VIDEO: How RGB Color is Converted to CMYK in Photoshop
I have been asked the question many times before: why does my photography/image look so flat when i convert RGB to CMYK color?
Well, the answer is science.
RGB is additive color theory. Red, green and blue light when combined produce white light. When red, green and blue light are turned off, there is no color therefore resulting in black. This is how your monitor and television function.
CMYK is subtractive color theory. Cyan, magenta and yellow ink act as filters to absorb and reflect light that is reflected off paper. When light reflects off paper where no ink is applied, this is white. When light reflects where all three colors are present, no light reflects back resulting in black. Cyan ink absorbs red light; magenta absorbs green light; and yellow absorbs blue light. This is the basic theory of subtractive color.
It’s important to note that a fourth ink (black) is used to create more contrast and deeper blacks/shadows in images. The amount of black ink is dependent on the conversion process that is used, for example medium GCR (Gray Component Replacement).
RGB color uses projected light which is much more brighter than light reflecting off a substrate with CMYK color. The additive light (RGB) creates a color gamut that is much larger than subtractive (CMYK) color gamut.
Does this explain RGB to CMYK conversion for you? Please place your comments below…
Transcript of Video
Hi. This is Rick Rys from HiDefColor.com. Today we are going to discuss the conversion from the RGB color space into the CMYK color space.
[0:14] RGB is an additive color space meaning that red, green, and blue light together will create white. When red, green, and blue light are off, they will be black when it is projected onto screen, or onto a monitor.
[0:31] CMYK is the subtractive color theory meaning that the cyan, magenta, and yellow inks act as filters. As light bounces off of the paper, it reflects up through the cyan, magenta, and yellow inks, which in turn will either absorb or reflect different color wavelengths.
[0:52] The opposite of red is cyan, the opposite of green is magenta, and the opposite of blue is yellow. The subtractive colors are the gray components of the additive colors meaning that when they’re put together, they create gray, or black, or white.
[1:07] The LAB color model–let me turn this off here–is a 3-axis color system, and the LAB colors are absolute meaning that the color is identical. It’s across what’s called a device-independent, meaning that the LAB color space is the only way for you to communicate different colors across different devices.
[1:35] Now, it is a 3-axis system. The first axis, the L-channel, or lightness goes up and down the 3-D color model, and it consists of white to black, and all of your gray colors will be exactly right down the center.
[1:49] All your neutral colors will be relatively in the center of this axis. The A-axis goes from a cyan/blue color across to a magenta/red color, and the B-axis goes from blue to yellow.
[2:07] Within this area, we’re going to plot our visual or reproducible colors based on the gamut or the profile of the device we have. I’m going to turn on the sRGB color profile. Most monitors display in sRGB; sRGB is preferred for any type of Internet or Web application.
[2:27] And look at this thing spinning here. And you can see the volume of the colors that you can reproduce from this additive color model. Obviously, since it’s dealing with projecting light, they’re very bright colors and they’re very saturated.
[2:44] Now when we bring in and display the CMYK GRACoL color profile, you’ll notice when I turn it on the sRGB encompasses the whole CMYK color gamut beside this area of cyan and greens through here.
[3:03] If you look down on the color model, you’ll notice that the circumference of the model is projected along the bottom here. You can see the outside perimeter of the sRGB color profile.
[3:17] The brighter colors are just not capable of being reproduced with the CMYK color gamut. You can see what happens when you get these real dark blues. There’s no blue for you to hit in a CMYK color model.
[3:30] What I’m going to do is I’m going to take our sRGB color profile, and I changed the opacity, so you can see the difference that we’re dealing with here. As you can see the volume of color on the RGB color profile is nowhere near what can be reproduced in a CMYK color profile.
[3:53] So what we have to do is we have to do our best job of remapping these colors, or what is known as tonal compression, to bring this sRGB color model into the CMYK color space.
[4:06] This is why–I’ll stop right here–when you look at a blue sky, you may always be disappointed with the results you get because when the photograph is in RGB, you’ve got all these deep bright blues and more of the colors you see in the horizon.
[4:30] When they’re converted to CMYK – you’ll notice when I change opacity, all those bright blues have to be condensed into this little area here of the blue hue that’s reproducible in CMYK.
[4:42] There’s a sacrifice there, and that’s where you get into using either relative rendering intent, or the perceptual color intent. That will help you resolve some of issues you have with converting your dark blues into the CMYK color space.
[4:58] Let’s turn this opacity back up, and you’ll get an idea, again, of what we’re dealing with here. We need to take all of this color and condense it into this little area right here.
[5:12] And this is the GRACoL color profile. The GRACoL color profile has more colors than the SWOP profile, so we’re going to get a better representation of some of those more juicier RGB colors when they’re converted to CMYK.
[5:27] So let’s bring this up again and show you the difference of converting all of this into this little area here. This is why color management is so important and knowing what profiles you’re dealing with.
[5:41] Your safest bet is using the sRGB color space and converting into the coded GRACoL profile. Keep in mind that dealing with your print provider, they will produce, or they will provide the correct color profile based on their printing condition.
[5:58] A profile is a recipe, or the characteristics of a particular printing condition. Based on the press, the inks, and the paper that they use that will produce a profile.
[6:10] Well thanks again for tuning in. I hope this clears some things up. If you have any questions, please feel free to leave a comment on the blog. And we will see you next time.
[6:21] Have a great day.