Anti-alias fringe issue

In relation to this discussion here is a specific bug report.

When rendering mixed transparency filters, color bleeding from 100% transparent areas in the input image occurs when anti-alias process kicks in.

The attached filter expose the problem:

1. New Image with background color ARGB = 0,0,0,0
2. Render filter
3. Inspect ellipse edges - there is black mixed into the edge region..

AFAIK, there is no way to get around this in Filter Forge alone.

Another perspective on the problem:
A filter sample output value of ARGB = 0,1,2,3 is a valid output, but there is no possible way to save that to a file, since FF will "blend" the result onto the preinitialized image background.

To verify this, use the attached filter, save the output to a format that supports alpha, reload it in Filter Forge and reset the alpha channel to 100% opaque using Set Alpha. You will now see the color from "New Image.."

Fringe Problem 1.ffxml

Quote
Another perspective on the problem: A filter sample output value of ARGB = 0,1,2,3 is a valid output, but there is no possible way to save that to a file, since FF will "blend" the result onto the preinitialized image background.

Second thoughts: I'm not so sure about the validity of that statement, however colors from zero alpha samples should never influence the outcome of an operation. Zero alpha means that the color is not visible.

I created a script that do anti-aliasing correctly, let me know if you're interested.