Working with various color formats

As we have seen, OpenCV and the iOS SDK work with various formats for color and grayscale images and sometimes we need to convert between formats. Let's step back from the code for a few moments to discuss the differences between formats and the problems that can arise if we do not perform the correct conversions.

RGB, BGR, RGBA, and BGRA

You probably learned the 24-bit RGB color format long ago, the first time you picked a custom color in a paint program or word processor. A pixel's color is represented by a sequence of three values, each with a range of 0 to 255 (that is, 8 bits or 1 byte). The first value is the color's red component or channel, followed by green, and lastly blue. For example, the color of an amber traffic light is (255, 126, 0), a mixture of lots of red and some green, but no blue. A series of pixel data makes an image.

The 24-bit BGR format simply reverses the channel order. For example, the color of the amber traffic light is (0, 126...