Thanks Frugal, but you quickly reached the edge of my
understanding ;-). I shall stick with Graphic Converter or PS,
albeit now knowing that the quality levels might bear little relation
to each other.
Chris
~~ >-)-
C M I Barker
Cambridgeshire, Great Britain.
+44 (0)7092 251126
www.threeshoes.co.uk
homepage.mac.com/zuiko
On 17 Jul 2005, at 15:22, Andrew Dacey wrote:
> As for sub-sampling, if memory serves, that's a method where the
> chroma values of adjacent pixels are averaged in order to reduce the
> file size. At a setting of 1x1 there should be none (you're averaging
> a grid of 1 pixel by 1 pixel). A common setting (at lower jpeg quality
> settings) is 2x2 sub-sampling. Normally, each pixel would have 24-bits
> of data (8 bits each for Luminance, Chroma A and Chroma B) so for a
> 2x2 grid you would use 96 bits of data (4 x 24). When sub-sampling,
> you keep the luminance value for each pixel but only store 1 chroma
> value for A and B for the whole grid. This results in each 2x2 grid
> only needing 48 bits of data (4 x 8 + 8 + 8). For images with lots of
> solid colour this is a great way to get small files (50% savings right
> there) but for images with lots of fine detail the results are usually
> not good.
>
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