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When color isn't just color anymore

Posted by darkpegasus, 23 December 2011 · 502 views

My background has been primarily dealing with digital devices which has allowed me to only work with RGB, blissfully unaware that anything else existed. Sure, I would see that there were other alternatives in dropdown lists of Photoshop, but what were those for anyway?


And then I started working on Genegrafter from a different angle: http://www.genegrafter.com. Moving from digital to print media meant that I needed a crash course in how they were different. So for anyone that doesn't know the difference, here is a quick run down.

What is a colorspace?

Wikipedia defines a color model as "colors [that] can be represented as tuples of numbers, typically as three or four values or color components". So the colorspace is a mapping between this model and our implementation of color. This is a fancy way of saying that we look at color differently depending on which color space we are using.


CMYK vs RGB

I mentioned RGB before, and this is what I've been used to. The reason is because RGB is what electronic devices use. It is an additive model, Red, Blue, and Green are added together to produce colors. You can't use this when printing in the real world however because this method would just give you a bunch of mottled browns and blacks.

CMYK is able to get around this because it is subtractive. Since the color white is actually the presence of all color (and conversely black means that all color is absent) we can add Cyan, Magenta, and Yellow (the CMY) to strip away (or subtract) colors from the white paper in order to get the colors we really want. This process can actually be affected by the starting color of our paper as well; printing on an off-white page would not be the same as if it had been pure white. K in CMYK is black and can actually be obtained by combining the cyan, magenta, and yellow but then we would have to use three times as much ink, so it's generally better to have a separate mix of black available.
The following is the same image but one is using CMYK and the other is RGB Posted Image
The moral of the story is to make sure that if you ever plan on printing anything, you ready it as CMYK but use RGB for anything digital. Most of the time you will use one or the other, but every once in awhile you may want to cross over to the other side, and when you do, there are considerations to be made as seen in the image above.




It's been well over 10 years since I worked at the print shop... but I'm surprised your RGB image is so much more vibrant. I always thought the stuff i worked on at work was always brighter, then anything I did at home. CMYK at work, RGB at home.

Also, they do make machines that can handle 7 colors, (still uses subtractive blending), takes a little extra work setting things up before making a screen... but they printed faster, and used less ink all together. Though that was a silk screening beast the size of a honda.
Well if you show CMYK on a screen, you would have to do some conversion of it to RGB first. (since that is what the screen uses)
Shouldn't any color difference in the images only depend on this CMYK->RGB mapping going on in the left image?

you use additive color mixing when you display colors using light, as on a screen, because that is how light mixes.

When you print, you depend on the light bouncing off the paper, and use subtractive color mixing, because the only thing you can control is what light does _not_ bounce off by coloring the paper with the complementary colors

Its more a question of what you have to do, then a choice...
If you work in any TV medium, YCbCr (could beinterchangeable with YUV) is a color space that is used often. The Y is the luminescence component (basically, brightness) and the Cb and Cr are chrominance colors. However, YCrCb can be converted to and from RGB fairly easily.
One thing to add is that when you are working with RGB on the PC what you are in fact working is is sRGB space.

This is important as sRGB isn't linear so any maths you do on two sRGB images, without converting to linear space first, is technically wrong :)

This also applies to everyone out there doing graphics; all your diffuse textures are sRGB and unless you tell the GPU otherwise they get sampled as linear and then combined with linear lighting equations giving you the wrong image :)

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