What exactly is BGR color space?

Question

An RGB color is composed of three components: Red (0-255), Green (0-255) and Blue (0-255).

What exactly is BGR color space? How is it different from RGB color space?

Answer 1

It is about the way the color components are laid out in memory. For BGR the order is BGRBGRBGRBGR..., and for RGB the order is RGBRGBRGB... For BGR, the default order for OpenCV, it is like this:

(See the OpenCV documentation for How the image matrix is stored in the memory?)

Note the other answers have referred to certain colors being least or most significance, but that actually depends on the endianness of your machine. And the packing order of components in an unsigned int, e.g, actually depends on your software, or the library you are using. However, regardless of your library, or the endiannes of your machine, for BGR pixel the byte address of the B color component will be one less than for G and and two less than for R (and for RGB it is just the other way around).