Alain <aze### [at] qwertyorg> wrote:
> I started with a true colours JPG file about 16K.
> I converted it as GIF using dithering and optimised palete. The result 
> was rather poor, and the file size jumped to around 150K! Almost the 
> same as the 24 bits BMP version...

> Yeah! Right, dithering plays havok on compression. It introduce 
> non-compressible noise, most of it is essentialy random.

  Converting from JPEG to GIF (or PNG) also has the drawback that JPEG
artifacts will worsen the compression (compared to the original).

  When I want to, for example, show a screenshot of my desktop or some
program to someone (and assuming there aren't very many colors overall),
I always convert the screenshot to a 256-color PNG undithered (and then
optimize the PNG using one of several PNG optimizers out there). It usually
gives by far the best compression for a lossless snapshot (well, at least
if 256 colors is enough to represent your screen or the program you are
taking the snapshot of).

  Avoiding the dithering is, indeed, a very crucial step in this process.

-- 
                                                          - Warp

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