jpeg in 8 or 16 bit?

[mbalensiefer]

--?Question: DOES JPEG only allow 8 bits of data per channel?--

I thought that having a 24-bit JPEG image really means that one has 8 bits per standard (RGB) color channel.

But in Photoshop, under
 Image > Mode >
 and with the "RGB Color" dialog checkmarked: either an 8-bit-per-channel or a 16-bit-per-channel option can also be selected.
 Under both of these options I can export as .jpeg.

 What gives?

Thanks!
Michael

[fdisilvestro]

Jpeg supports only 8 bits. I assume you have CS5, which will do the conversion to 8 bit as part as the save as command. In previous versions you had to convert first to 8 bit to be able to save in jpg.

[bjanes]

Jpeg supports only 8 bits. I assume you have CS5, which will do the conversion to 8 bit as part as the save as command. In previous versions you had to convert first to 8 bit to be able to save in jpg.

This change is useful to know, and I was not aware of it. JPEG2000 is supported by Photoshop and it supports 16 bpc files.

[joofa]

If my memory serves me right JPEG (yes, the old one, not the new JPG2000 necessarily) does allow more than 8 bits. Is it 12 or 16 bits, I can't seem to remember, but can verify later. IIRC, interestingly, JPEG has a mode for lossless compression also!

Furthermore,  JPEG is an algorithm, which in principle can be applied to any number of data bits, the standard just defines what somebody picked (stuff such as bit depth, color space, transforms, etc.) and canonized. Furthermore, many times people mean JFIF, which is a JPEG file container, when they say JPEG, and mix the JFIF file container properties with the JPEG algorithm.

Joofa