Saving 16bpc to JPG - first convert to 8bpc?

Forum|Forum|9 years ago
January 27, 2017
2 replies
3507 views

I'm sorry if this is well known but I've only just noticed that if I save a flattened 16bpc image directly to JPG then no dithering is applied to the 8 bit conversion, even though I have the 'Use Dither' option checked in Color Settings. Is there another setting somewhere to enable dithering on saving a 16bpc image at 8bpc? Otherwise, the solution looks like converting to 8bpc mode prior to saving the JPG....i.e. an extra operation in the workflow.

Normally I want to enable dithering when converting down to 8bpc, so as to retain a bit of extra shadow detail and reduce perceived banding. It might explain some of the effects I've noticed that, up until now, I'd blamed on JPG artefacts.

dither

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Randy Hufford

Community Expert

Yes you have to change file to 8bit before saving as jpeg

Trevor.Dennis

Community Expert

Randy, they made saving directly JPG from higher bit depths a JDI feature with CS5 (I think). Are you saying that using that feature does not work as well as doing it step by step?

richardj21724418Author

Known Participant

I have an instinct, not confirmed, that dithering an image is going to introduce discontinuities in the image data which will interact poorly with JPEG compression, which is designed to optimize continuous tone. This means that the dithered version may compress less well or be more prone to artifacts.

This might be why Adobe don't dither. I'd recommend examining compressed size and quality if analysing this issue.

I've now tested with a few real life images. Saving at a JPEG quality of 10 it makes very little difference to the file size if the image is dithered down from 16-bit or not. Dithering generally adds a fraction of one per cent to the file size, although surprisingly in one case the dithered image file was very slightly smaller.

I doubt Adobe had concerns about applying dithering prior to JPEG compression. After all, if you work at 8bpc you probably enable dithering when applying grads and when performing colour space conversion, don't you?

Whilst the lack of dither is glaringly apparent on grads, viewing real life images I have to say that I have to look quite closely to notice any difference between a dithered and non-dithered conversion from 16bpc to 8bpc. I'm therefore surprised I ever spotted it. As you'd expect, the differences are most apparent in shadow areas, but really only when you've got a good clean source image. In other words, dithering is probably only of practical benefit when you're dealing with a clean, low ISO shot and/or have incorporated clean tones post process.

I'm still surprised that Adobe don't provide a dither option when saving 16bpc to JPEG. Or at least follow the option setting for 16bpc to 8bpc conversion.

D Fosse

Community Expert

Jpeg compression in itself creates color banding in soft gradients. That's most likely where the banding comes from - unless you can demonstrate that converting to 8 bit prior to saving as jpeg eliminates it.

richardj21724418Author

Known Participant

Yes, when I pixel peep I can clearly see that banding is eliminated (or at least it's dither distributed by the lower order bits) if I drop down to 8-bit before saving as JPG. It's how I spotted this effect in the first place. If I drop to 8-bit mode with dither disabled the screen looks remarkably like the JPG image saved from 16-bit mode!

It's not something I was aware of before. I'd naively assume Photoshop would dither down to 8-bits when saving to JPG. Still learning something new every day.

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