I am not aware of any options during a 16 bit to 8 bit conversion in any app that will guarantee no posterization in the 8 bit version if there was no posterization in the 16 bit version.
Can you suggest an app and the options/settings within it if an app exists?
You can edit the image to avoid areas that are problematical for posterization given the output color space and the image. Or you can apply dithering in the 16-bit precision space before conversion. Ps has ways to generate dither. But if not further editing is to be done on the 8-bit image and you avoid wide gamut spaces like CIElab and PPGRB, posterization is unlikely if the editing is performed in the 16-bit space.
And even before that, image comes with noise, often providing good initial dither.
when i used to shoot m43 i used to convert raw to 16 bit then save both the 16 bit and a 8 bit jpeg. and see no posterization. on the rare occassion though when printing posterization would be seen in the print so i would have to print the 16 bit file.
Good point. Sometimes photon noise is our friend. Synthetic images are more susceptible to posterization than photographic ones.
But now we have gone full circle and back to my original comments that if the posterization is not noticeable or objectionable then it really doesn't matter and that editing out posterization might result in the original goal for the image to not be achieved.
Why and how one would introduce posterization in the first place, could you show?
You have not made a case for the assertion that the original goal might not be achieved. Please explain how that could happen.
I did not suggest editing it out. I suggested keeping it from occuring.
Yes I know you did.
It was I who suggested editing out posterization in the 8 bit version after being converted from an original 16 bit version with no posterization might not allow the original goal for the image to be achieved with the 8 bit version.
I also said that if the posterization is not noticeable or objectionable then it really doesn't matter anyway.
I explained earlier the scenario where posterization introduced by converting to 8 bit from 16 bit might not allow the image's goal to be achieved if the undesirable posterization is edited out.
It will depend on the image and the goal for it.