Cut Out a Subject and Keep the Edge Honest

How it works

Your image is first resampled up to about four megapixels — deliberately larger than most pages on this site use — and then passed to a segmentation model that produces a matte of the subject. That matte is inverted into an alpha channel and attached to the image, and the result is written as a PNG with real transparency.

There is no diffusion model and no sampler anywhere in the chain, so nothing in your picture is redrawn. The pixels in the output are the pixels you uploaded, resampled, with an opacity value attached to each one.

The problem lives at the boundary

Consider what a pixel at the edge of a subject actually contains.

A camera sensor integrates all the light that falls on one photosite. If half of that area was hair and half was the wall behind it, the recorded value is a mixture of the two. There is no arrangement of a single number that separates them again — the pixel is genuinely both.

For a rigid object this affects a thin line one or two pixels wide and nobody notices. For hair, fur, feathers, lace, smoke, or a glass edge it affects a band that can be dozens of pixels deep and contains most of what makes the subject look like itself.

The honest representation is therefore not a cutout at all. It is a matte with intermediate values, where a pixel that was sixty percent hair is sixty percent opaque. That is what this page produces, and it is why the result composites smoothly onto a new background instead of arriving with a cut edge.

Why the resolution is set high

Since the difficulty is that detail is finer than the pixel grid, the most effective intervention available is to make the grid finer.

Resampling does not add information about the scene. What it does is give the segmentation model more samples across the transition region, so the matte it produces has more room to express a gradual change from subject to background. The difference between a mask that steps from one to zero over two pixels and one that ramps over eight is very visible on hair and almost invisible on a bottle.

The cost is time and memory, which is why not every page here works this way. For a cutout it is worth it, because the edge is the deliverable.

Checking it properly

The universal mistake is checking a cutout against white.

Almost every fault this job produces — a fringe of the old background, a halo, an over-eroded edge that has eaten a millimeter of shoulder — is low contrast against white and therefore invisible. You approve it, place it on a dark brand color a week later, and the outline appears.

Open the PNG on something that clashes: if the original background was pale, view it on near-black; if it was a blue sky, view it on orange. Look along the whole silhouette, and pay particular attention to hair, to the gap between an arm and a body, and to anything transparent or reflective.

Thirty seconds there saves the version of this that gets noticed by somebody else.

Publication gate

This page ships once the workflow has been run against a rigid product on a plain surface, a portrait with flyaway hair against a bright window, a subject holding something with a gap through it, and a glass object, with every output composited over black, white, and a saturated color.