Thanks Mick & Dave
I am trying to provide the info in bite size chunks as and when I get a few free moments.
A point to mention about the Chroma down sampling is that the colour information is lost forever. Plus further opening and resaving of the jpeg file will cause further lose of colour information. How important colour information is in our lives is open to debate. When you consider that 1 in 20 men are genetically disadvantaged and suffer from some form of colour blindness. The best way of explaining colour blindness is the restriction (compression) of the range of colours in a rainbow. Women ( only 1 in 200 suffer from colour blindness ) will see all the hues of reds and greens. Whereas men may just see a red and a green and in severe cases not be able to tell the difference at all.
Moving on to the next process step.
The luminance and chrominance components of the image are divided up into an array of 8x8 pixel blocks. Padding is provided if required to ensure blocks on the right and bottom of the image are full. This segmentation into blocks is in preparation for the next stage which is the most processor intensive and difficult to comprehend. But don't let that put you off
