Quote:
Originally Posted by eschwartz
I don't know why you would want to use the lossy JPG format (and do a conversion in the process),
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Replying to an older post but I have a rationale for wanting to change format and re-size.
Many eBook novels are effectively 100kb+ of compressed text attached to a 3mb+ of image(s). The images, in all their glory, are often viewed for seconds or in thumbnail while the text is pored over for hours. A lossy reasonably sized jpg can give an excellent image presentation on the screens they are viewed on, whereas large lossless print-quality 24bit PNGs are something I consider overkill in many casual read/paperback-like cases, though desirable for archives and art books.
Would be nice if Calibre's Polish, or a Plugin, or the Editor could batch resize and change format to a "paperback" spec, reliably without artifacts (bicubic, etc...). Throw in a "combine identical images" function and a lot of epub novels could go from ~3mb to ~300k. A 90% filesize reduction that doesn't noticeably affect readability is nothing to sneeze at.
RIOT Radical Image Optimization Tool, used judiciously, is a nice in-place batch tool for Unpacked images. Format changes in the epub still have to be managed individually in Edit eBook though. Take care on black-and-white gifs/pngs, it can try to optimize them all the way down to 2-color dithered. Major size changes can produce ugly compression artifacts as well.
Photoshop's "Image Processor" script is still the consistently cleanest batch resize/resample that I've seen.