The red, green, and blue primary color signals (R′G′B′) are weighted and summed into a single luma signal, designated Y′ which takes the place of the original monochrome signal. The color difference information is encoded into the chrominance signal, which carries only the color information. This allows black-and-white receivers to display NTSC color signals by simply ignoring the chrominance signal. Some black-and-white TVs sold in the U.S. after the introduction of color broadcasting in 1953 were designed to filter chroma out, but the early B&W sets did not do this and chrominance could be seen as a crawling dot pattern in areas of the picture that held saturated colors.”
While YIQ is not actually commonly used beyond NSTC, you are more likely to run into YUV, or one of its other variations. In fact, the HAP Q codec makes use of a similar colorspace, YCoCg, using a shader to quickly convert the pixel data to and from RGB as part of the pixel pipeline.
When we are talking about color representations in digital image / video codecs, the choice of a color space is part of the design considerations. When storing pixel data as YUV, more data is stored for the luma level than the chroma values when compared to RGB based codecs. This typically results in slightly lower color accuracy with the benefit of improved brightness and contrast accuracy. As the human eye is more sensitive to differences in brightness levels, it can be beneficial to use color spaces that are more tuned to these qualities in some cases. Similarly, when creating metrics that help us evaluate how noticeable image compression is to the human eye vs how well it performs numerically, these types of characteristics are taken into account. The amount of time it takes to convert to and from RGB is also something that needs to be considered when optimizing compression algorithms for real-time usage.
Beyond all this, oftentimes certain image processing FX may use a different color space for a variety of different reasons. For example, performing a hue shift is a difficult operation to perform using RGB values, but very easy to do when working in an HSV or HSL values. If you look at the code behind the Color Controls ISF, you’ll notice that each pixel is converted from RGB to HSV, adjusted based on the input parameters, and then converted back to RGB for the output.
