We hear a lot about color space. RGB, Y'U'V', Y'/R-Y/B-Y, Y’/Cr/Cb…it ranges from logical to alphabet soup. In the end, the color in our images is made from three component signals combining to create a color image. What those three component signals are…well, that depends. (There are colorspace definitions outside of the colorspaces I've listed here, of course. I just tried to include the colorspaces most media production pros will encounter...)
RGB
The easiest place to start might be RGB. It will be review for almost anyone with any computer background as RGB logically stands for “Red/Green/Blue”. RGB color images are produced by combining three channels of image information, each containing the intensity information for each individual color, Red, Green, and Blue. This colorspace is typically used in computer displays and projectors. It’s used in the creation of the visual content developed for video games or the web. Conventional analog and digital video has traditionally not used RGB color space, causing many long-time professionals to discount the general accuracy of the video your NLE shows you on your computer desktop monitor. However, with the growing use of large LCD and Plasma displays RGB is becoming at least one of the display methods you may want to (or be forced to…) employ through the post process.
HSL
“Hue, Saturation, Luminance” is found most often inside computer software for image manipulation or video color correction. It is fairly simple to picture as a color wheel where the color red is in one direction, moving around the wheel to yellow, green, cyan, blue, magenta, and back to red. While the color order would seem to directly mimic the vectorscope display, in HSL the individual colors are evenly spaced like numbers on a clock face while a vectorscope is showing a color difference signal where the positions of the primary and secondary legal colors are not equidistant.
However, the vectorscope model is indicative of how the hue and saturation portions of an HSL color wheel work. The “compass” direction from center indicates the hue and the distance from center indicates saturation. Also similar is the way the HSL color wheel doesn’t display luminance changes, but the brightness is adjusted via each color channel somewhat similar to an RGB model.
CMYK
For those of you who have dealt with this color space strictly as an irritant when you receive your client’s logo art from their printer and forget to convert it before spending 10 minutes trying to figure out why it won't load into After Effects, we’ll just touch briefly on what it is. Printing ink is based on reflected light whereas the RGB color model is constructed to create an image from a light source.
RGB is based on the color we perceive when we look directly at a light source (spotlight, television screen, LCD monitor…etc.) and is referred to as “Additive Color.” The primary colors are Red, Green and Blue while the secondary colors are Cyan, Magenta, and Yellow. When all colors are combined in an additive color environment (all colors of light are 'on'), you get white, when all color is gone (lights off), you get black.
CMYK turns everything on its head, as it's based on “Subtractive Color.” This is color we perceive once light has bounced off of something. Usually the color we see is the only color not absorbed by the object. So the car isn’t really green, the surface of the car absorbs most of the spectrum of light that hits it, but rejects green light and it bounces off, becoming visible to us. Since we are turning everything around, you can picture combining all the colors in a paint store (or as a kid, in your water color tray...or on the living room carpeting...) and getting something that is usually undesirably close to black. Removing all pigment from paint gives you…white. Just to cap off the confusion, the primary colors are Cyan, Magenta and Yellow, hence the “CMY” with “K” indicating black ink. Of course since all else is opposite, Red, Green and Blue are the secondary colors in subtractive color.
Color Difference Systems
I explained my rather unconventional way of visualizing Color Difference systems in general. (see the explanation and illustration from the post "Y's and Whats..." below). What follows is a description of the various common designations we all encounter in the field and their intended definition.
Y’UV
I’ve even spouted this designation off when I am referring to all color difference, non-RGB television color space, but it’s just laziness on my part as it really isn’t correct.
It does refer to color difference video color space, but it specifically refers to the color difference signals B’-Y’ and R’-Y’ after scaling used in an intermediate step in the creation of an analog composite NTSC or PAL signal. Though “Y’UV is frequently misused in referring to digital video, and particularly incorrect in cases where it is paired with a subsampling designation such as “4:2:2”, as it has nothing to do with component video or digital video.
Y’ PB PR
Y’ PB PR is the description for the coding for analog component video. PB and PR correspond to the B’-Y’ and R’-Y’ signal after scaling to this standard. In the case of analog, digital color sampling ratios like “4:2:2” don’t apply, but in this system the color difference components are lowpass filtered to approximately half the luma bandwidth.
Y’ CB CR
Digital component video changes the designation somewhat. Y’ CB CR references the scaling method used in digital video files on disk or tape. While the analog component color difference signals would be lowpass filtered to preserve bandwidth, their digital counterparts would instead be subsampled if efficiency was required, creating the basis for the typical ratios expressed with numbers like “4:2:2” or “4:2:0”. (More on that in later posts.)
Y’ I Q
An obsolete system established in the early days of color television. What was intriguing about this system is the normal vectorscope color orientation was tilted 33 degrees, and the “I” and “Q” axes (which correspond to the ‘U’ and ‘V’ components, but the axes are actually switched) can still be seen on some vectorscopes to this day in that configuration.
There's some stuff to keep your mind working over the weekend...
TimK
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