October 25th, 2003

Color, food and your fridge...

An interesting question and answer from the dpreview.com forums....

SteveZachary wrote:
> After owning 3 digital cameras, I don't still fully understand what
> is meant by "color space" and the difference between Adobe and RGB.

If you wanted to describe every point in space inside your refrigerator, you might choose three coordinates: X, Y and Z. If the egg salad is near the left wall, it has a low value of X. If the food is near the back wall, it has a low value of Y. If the food is on the bottom shelf, it has a low value of Z. These choices in this example are completely arbitrary, but they are self-consistent. Three coordinates can define a cartesian space.

Now, let's look at color as registered by your eyeball and by the digital camera. They're slightly different, but they both share a common principle. They don't measure the wavelengths of incoming light, they measure how strong the R (red), the G (green) and B (blue) receptors are tickled by the incoming light. The actual color depends on the mix of those three variables.

Since you have three variables, a lot of people call it a "color space," just like the space in your fridge. Less R to the left, less G to the back, and less B to the bottom. More R to the right, more G to the front, and more B to the top.

If you wanted to sort your foods by this scheme, you'd put cherries in the right back corner of the bottom shelf. You'd put oranges near them, but closer to the front. (A lot of Red plus a little Green looks orange.) Blackberries would go in the bottom left rear corner. Vanilla ice cream belongs in the top freezer, near the front right. (White light is a lot of R, G and B mixed together.) Lettuce belongs at the front of the crisper drawer on the left.

CMY (cyan magenta yellow) is a completely different color space. It can describe the same positions in your fridge, so don't move those foods! But the values for C, M and Y are different. As it turns out, it's trivial to convert from RGB to CMY numbers. You subtract colors with absorption instead of adding colors with lights. Cyan is the absence of Red. Magenta is the absence of Green. Yellow is the absence of Blue. Those blackberries are lacking in a lot of colors, so they are rich in C, M and Y.
Same fridge, different schemes to describe where the food is.

If we described sRGB versus Adobe RGB, it would just alter how the values relate to the exact colors. sRGB tries to keep things linear for computation's sake. AdobeRGB bends the values non-linearly to adapt to the way the eye sees color a bit more. This won't answer your question adequately, but mapping between these schemes doesn't actually move your food, it distorts and bends the fridge instead. The R G B value ranges stay the same, but the shape of the fridge changes so there's more room in the fridge near the most important color positions.
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