Seed-Nodes: Screen模式去黑底

Here's a classic compositing problem: you've got a fire explosion, a particle burst, or a light streak rendered on a pure black background, and you want to drop it over your scene. Chroma keying won't touch it - there's no green screen, just black. The standard answer in Photoshop is the Screen blend mode, which makes black disappear while keeping bright pixels. This node is that operation, distilled into a zero-parameter box: feed it black-background VFX and get back an RGBA image with the black turned into transparency.

How it works

The implementation follows a specific recipe, and the source code makes it easy to describe. The node splits the image into its R, G and B channels, and treats each channel as its own grayscale sprite where the channel value doubles as alpha. Then it composites the three - red first, then green, then blue - onto a black base using the real Screen formula:

result = 1 - (1 - base) × (1 - blend)

The alpha channel goes through the same Screen math, so brightness drives opacity: black pixels end up fully transparent, bright pixels stay opaque, and mid-tones get a natural soft falloff. That last bit is the whole reason this beats a hard threshold - you get a gradient of partial transparency instead of a crunchy edge.

Input is a single image; output is an RGBA image. There are genuinely no other parameters, which is either a blessing (nothing to mis-tune) or a limitation (no strength control) depending on how you look at it.

Where it shines

Sequence frames, particle effects, glow and light overlays, fire and explosion sprites - anything with a black background that you want to composite over a render. It's also surprisingly good for fixing the classic "my VAE decode put the fire on black" problem. One honest caveat: it's tuned for black-background bright effects. A dark-but-not-black image will come through with a murky, semi-transparent background that's worse than what you started with.

How to install it

Part of the ComfyUI-Seed-Nodes pack:

cd ComfyUI/custom_nodes
git clone https://github.com/Aerse/ComfyUI-Seed-Nodes
# restart ComfyUI

Or ComfyUI Manager → search "ComfyUI-Seed-Nodes". No models, no downloads, pure numpy math on the CPU. Note the README's clone URL is stale - the real repo is Aerse/ComfyUI-Seed-Nodes.

Common issues

• Output looks transparent in the preview - preview nodes bake alpha onto black, so a correctly-transparent result can look like nothing happened. Preview it over a background or save as PNG and check.
• Gray haze where black should be - your "black" background isn't actually black (compressed video footage, for instance). This node has no tolerance dial, so a slightly-off black stays slightly visible. Fix it upstream with a level/contrast node first.
• Not found in the menu - it's labeled "Seed-Nodes: Screen模式去黑底" (Chinese for "remove black background"); the pack needs a restart after install.

For anyone compositing effects, this is the node in the pack with a real job to do. It's niche, it's opinionated, and it does its one thing well.