> 3. JPEG can compress image drawn from a paint program equally well as
> photo-realistic images although the pixel structure that
> cause artifacting are
> considerably more common in hand-drawn low palette-size
> images (number of colours
> in images, NOT whether or not a palette is stored since
> JPEG does not work on
> pallete based images).
Not from my experience. If there is text, or if the image is very detailed
with lots of lines and harsh borders (like on a typical advertisement) on
the image, JPEG performs really poorly. JPEG was designed to handle the
gradual tonal change in photographic images. The energy in the spectra
(output of the cosine transform) is filtered by the quality factor,
resulting in compression. Gradual tonal changes results in spectra that
filters well. Text and harsh lines cause spikes in the spectra, resulting
in poor compression and big loss of image quality. This is evident from the
numerous web sites that display fuzzy "graphic" text.
> > GIF and PNG on the other hand provides much better
> > compression that JPEG with images drawn with a paint type program.
>
> Where small palette sizes or large contiguous areas of the
> same (or similar) color are
> evident... Usage of more powerful image manipulation software
> that handle smoothing,
> antialiasing, alphablending and colour shifting for you will
> soon generate images that do
> not compress well in PNG and can't be stored in GIF
Depending on the transfer function of the smoothing and antialiasing
operations, the result could be more spikes on the spectra.
Furthermore, JPEG does not support transparency (no alpha channel) and
animations.
In short don't use JPEG for anything other than photo quality images.
> ... I
> would however suggest that PNG
> is the best portable format to invest in for Loss-less and
> work-in-progress formats (other
> than the native format of your image software (eg PSD))...
I would agree ... except that if there is to be significant image
processing, then a non-compressed format will deliver better speed (no need
to decompress / process / recompress). This is a speed / space tradeoff.
Dennis.
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