[robski]

JPEG uses a lossy compression method that throws useless data away during encoding. This is why lossy schemes manage to obtain superior compression ratios over most lossless schemes. JPEG is designed to discard information the human eye cannot easily see. The eye barely notices slight changes in colour but will pick out slight changes in brightness or contrast. Therefore JPEG's lossy encoding tends to be frugal with the gray-scale part of an picture and frivolous with the colour component. A typical photographic quality image maybe compressed by 20:1 without experiencing any noticeable degradation in quality.

If we look at the original RGB image we will find 3 high definition images one for each colour.

The first task of the JPEG encoder is to convert the RGB image components into a monochrome component ( luminance), a blue chroma component and a red chroma component. For the more technically minded it have converted from a RGB colour space to a YCbCr colour space used in video systems. Those who use Photoshop may of bumped into changing the mode of a RGB image to Lab.

Lab is a similar idea to YCbCr

Where L is the brightness component, a is the red/green component, b is the yellow/blue component

The purpose of the separating RGB into mono and colour it to allow different processing to the luminance and chroma channels.

Below we can see the 3 high definition RGB channels.

I have also added an illustration that shows the luminance (gray scale (L channel)) and combined chroma (a + b channels ) components separated from a RGB image using Photoshop. From this we can see that the gray scale is high definition and the chroma is very low definition. In fact if you blurred the a and b channels and converted back to RGB you would barely notice any difference. A completely different story if you sharpened or blurred the L channel.