A selection is a number per pixel, not a line
The marching-ants outline lies to you. It draws a boundary, but underneath, a selection is exactly what a mask is: a value from 0 to 255 for every pixel saying how selected it is. That is why selections and masks convert into each other freely, and why "the selection is off by one pixel" is usually the wrong diagnosis.
Choose the tool by the kind of edge you are dealing with.
The four tools and when each is right
Magic Wand — click a pixel, and it takes every connected pixel within a colour tolerance of it. Tolerance 32 by default; raise it for a gradient sky, lower it for a busy one. Only sane on flat, even backgrounds. Turn *Contiguous* off and it selects that colour everywhere in the frame.
Quick Selection / Object Selection — you drag, and the tool grows the selection outward, stopping where it detects an edge. Object Selection and *Select Subject* go further and use a trained segmentation model to find a person or product in one click. This is the everyday tool and it is genuinely good. Photopea has a quick-selection tool and an automatic subject cut-out; GIMP has the *Foreground Select* tool, which works on the same rough-then-refine principle and takes longer to get to the same place.
Colour Range (*Select > Colour Range*) — samples a colour and selects by distance from it, with a Fuzziness slider and a live black-and-white preview. Better than the wand because it is not limited to a connected region and gives you partial selection rather than in-or-out. It is the right tool for a red dress across a whole frame, or for a sky through tree branches.
The Pen tool — you draw a vector path by hand, then convert it to a selection. Slow, and unbeatable for manufactured objects with hard edges: a bottle, a phone, a shoe, a box. Because it is a mathematical curve rather than a pixel guess, the edge is smooth at any zoom, and every e-commerce cut-out you have seen that looks expensive was done this way. Inkscape logic applies here too — see vector and print. Photopea and GIMP both have a pen/paths tool with the same behaviour and roughly the same shortcuts.
Two habits worth building: hold Shift to add to a selection and Alt/Option to subtract, and select the *simpler* region then invert. Selecting a plain wall and inverting is far quicker than tracing a person.
Hair, and why refine-edge exists
Here is where beginners lose an afternoon.
At the edge of hair, a single pixel is not hair and it is not background. The lens and sensor averaged both into one value. That pixel is genuinely, physically, 40 per cent hair and 60 per cent wall. No boundary exists to find, so any hard selection is a lie in one direction or the other.
The answer is partial alpha, and that is what Select and Mask (Photoshop; earlier called Refine Edge) is for. The important control is the Refine Edge brush — you scrub roughly over the transition zone and the tool estimates, per pixel, what fraction of it belongs to the subject.
The second control is the one people miss: Decontaminate Colours. Consider a person photographed against a green wall. The semi-transparent hair pixels literally contain green — the wall's colour is baked into their values. Place them on white and you get a green fringe. Contracting the selection does not fix this; it just eats the hair. Feathering does not fix it either; it smears the green. Decontaminate Colours rewrites the colour of those partial pixels to match the subject while keeping their transparency. It is the only control that addresses the actual cause.
This is also why professionals shoot cut-outs against grey or white rather than a colour, and why a green screen is used for video and rarely for stills.
The free paths, honestly
- Photopea's automatic cut-out handles most product and person shots to a standard that passes on a marketplace listing. Its hair refinement is weaker than Photoshop's.
- remove.bg and similar web services do one-click background removal. The free tier gives you a small image; the full-resolution file is paid. Also: you are uploading a client's photo to somebody's server, which is a decision, not a convenience.
- rembg is the open-source version of the same idea. It runs on your own machine, on the CPU if you have no GPU, and it costs nothing per image. Slower, private, unlimited.
- On a phone, most recent Android and iOS galleries let you long-press a subject and lift it out. The alpha is rough at hair but perfectly good for a social post.
- GIMP with Foreground Select plus hand-painting on the mask will get you anywhere Photoshop will. It will take three times as long on hair.
How to spot a bad cut-out
Look for these before delivering, at 100 per cent zoom:
- A pale or dark halo tracing the whole outline — the selection included a rim of the old background.
- An edge that is sharper than the rest of the photo. If the subject was shot at f/2 and slightly soft, a razor edge reads as fake instantly.
- No contact shadow. An object with a perfect edge and nothing under it floats. Keep the real shadow or rebuild one.
- Hair that has turned into a helmet — the fine strands were all sacrificed to get a clean line.
Today: cut one thing out with Quick Selection and the same thing with the Pen tool. Time both. You will know from then on which job deserves which.
Before you move on