[robski]

The next stage is to apply Chroma sub sampling (down sampling - reduce the resolution).

The simplest way of exploiting the eye's lesser sensitivity to colour information is to use less pixels for the two chrominance channels

The luminance channel is retained at full resolution.

Both chrominance channels are typically down sampled by 2:1 horizontally and either 1:1 or 2:1 vertically

For example a colour image of 1000 x 1000 pixels, the luminance (grey/mono) channel will remain at 1000 x 1000 pixels but the chroma channels would be either 500 x 1000 pixels or 500 x 500 pixels. So a single Chrominance pixel would cover either cover 2 x 1 or 2 x 2 luminance pixels dependent on a jpeg encoder setting. This maybe under user control as part of the quality setting or just hard coded by the programmer.

So in terms of compression the higher down sampling will give us 50% compression with very little perceived lost of quality with photographic images .

i.e. we store 6 pixels values for each 2x2 block ( 4 luminance values and one for each of the 2 chrominance channels) instead of the 12 needed to store at full resolution in each channel.

The existence of chroma sub sampling in JPEG compression explains why better compression ratios are achieved (ratio between original file size & compressed file size) with colour photos than with greyscale (mono) photos.

Most DSLR cameras use 2x1 chroma down sampling while some graphic editing programs and point and shoot digicams use 2x2.