web-safe colours.

The more I get into making images for the web, the more I get
confused.  I think this comes from a fundamental confusion
concerning web-safe colours.  My questions to you are:

1) What happens to colours in a GIF that are not in the colour-cube?
 Does a harmless midtone blue change to some sort of green or does
the pixel not show?  If these in-between colours are dithered then
why dither them first?  Surely it is better to allow some people to
see the higher quality work.

2) Does JPEG's "need" to be dithered to a web-safe palette?  I've
heard that they don't.

3) My understanding of GIF's is that they contain some sort of
palatte index ( eg 1 = #ff6699, 2 = #ffffff, etc ) and then some
image data.  If JPEG's don't need to be dithered then why do GIF's
need to be - they both carry colour information (ie - gifs don't
really have to conform to some sort of standard palette)?

One of my reasons for these questions is that when I come to dither
an image to 256/216 colours for saving as a GIF - I achieve much
greater quality when using adaptive-dithering rather than web-colour
dithering.  In many cases an image made of 64 adaptive colours is of
a much greater quality than one of 216 web colours - and it is
smaller! 

PS - is there some benefit of dithering first to one type of palatte
and then to another, or do you get the same results?

Thanks in advance, Warrick.





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