System/Image Filter/Type, how does it affect the render?

[SharpEars]

I tried setting it to None, Tent, Hann, and Blackmann-Harris and rendering very high contrast thick diagonal reflections of lights to see what these various anti-aliasing settings do. I get exactly the same results, even after hundreds of passes, as if this setting affects absolutely nothing. Highlight clamping was set to 4, but highlights were still blown out to show very bright and aliased highlights on a dark surface. Anti-aliasing was clearly visible in the resultant images which were rendered at 3840x2160 BTW, especially at 2x magnification of the pixels, but it was constant and independent of the filter type setting. It seems that the AA scheme chosen is fixed (to what?) and cannot be changed/affected by the Filter/Type setting.

Is this one of those completely useless legacy settings that did something at one point, but now does not effect the render at all or does this setting actually do something and if so, what?

[romullus]

Image filtering definitely is working, but don't expect that it will help you with pixels that has value >1. Adjust exposure and or highlight compression until pixels are <1 and then do your test.

[SharpEars]

Image filtering definitely is working, but don't expect that it will help you with pixels that has value >1. Adjust exposure and or highlight compression until pixels are <1 and then do your test.

OK, I tried what you said and once I got the output exposure levels to values less than one, the filter types did in fact have an effect on anti-aliasing. Here is the order that I came up with from least AA to most AA:

(None ?? Blackmann Harris) < Symmetric Tent < Hann < Tent < Parabolic < Box

This is a pictorial diagram that I created to help myself and others make correct future choices from the filter types (since they seem to be undocumented - I wish a diagram like this was in the official Corona manual). The image shown is of an octagonal light source (with minimal rounding of corners) reflecting in the center of a highly-reflective large sphere (there is very little to no spherical distortion of the light source present). I made sure to keep all output RGB values < 1.0 (approx. 0.97 max or so). You can click on the image to expand it and scroll it to see the detail along the edges of all modes:



In my opinion, Symmetric Tent is best for minimal AA (but still a reasonable amount) and Box is best for maximal AA, at the expense of some loss in high-contrast sharpness around edges. Blackmann-Harris was skipped, because it is overly biased towards anti-aliasing horizontal lines and doing next to nothing to vertical lines, to be useful for all scenes. It may be useful for those scenes that have large amounts of aliasing around horizontal edges, however (e.g., sky/land/horizon transitions).