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Old 10-12-2007, 09:30 AM   #16
wallcraft
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Join Date: Mar 2006
Location: Mississippi, USA
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Quote:
Originally Posted by andym View Post
2. convert jpegs etc to gif
The problem is that, although gif file sizes vary, the "typical" 64KB gif image from a full color illustration is about 320x320. The gif optimizers I have looked at reduce the file size (for a fixed image size) by reducing the quality of the image. So, why not instead use a format like jpeg which is designed for lossy compression in the first place?

This seems to be what MobiPocket concluded too. Their original approach to importing images larger that ~64KB was to first convert them to .gif and then reduce the number of pixels in the image until the gif was small enough to fit in ~64KB. In addition to small images (itself a loss of image fidelity), you could also get variable image sizes from the same original image size in pixels. MobiPocket's new approach is to leave jpegs in the jpeg format but (presumably) increase their compression without reducing the image size to get them to fit in ~64KB.

There are cases where gif might be superior, e.g. sharp-edged line art with a limited number of colors. These may already be smaller than 64KB, and if not reducing the size of the image may be the best lossy compression technique available.

I am embarrassed that I did not reference the Illustrations in mobipocket thread, particularly since I posted to it. However, its example ebook illustrates the problem. Most of the internal images are ink drawings and are less than 64KB in .png or .gif, but the cover image is a 110KB jpeg. Once again, mobigen without -jpeg produces a much smaller cover image.
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