lossless JPG rotation

[Guest]

This is a reply to the thread named: "IMAGE ROTATION" - please read it before.

When I rotate forward then back a jpg file (multiple of 16), a binary file compare gives some differences. Is it just some metadata that changed, or.... ?

Is the statement about the mathematically lossless character of jpg rotation of the Image Preview app of XP an official product group statement ? There's no KB article saying anything about it...and beside this: (entry #46 in http://sylvana.net/jpegcrop/losslessapps.html), I couldn't find any statement about it...

I wonder if the Image Preview code implements this:
http://msdn.microsoft.com/library/d...formingaJPEGImageWithoutLossofInformation.asp

 

[David Candy]

When you rotate you change the file. If it's 16 x 32 then it will become a 32 x 16 image. GDI only does bitmaps and metafiles (emf, wmf). So it should be using GDI+ as it was written for jpeg etc.