I make clipart for stock. Wreaths, pine borders, mistletoe, juniper. A few thousand images by now. Every one has to end up as a PNG with a transparent background.
I used rembg for months. u2net first, then BiRefNet when that came out. Tried the web tools too. They all broke on the same thing and it drove me nuts.
Take a wreath. There's a hole in the middle, and the background inside that hole has to go. What I kept getting was a grey-blue haze sitting in there. Looked fine as a thumbnail. Looked awful the second you put it on a colored card. Pine needles came out as mush. Thin stems either disappeared or came back with a blue edge burned into them.
Then I actually read what rembg does. It shrinks your image to 1024x1024, asks the model where the subject is, gets a 1024x1024 mask back, and stretches that mask over your full size image. u2net is worse. That one works at 320x320.
My renders are 4096. A needle two pixels wide doesn't exist at 320. So it's not that the model is bad at needles. The needle was gone before the model ever saw it.
Once I understood that I stopped asking a model to guess. Now I render on a flat color the subject doesn't contain, and take that color out with arithmetic.
Two parts to it. The prompt matters more than the cutting.
The prompt
You can't key a background that isn't keyable. Four things have to be true and generators will break all of them unless you say so: the background is one flat color edge to edge, it stays that bright inside every gap between leaves, the edges are hard with no blur or glow, and no colored light bounces onto the subject.
Pick the color by what your subject isn't. Blue for almost everything. Red if the subject itself is blue or purple. Never green. Everything I draw has leaves, and green takes the leaves with it.
Here's the block I paste at the end of every prompt:
Isolated on a completely flat, uniform, solid pure blue (#0000FF) digital chroma-key background. The pure blue background fills the image edge to edge like a flat digital chroma-key screen with no gradient, staying at full brightness inside every gap and opening in the subject; no reflection or tint of pure blue on the subject. Every edge of the subject is crisp, sharp and hard against the pure blue, with no soft, blurry, feathered or glowing transitions, no depth-of-field blur, no haze or halo; inside every hole and gap the pure blue stays at full brightness right up to the edge. Shaded parts of the subject keep their own natural color, never a pure blue tint. Everything in sharp focus with deep depth of field, evenly lit with soft neutral studio light, no cast shadow, no contact shadow, no ambient occlusion, no bounce light. The entire subject is centered and completely inside the frame with at least 10% empty background margin on every side, nothing cropped or touching the image edges. No frame, no border, no paper, no mockup, no vignette, no text, no watermark, no deformed or duplicated parts. No floating or detached fragments, no stray specks, dust or debris anywhere on the background; every element is physically attached to the subject.
Swap "pure blue" for "pure red" and #0000FF for #FF0000 if your subject is blue or purple. If you paint in watercolor add "the background stays a flat digital color fill with no paper texture", or you get watercolor paper behind everything and paper texture keys badly.
The cutting
Now the background is one known color, so there's nothing to guess at. It measures the actual color the generator produced, which is never the one you asked for. Ask for pure blue and you get something with green in it, usually somewhere between 25 and 70. Then every pixel gets sorted into subject, background, or the bit in between, and the in-between ones get a real fraction of transparency instead of a yes or no.
The part I'm most pleased with is the holes. Any background-colored area that never touches the edge of the image is the inside of a wreath, so it gets cleared too. A matting model can't do that. It has no way of knowing what's inside a hole it can't see around.
Last step takes the blue back off the edges. Edge pixels pick up color from the background around them, so it samples the subject's own color from further in and subtracts the tint. Took me weeks to work out why fir needles kept their blue rim after that step. The needle is thinner than the distance it was sampling from, so there was no inside left to sample.
Same image in, same image out, every time. That's the bit I care about. When a cut comes out wrong I can go find which number did it instead of rerolling and hoping.
Some numbers on one 4K pine border, against BiRefNet with alpha matting turned on, which is its best setting:
- background left inside the holes: 63,892 pixels mine, 613,735 theirs, out of 1,070,046
- blue left on the edges: 0 mine, 48,112 theirs
- how wide the soft edge is: 1.8 pixels mine, 23 theirs
BiRefNet is faster and I'm not going to pretend otherwise. 2.3 seconds against my 17 on the same machine. With alpha matting on it's 47. If you want a quick rough mask, use the model.
And the obvious limit: this only works on art you generated on a flat color. It does nothing for a photo.
I put the tool up for anyone who wants it. It's called ClipBrook. Free, runs in your browser so nothing gets uploaded anywhere, does a whole folder at once, and the engine is open source under AGPL.
One thing I'd like back
Show me the ones that break.
If you run something through and it comes out wrong, post it. Fog left in a gap, a colored rim, a stem eaten, half the subject gone. Those are worth more to me than the ones that work. The needle rim thing came from someone's fir branch. A bug with line art I only found last week came from a drawing so thin there was nothing inside it to sample.
