Lossless rotation of JPEGs

[herman]

Does PixInsight perform lossless rotation of JPEGs?

[Nocturnal]

As Juan would say "of course".

 

[georg.viehoever]

I think the answer is "no". When you load a jpg to PI, it is decoded. You can then rotate the decoded data (without data loss). But when you store it again as JPG, it needs to be encoded again (with the associated losses).
Georg

[Nocturnal]

But by that definition no software would have lossless rotation. The way jpg is encoded (macroblocks and all that) you have to decode to pixels before you do anything.

[pfile]

but there's a trick for rotating jpegs without decoding them... that's probably what the OP is asking about.

[georg.viehoever]

I think it is possible to rotate jpg without decoding it first -some tricks with rotating the coefficients. See http://graphicssoft.about.com/cs/digitalimaging/f/rotatequality.htm
Georg

[Nocturnal]

Another educational day on the PI forums

[Carlos Milovic]

Yes, it should be possible... at least, rotating the 8x8 blocks is easy. The problem arises with the coefficients inside the block. A 90º rotation is also a 90º rotation in the frequency domain (fourier, or dct), so that is not hard. But, I'm not sure that the lossy compression always keeps "simetrical" coefficients, or it has a prefered direction. I would say that some of them, after the rotation, will fall on ignored positions, so at the end we'll have either loss of data, or/and artificial coefficients (= zero?).

Please correct me if I'm wrong.

[herman]

but there's a trick for rotating jpegs without decoding them... that's probably what the OP is asking about.

Yes, that is what I am asking about.