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From what I understand, a JPG file stores each 8x8 pixel block of picture data intependently. It would be nice to have a program which could perform certain manipulations on a JPG file and write out each 8x8-pixel block exactly as it was in the original file (avoiding file-size-growth which would
result from rewriting the JPG with too high a quality setting and quality loss which would result from using too low a quality setting).
Functions I'd like to see supported: cropping off multiples of 8 pixels from any side, and rotation (I think the data munging required for rotation could be done losslessly). Additionally, for an image editor, I'd like the editor to keep for each unedited 8x8 pixel part of the picture the information required to rewrite that part of the picture exactly the same as it was in the original.
Does anyone know of any programs that do that?
JPEG Wizard
http://www.jpg.com/products/wizard.html
Rotates, mirrors, crops without loss. I haven't actually used this product, but the description seems close to what you describe. [jutta, Dec 20 2000]
The Independent JPEG Group
http://www.ijg.org/
Their jpegtran program (included in jpegsrc.v6b.tar.gz) performs various lossless transformations of JPEG files by working directly on the compressed data. This is part of the standard JPEG package that includes libjpeg, cjpeg and djpeg. [Ed Avis, Oct 11 2002]
[link]
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What bad experiences have you had with JPEG? It's not the whizziest format around, but within its domain (lossy compression of photographic images), it does a pretty good job. |
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A refinement I'd like to see with a jpeg editor would be the ability to change the amount of loss in different parts of an image. For instance if you've got a picture of a car, you could draw round the car and have that bit compressed without too much loss while the background is highly compressed.
Ah, just checked out jutta's link and lo and behold the feature regional compression. |
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Gordon, Jutta: The JPEG wizard utility does precisely what you (and I) were looking for. It allows you to define up to 15 regions on a JPEG and adjust the sliders separately for each region. Additionally, it allows certain manipulations to be done to JPEGs losslessly (resizing, merging of JPG's, color adjustment, etc.) Way cool. And the best part of all is that the basic version that provides everything except automated batch processing is FREE. |
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