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Re: [BKARTS] image compressions....



On Monday, January 27, 2003, at 06:24 AM, Julie Sadler wrote:

.jpg is a compressed file and therefore probably isn't the best choice
for saving images of quality.
.tiff is a file with more complete image data, and therefore would be
the best choice. But beware, these files are going to be very large in


JPG is a lossy compression that is user adjustable on a long scale. The highest quality jpeg's are compressed only a little, while the file size doesn't shrink much, the image degradation is (to my eye) imperceptible. The deepest compressions result in tiny files and brutalize the image. In these images you can clearly see artifacts from the 8x8 pixel grids which are used as the compression block in the image (zoom in several 100% to get a good look at these blocks). If your image was at 75dpi these artifacts will be large and noticeable (about 1/10 of an inch). If your image was at 300 dpi the artifacts will be small (~.026") resulting in less obvious damage to the image.

Each generation of jpg compression will further damage the image. While
I'm cautious about using jpegs as archived images, an understanding of
the above information and some experimentation can open some options.

Also, unless standards have changed since I last checked, the only
reliable (and proper) color space for a jpec is rgb. Mac browsers and
software seemed pretty flexible on this point but Windows machines
pretty insistent. Color space changes can cause lots of color matching
problems. TIFFs seem happy as greyscale, rgb or cmyk.

TIFFs offer a lossless compression called LZW. This can cut your file
size significantly (close to 50% in some cases) and results in
absolutely no loss of image data. When I left the graphic arts a couple
of years ago the support for lzw compressed tiffs was substantial. I
don't recall any major software players, mac or pc, that didn't allow
the direct use of compressed tiffs as placed images.

Don Drake
Dreaming Mind

ddrake@xxxxxxxxxxxxxxxx
IM: stitchbyhand
http://www.dreamingmind.com
http://www.hughstump.com
(510) 537-9711

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