A better estimation of rendertime

Often I need to know how long a render will take but Cycles doesn't know exactly how long each tile will take to render, so it estimates the remaining time based on current speed*number of tiles (a little guessing here). My proposal would solve this issue and give a further bonus.

Cycles should first render 10 samples (edit: or maybe tiny% of total samples) for each tile, so that it can gather information about their "speed" and then calculate in proportion with the number of total samples, to get the correct estimation. The bonus: a quick glimpse of the full image, that often can tell - even if grainy - if something's wrong (forgot some layer, some lamp or modifier is turned off for render, etc...). Optionally this fast preview could be temporarily treated as a final render (postpro, and save in case of commandline), while Cycles proceeds with the render.

EDIT: developer.blender.org/rBf8b9f4e9bbc10675de9bbc3088f2841381e23f78 here's the commit that should allow for a feature like this. since EXR allows for meta-data information, "N" and "M" info on samples could be written there.

The concept could be also expanded picking ideas from here blenderartists.org/forum/showthread.php It would be interesting to have a final image “progressively” saved over itself every x samples for example. Could be expanded further for preliminary animations, to have quick access to rendered (noisy) frames and throw them in compositor/sequencer. Upped!