> 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.
The DCT produces a stream of codes reducing the most common tones of the
8x8 pixel area. This produces a stream of 64 codes to give lossless results,
however the quality level indicates that only a certain number of the most significant
of these codes are stored... These codes are then huffman coded and stored with
a runlength encoding for zero values... The artifacting causes are simple enough
and your example gives one but does not ome under my definition 'done with a
paint program'...
> Depending on the transfer function of the smoothing and antialiasing
> operations, the result could be more spikes on the spectra.
My definitions of smoothing and antialiasing must be different to yours then - these
reduce colourspace movement usually by interpolated colourspace positions gained
from weighted averages between two colourspace locations...
> 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 don't use animations or need transparency for all of my non-photo images...
And if you're requiring photo 'quality' images don't use JPEG use PNG...
> 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.
sounds like we're very agreed that there are horses for courses ;)
--
Aaron Scott-Boddendijk
Jump Productions
(07) 838-3371 Voice
(07) 838-3372 Fax
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