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Old 09-05-2026, 07:18 AM   #16
kovidgoyal
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compress images does have options to convert image formats and it uses optipng as well.
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Old 09-05-2026, 05:42 PM   #17
JSWolf
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Quote:
Originally Posted by k12b5 View Post
You're right - Calibre's "compress image" only re-encodes in the current format, and I've never seen a plugin that converts PNG to JPG either. The usual approach is to convert outside Calibre and put the results back in:

1. Extract: EPUBs are just ZIPs, so unzip book.epub -d book/ - the PNGs will be under book/OEBPS/images/.
2. Convert: drop that folder into a batch converter. If you have no qualms about installing, mogrify -format jpg -quality 85 *.png (ImageMagick) does the whole folder at once. If you'd rather not, Handio's PNG to JPG converts all uploaded images in one click with a quality slider and a "download all" button - files are processed in your browser, nothing gets uploaded.
3. Rebuild: swap the converted JPGs back in, then re-zip: cd book && zip -X -r ../book.epub mimetype * (zip the mimetype file first, uncompressed).

Two caveats: only convert photographic images - PNGs containing text or diagrams will look worse as JPG. And keep quality at 85+, since ebook readers stretch images and artifacts show up more than on screen.
Your wrong. The current version of calibre does support converting PNG to JPG. You have to do this in the editor.
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