On April 1, 2004 16:12, Winsor Crosby wrote:
> You also have an advantage if you use Photoshop. Adobe changed the way
> PS handled jpeg in one of the more recent versions. The way I
> understand it is if you open a jpeg image and make changes PS preserves
> the original parts of the file that were not changed and jpeg only
> operates on the pixels that were changed. So deterioration of the image
> by the lossy compression process is minimized.
But that would be most operations done to _photographs_, except rotating (by
multiples of 90 degrees) and cropping (and cropping only when done on even
multiples of 8 pixels.) All the other common operations, levels, curves,
sharpening, contrast and colour adjustment, are whole image operations, so
this PS optimization is hardly ever applicable, except if doing something
like writing "Happy Birthday" on an image. Oh yes, and spotting dust, unless
you use automatic de-spotting, which will affect many pixels, dust or not.
The same optimization can also apply in the case of Jpeg200, as it, like
Jpeg, is done in blocks.
If you want to get some image compression, be lossless, support layers, image
metadata, and variable pixel precision, try PNG (Portable Network Graphics).
Does much more than TIFF, with smaller files, is not proprietary, and is
generally widely supported by both commercial and open-source software.
> I have not used the JPEG2000 that is included in PS now. When I tried
> it before with one of those freeware converters it was achingly slow.
Wavelet compression (jpeg2000) takes a LOT more computation than discrete
cosine transform (jpeg). So it will take longer, but computers are REALLY
fast, so it might not matter a lot.
I don't use PhotoShop (I use GIMP and PaintShop Pro), and I don't use
Jpeg2000, because other folks are not generally set up to read this format.
Have to wait until the big guys put up lots of jpeg2000 images, that the
little guys want to see.
--
Parzival Herzog
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