Removing Black Background from stock footage using add/screen blending mode not working

[Chantal Y.]

Hello, 

I'm trying to remove the black background from this particle stock footage using the blend mode add or screen. I tried everything precomposing, using the 2 blending modes and it's not working... In other projects the blending mode works fine on these footage. But the problem is just with this one... 

Here's a screenshot of my timeline

 

[Rick Gerard]

Add, and Screen do not produce transparency; they only blend the pixels on the layer with values higher than zero with the pixels on the layers below. You need a layer below your stock footage before blend modes will do anything. Layers 8 and 9 are both set to add. There is nothing below them, so there are no pixels to blend with. I assume from what I see in the screenshot that both layers are stock footage of particles. If you turn off layer 8, the image will change because the pixels with a greater value than 1 on layer 8 are blended with the pixels in layer 9. Put a dark red solid below layer 9; you should see white particles on a red background. I don't think there is anything wrong.

If you want transparency, you need to create a procedural matte. You might try Set Channels and set the Alpha to use Luminance. You might need to apply a Curves effect above the Set Channels to the layer to increase the contrast of the layer to get the transparency to work the way you want it to work.

[chanlr]

Thank you so much, Rick! This really helped me a lot. 

[Mylenium]

Agree with Rick. There appears to be no layer underneath to actually blend with, so this method won't work. The comp background doesn't count since it's treated differently. Using a Shift Channels effect to convert the luminance to transparency would be the quickest way to resolve this.

Mylenium 

[ShiveringCactus]

If you're not sure how to use Shift Channels for the best results, if you search Effects & Presets for "Unmult" you'll find a preset there which should do this for you.

Alternatively, you could use a Luma Track Matte pointed at a copy of the stock footage.