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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.

enter image description here

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

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    $\begingroup$ Can you specify what "video layers" are you referring to? Are they planes with animated video textures in the 3D environment, or VSE strips, one of which is smaller, overimposed and moving (and how)? $\endgroup$ – m.ardito Dec 11 '15 at 11:29
  • $\begingroup$ Ok, thanks for the comment, I edited my question. I'm not sure this is possible, and I'm still between beginner to medium level in Blender, but smoke billows and curls much better in Blender than any other program I've tried, so I'm hoping this can be done... $\endgroup$ – MicroMachine Dec 11 '15 at 11:34
  • $\begingroup$ ok, I see the edit. If they are "images as planes", then, you can make the "planes" to emit particles, or maybe use another (parented invisible) object to emit particles following the plane motion, perhaps? $\endgroup$ – m.ardito Dec 11 '15 at 14:30
  • $\begingroup$ When you say that particles should come from the edge, is that the frame borders or the ball borders? $\endgroup$ – lemon Apr 19 '17 at 11:42
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    $\begingroup$ Another question (just for the principle): do you need the effect comes from edges or from the whole surface? $\endgroup$ – lemon Apr 21 '17 at 18:15
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enter image description here

I've failed to use a movie as direct texture density input for both particle system and smoke simulation. It seems that this information is still right or simply my setup is wrong somewhere.

So, as a workaround, I used the following modifiers, starting from a subdivided plane:

  • Vertex weight edit: this allow to color a vertex group depending on a texture (and this allows to use an animated texture or video)
  • Mask: the mask of the previously create vertex group keeps vertices of the mesh depending on the vertex group. We could use the vertex group directly but keeping vertices allows to round the geometry in the next modifier
  • Subdivision surface: to round a bit the remaining vertices corners
  • Smoke simulation

Vertex weight edit

enter image description here

It is based on the movie texture and adjusted (default weight, add threshold) so that the plane's vertex group are assigned depending on the alpha channel of the texture. The vertex group has to be created before with no vertex assigned to it.

The texture itself is defined as follow:

enter image description here

This first modifier give this result (weight paint mode):

enter image description here

Mask

enter image description here

The mask simply uses the previously defined vertex group, so that the plane geometry is restrained to this:

enter image description here

Subdivision surface

Not really useful, depending on the base subdivisions given to the plane. It rounds a bit the corners of the remaining geometry.

Smoke

  • Smoke flow:

Nearly the default settings, except for the smoke color:

enter image description here

  • Smoke domain:

A cube surrounding the previous plane. The resolution has been adjusted using 'smoke high res' and 3 divisions (if you don't do that, the smoke will appear crenelated). The dissolve/life time is set to 5 here.

enter image description here

Note

From the same base setting we could use a particle system, producing itself the smoke. But when there is no geometry (full alpha image), it seems that particles want to appear anyway from the plane origin. So if you want a setting using a PS, place the origin outside of the camera view.

Blend file

You may have to bake the smoke before usage.

Additional note

MicroMachine indicated in the comments that it was not an obligation that the flow comes from the circle edges and could be from the whole surface. Nevertheless I've tried to do it using 2 vertex weight edit modifiers: the idea was to create a kind of surrounding mask from the circle. But I failed to do it correctly.

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  • $\begingroup$ This looks really good, thanks for the efforts! $\endgroup$ – MicroMachine Apr 24 '17 at 0:12
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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.

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  • $\begingroup$ Hi @Paul B., thanks for the answer and sorry for the delay. Could you add a screenshot of a suggestion of nodes? I'll try this soon. Thanks! $\endgroup$ – MicroMachine Jul 13 '16 at 21:43
  • $\begingroup$ This answer doesn't respond to the OP's need for smoke emission from the keyed edge. $\endgroup$ – 3pointedit Apr 19 '17 at 22:34
  • $\begingroup$ Tested it and that does not work (or you should explain a bit more). But for now, I think this issue is still true developer.blender.org/T34063 $\endgroup$ – lemon Apr 20 '17 at 10:55

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