Single Pixel Border Artifact with Overlapping Masks?

Hi, I have some masks that I'm using to simply color a map using background nodes. The entire Fusion project is very complicated, but I'm dealing with what I think is a simple problem. One of the masks that is overlapping another is creating a noticeable "edge" where neither mask is being used, and it seems to be an extremely small edge that looks to be a single pixel. What might cause something like this?

Subpixel sampling is the most likely culprit.

Subpixel sampling is the most likely culprit.

I'm not sure what that means, can you elaborate? Google isn't offering up many results with respect to Fusion. How am I to address this issue?

What do you mean by ”overlapping” here, specifically? Do the masks literally overlay (one covers another) or is their border ”shared”, for example as if one mask was the inverse of another? This kind of fringe happens when you have ”shared” border as the contribution of both masks does not amount together to what you think it would. Two usual cases are 1) merging together two elements that share a border, which results in not fully solid alpha on the fringe area and 2) using masks for limiting some color grading etc operation where the fringe area does not create proper transition from one to another. Solution is to make one of the elements bigger so that there is actual overlap and then merge or mix using the mask of another element.

I don't know what your set up is, but if the masks are truly overlapping, you should not be seeing that, which only happens when you have masks close to each other and you are dealing with semi transparent pixel data, or if you have mismatch between alpha multiplication and divide somewhere in your comp.

I can reproduce it by simply putting one mask to the edge of the other, and make it go away by actually overlapping mask.

sshot-952.jpg (4.08 KiB) Viewed 421 times

sshot-953.jpg (4.48 KiB) Viewed 421 times

Or you can reproduce it by adding alpha divide node.

sshot-954.jpg (121.13 KiB) Viewed 421 times

So you are probably dealing with mismatched multiplication operations somewhere in your comp.

Transparency and Color - Alpha Multiplication in Resolve/Fusion

Understanding Premultiply, Unpremultiply and Alpha Divide ( Fusion 8 )

If the masks share a border, as described, usually the seam will go away if you try a different method of blending. I won't go down into the math behind it, but you might check to see if turning Alpha Gain to 0 in the Merge fixes it, or try a ChannelBooleans in Maximum mode.

Both options should prevent the seam, if that's what's actually going on. Of course, it's possible something else is to blame, but there's no way for us to know from a screenshot.

If you copy a relevant portion of your comp and paste it here between [code] [/code] tags, then someone can paste it into their own Fusion to take a look and give more useful advice.