If you’ve ever replaced the ink or toner in a color printer, you know the three colors Cyan, Yellow, and Magenta are used in various combinations to print any color. On your computer screen it’s mixtures of Red, Green, and Blue.
Each tiny dot in a picture, called a pixel, has three numbers: How much red, how much green, and how much blue get mixed together to form the color of that specific pixel. Typically the amount of each color is stored as a number ranging from 0 to 255.
On a computer, a picture is really just a whole lot of numbers representing the reds, greens, and blues of every dot in the picture.
Then, along came digital video…
Neither RGB or CMYK was optimal for it digital video. There needed to be a more efficient way of storing and transmitting images and that is YUV — sometimes called Y’UV or YCbCr.
YUV is partially based of the tradition that televisions started broadcasting in black and white only, and partially because light and dark is the most visually important aspect of an image. The first number in a pixel of a YUV image is Y, the brightness or luminance/luma.
If you were to only have the luminance of the pixels in an image, the image would appear black and white. To colorize the luminance, a U/V graph maps all the possible colors. The U and V values are the coordinates on the graph identifying the color of the pixel.
The origin is colorless gray. Quadrant 1 (high U and high V) is pink, quadant 2 (high Y and low U) is red, quadrant 3 (low U and low V) is green, and quadrant 4 (high U and low V) is blue.
Give it a try to see the differences:
Side-by-Side: Color, CMY, RGB, YUV
Here’s a comparison of storing the same simple 4 pixel wide by 5 pixels tall image as CMY, RGB, and YUV:
Compressing Digital Video Images
One huge benefit to storing images as YUV instead of RGB or CMYK is the ability to easily compress the image without losing perceived quality. Scientists have concluded that the human eye is most sensitive to changes in brightness/darkness. Color is less important. As a result, the U and V values of a YUV image can be compressed with minimal affect to what we see.
In a UYVY or YUV422 image, only 75% of the image remains. All of the luminance/luma values are used, but only half the color numbers — every other U and every other V. The moniker 4-2-2 comes from every 4 Y luminance numbers have 2 U and 2 V chrominance numbers.