucelindbloom.com
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