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

Forum|Forum|9 years ago
January 27, 2017
2 replies
3506 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?

D Fosse

Community Expert

Thanks for taking the time to check this for yourself. I wonder why we're seeing it differently. There must be a reason. Some other setting somewhere?

I saved out with and without dither checked in Color Settings, from 16 bit and 8 bit originals.

Just checking but by that you don't mean you drew the grad in 8 bit mode? To observe the effect you must draw the grad in 16 bit mode, then switch to 8 bit mode through Image>Mode. If dither isn't enabled in 'Color Settings' you will immediately see the 8 bit steps. Hit Ctrl+Z to undo the 8 bits/channel conversion, enable dither and switch to 8 bit mode again. When dither is enabled you will probably still be able to pick out the steps, but they will be a lot less obvious if they are dithered (less obvious than they are in 16 mode on an 8 bit monitor). At least, that's what happens on my system. There's no need to output any JPG files to test this, it will be observable on the screen. as you do it.

Also, as you have a 10 bit monitor, best do all this at a display zoom of 66.7% or higher.

Not to question what you're saying, just curious - but is this something you know for a fact, from other sources?

It's not something I've read about elsewhere, otherwise I wouldn't have brought it up. I'm just reporting the way I observe it working on my systems.

I'm running on a PC. I don't know if there are any functional differences running a Mac.

richardj21724418 wrote:
Just checking but by that you don't mean you drew the grad in 8 bit mode? To observe the effect you must draw the grad in 16 bit mode, then switch to 8 bit mode through Image>Mode.

Yes, that's how I did it.

I'm viewing everything at 100% consistently. And I'm on Windows.

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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