Remove the background from an image

The model does not draw an outline. It looks at a reduced copy of your photo and gives every pixel a value between zero and one: how much of that pixel belongs to the subject. Solid areas come out as all or nothing, and the edge comes out as a soft gradient. A second step then compares that gradient with the photo at higher resolution and pulls it onto the edges that are really there, which is what rescues strands of hair and fur. The finished mask is scaled to the full picture, so the result has the same width and height as your original.

The mask is kept while you work. Changing the colour, the gradient or the feather only repaints the background, so each new version appears at once without the model running again.

Three things decide the quality of the edge before the model has seen anything. Contrast: a dark jacket against a dark doorway gives it nothing to separate. Focus: a sharp subject has a definite edge, a motion-blurred one does not. Framing: a subject that is whole and well inside the picture is recognised more reliably than one cut off on three sides.

If you can retake the photo, stand the subject a step or two in front of a plain wall in even light. If you cannot, expect to look hardest at hair, at fingers, and at the gaps between an arm and the body.