In DisplayButtons, Panel "Output", a new slider "Dither" allows to add random noise dither to rendered output. It works on sky as well as solid and transparent. Note however that in OSA render, the Unified Render gives much better results. The old render mode isn't well suited for this postprocess.
A dither value of '1.0' will exactly add maximum of 1.0/256.0 to the pixels.
Potential improvements for next releases;
Also note that this gives best results for print work or stills. Animating it gives slight visible noise. Also runlength compression wont really work, and Jpeg can look quite ugly with it.
The pictures show without (top) and with (bottom) dithering. You have to click the images to actually see difference!
Patch provided by Goran Kocov, thanks!
For compositing purposes, the Material option Only Shadow allows to render Material as alpha-only, which by default displays as black color with an alpha map to composit over the background (for example sky).
Since AO isn't really rendered as 'shadow' in Blender, but works on the diffuse light amount, a direct conversion from AO to alpha isn't fully identical. Pictures below show it works in 2.35.
Tweaking alpha rendering is still work todo for an upcoming release. :)
