How to properly mask from a movie clip

Thanks in advance to anyone who can help me with this issue, I have done a lot of reading but can’t find anything that combines all of the factors together with what I am trying to do.

I have a movie clip that I have tracked and masked a part of, as well as a smoke simulation. My goal is to have the smoke look like it is emitting from the palm of the person in the movie clip.


I masked around their thumb in the hopes that the thumb could look like it is appearing in front of the smoke, with the rest of it coming from the palm behind it. This photo is a scaled down simulation where the smoke isn’t as prevalent, in my ideal scene there would be much more smoke but when it gets close to the thumb the effect obviously doesn’t work unless it goes BEHIND the thumb and likely the index finger as well.

No matter what I try the thumb is either behind the smoke, pure white mask, absent, too dark, or not blending in front of the smoke. I attached the node setup below.

I am new to masking but I thought if I expanded the feather greatly near the part of the thumb closest to the palm, that it could be partially over the smoke and the other part blended better with the smoke. I am not sure if this is the correct approach even if I could get the plan to work. Any help with this stuff is greatly appreciated, have a good day.

LOL what’s that difference node for? The difference between two identical image buffers is always 0. That’s a fancy way of generating the color black. Just Invert the mask and plug it into the “Fac” of the alpha over node.

P.S. If you want a little smoke to pass in front of the thumb, use a “map range” node to make the mask partly grey.

Thank you so much. Those ideas worked, yet when I went to include the index finger into the mask, the smoke still overlays that finger? Images below.

Also what do the map range values adjust theres so many Max and min factor. Thanks again.

I’m glad it’s working for you but I’m looking at this node setup and frankly I have no idea how that even works at all. The values you entered into the Map Range node should only yield an output of 0.0 (another fancy way of generating the color black), and even then you still have it plugged into the wrong input. But you’re saying it works?

I shouldn’t say “works” I should have said it’s having some effect.

I don’t comprehend how it works and reading it on the blender site didn’t help as much.

I wish there was a way to just stack the mask on top of everything like photoshop and after effects.

If you have time I’d appreciate it if you know the way for me to correctly set this up.

I understand now. This will help:

  1. Try these map range values: From_min = 1 and From_Max = 0. This is one way to invert the mask. Then do To_Min = 0 and To_Max = 1. Start with that and then gradually try turning up the To_Min value so just a little smoke will show through the thumb.

  2. After the Mask goes through the Map Range node, it should be plugged into the Factor input of the OTHER Alpha Over node (the one that combines the smoke with the movie clip). That input determines how much or how little the smoke gets put onto the video, so that’s where the mask needs to go.

  3. Delete the lower Alpha Over node. There is no point in alpha-overing a thing with itself.

  4. You should verify that the vector blur node is actually making a difference. If not, I think that could be deleted as well.

If the above does not work, check the output of the Map Range node and remember that whatever is white will cause the smoke to be visible, and whatever is black will hide it.

That’s it! If you’ve already done the work of masking, you’re very close to being finished.

Thank you so much! Finally I have the masks working per your advice.

Yet now my smoke domain is rendering with a weird haze cube even though the smoke domain is much larger than that, and the scale is applied. Jeez it is always something, I cannot wait to have this program somewhat understood.

I’m afraid I don’t know much about smoke sim, but I thought that the domain was one thing that you should NOT apply the scale on. I could be wrong but maybe try using a cube and only scaling it in object mode

thank you again, will look into it!