Question:
Does AC pixellate at 1280x720 and 1920x1080? If it doesn't, I would suggest that perhaps it's got something to do with the way the engine scales resolutions.
No those resolutions do not have the problem, and that's the theory I put forth in my post at the top of the page.
As far as I can tell, any resolution that's a perfect 16:9 ratio will be ok. Both of those resolutions are exact 16:9.
I believe I have tracked down exactly when pixlization issue appears - it's occurs when the height of corresponding 16:9 resolution for whatever the width of the actual reoslution your using is odd.
E.g. the 16:9 resolution for the width 1680 is 1680x945. The height is an odd number. I'm betting the problem is the result of having a viewport that's not evenly visible by 2. Video cards don't like things that aren't multiples of 2.
All of the resolutions that Tanuki tested (1680x1050, 1360x768, 848x480, 720x576, and 720x480) that have this issue all have odd number heights in 16:9 - 1680x945, 1360x765, 720x477, 720x405 respectively.
1440x900 works because the 16:9 resolution for that width, 1440x810, has en even height - same with all of the other resolutions that don't have the issue.
How do you figure out the 16:9 resolution for your actual resolution? It's really simple. (X * 9) / 16 = Y Where X is the width value of whatever resolution your using and Y is the resultant the height for the the 16:9 resolution.