FastStone JPEG rotation and file size

Started Aug 26, 2013 | Discussions thread
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calumm
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FastStone JPEG rotation and file size
Aug 26, 2013

Hi

I'm using FastStone to losslessly auto-rotate my JPEGs according to the EXIF orientation tag. I'm finding that the file size decreases by more than I'd expect; for example, 4.66MB to 3.92MB. So I'm wanting to double-check that this process actually is lossless.

I tried the technique in the last comment in http://www.impulseadventure.com/photo/lossless-rotation-test.html to compare the original and rotated files (paste rotated as new layer onto original, layer differences, flatten, auto-level), and I see a black screen, implying that they are identical.

I also note the v2.28 release notes for FastStone - "Enhance JPEG Lossless Rotation - images are optimized for smaller file size" - suggesting that it is doing some optimisation while remaining lossless.

Should I be worried about the reduction in file size, or should I be confident that FastStone is doing the right thing?

Thanks - Calum

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