Particles emitted from video layer I am trying to compose a shot that combines video layers and 3D.

In the shot, there are two video layers, one is the background and one is an alpha layer of a ghost floating around.



They are both .mp4 / H.264 video layers shot with a DSLR, one of them made transparent with an alpha layer. They would both be 1920x1080.

They could be imported in Blender as "planes as images" but any other way that works is good.

I am trying to make the Ghost layer emit particles, from the border of the object (not the edge of the frame) resembling green smoke or steam. The particles should come from the edge (all around) of the ghost layer.
I looked into doing this in other software (After Effects with Particular), but haven't found a way to get the results I'm looking for, which have more to do with curls of smoke than particles.

Here is an alpha video with a ball instead of a ghost you can use to help solve this question.
And here is a .blend file where the video was imported already
The basic setup you are asking for is quite simple. You could just arrange the planes accordingly in 3d space (although I would render on a transparent background and later composite your 3D particles + ghost layer on top of your plate).

The part with only emitting particles from visible parts of the ghost layer could be resolved using a particle texture (just feed the transparent video into the particle system using the "Textures" panel under the emitter/particle system settings; a ramp could be used on the texture for additional transparency control).

On the behalf of the smoke I would try to utilize a low-res smoke simulation (source of which the particle system would be). That generally tends to look better than just faking it using particles.