Ok, I understand now. That's not a soft-edge between two projectors then.
That's what we call "Soft-Borders". In MM 2.5 beta you can choose a shader per surface. There's a "soft-borders" shader. But not on a mask. Mask have a different behavior.
You have two solutions for your problem: create a picture of the mask, with PNG with alpha channel, and use it on a quad, then you can do perspective.
Or you can make a quad, with a black picture (or testcard with front color and back color set to black), use the "soft-borders" shader, and use mesh warping (setting the "show input mesh" and "link input to output" and modifying the mesh in the UVs would do the trick), cf screenshot
- Screen Shot 2016-10-25 at 10.35.46.jpg (217.47 KiB) Viewed 1683 times