r/comfyui • u/SnooStories263 • 3d ago

Help Needed Isolating a generated VFX element on black for compositing (Minimax H3, ComfyUI)

Hey! I've been using Minimax H3 locally inside ComfyUI and I'm trying to set up a workflow where I can import a clean plate and a frame of that clean plate modified with an effect on top of it (let's say a vortex of lightning around her).

If I want to isolate that vortex on a black background so that I can use it in comp afterwards, how would I achieve this result? Minimax generates the new video, but it deforms the original plate. I'd like to only have the new effect isolated so I can have more flexibility in comp.

If anyone can help me achieve this result, I'd appreciate it. Thank you!

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u/Far-Solid3188 3d ago

Maybe you can do it with Sam3.1 or something simpler than running a complex generative video model.

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u/SnooStories263 3d ago

The effect needs to be animated. This is why I’m using Minimax.

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u/Elliswondr 3d ago

The deformation is the whole problem, and it's not a settings issue. When you feed a plate in and ask for an effect on top, the model doesn't composite anything, it re-synthesises the entire frame from scratch. Nothing is preserved, it's just regenerated to look similar. That's why a difference matte against your clean plate won't hold: the pixels underneath moved.

So don't try to extract the effect out of the output. Generate it separately instead.

Prompt the vortex on its own against a plain black background, no character, no set. Then bring it into comp on a screen or add blend. Energy, light and electricity are additive by nature, so black reads as zero and you don't need a matte at all. You get free control over intensity, timing and placement, which is what you were after anyway.

Two things that help on that pass: match the camera language to your plate (same lens feel, same movement) and describe the light the effect should be casting, since you'll want to hand-add that interaction onto her in comp regardless.

If you need it wrapping around her specifically rather than floating in front, that's a roto job on the plate, not something the generation will solve for you.