The basic effect is mirroring across the seams. All the stuff that makes it look good is the hard part. 
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Are there any artifacts when a foreground object (like a character) moves in front of a blended seam and obstructs the pixels behind?
As with all screen space effect it’s not perfect. But the blend fades when occluded by a foreground character. It’s subtle, but there. A bit like other screen spaced effects like ss reflections or ao/gi. You don’t really notice it on smaller stuff like grass, props etc -
I’m assuming you’re using depth to blend and mask and effect, are there any artifacts if a transparent object or effect, like glass or smoke, passes in front of a seam?
The effect happens before transluceny. So water/glass/etc works fine. -
What’s the performance like, or how many ms is the post process adding to your test scenes?
Currently sits at around ~0.45 ms for a 2560x1400 resolution. -
Do the screenspace blend sizes scale based on depth so they don’t appear to change width as the camera moves near or far? I’m guessing this is what the “Size” and “MinSize” values are for, with “Size” being the largest a blend can be up close while “MinSize” is for objects that are farther from the camera?
The size scales on depth * screen size * FOV. So the size is consistent regardless of render resolution or fov.
MinSize is an artistic setting to keep the size going for x distance. At 2 it will keep the same size in screenspace 2x further away from it. Helps make small pebbles slightly blurry up close, but still a bit blurry at distance. -
You mentioned a complex actor needs to process each mesh, how heavy or slow is this process? Is it processing at runtime for actors within view or is it a pre-process baking step that needs to be run beforehand? I’m wondering about performance or build times for large levels or open worlds in a full production.
The activator is runtime, and works on a processing budget. The current default budget is set to 0.3 ms.
I’ve partnered with a game company that is using it. So it’s getting battle tested on a real open world AA game with stupid amounts of actors. Overall I feel the plugin is coming along nicely both in terms of performance and fidelity! 