How background removal works, and which mode you actually need
"Remove the background" describes two genuinely different problems, and most tools quietly solve only one of them. Cutting a logo out of a white rectangle is a matter of arithmetic. Separating a person's windblown hair from a park behind them is a matter of judgement. This page offers a mode for each, and picking the right one is most of the battle.
Fast mode: what "uniform background" really means
Fast mode starts at the outer edge of your image and floods inward, marking every pixel that is close enough in colour to where it started, and stopping when the colour changes sharply. That is why it is instantaneous and needs nothing downloaded — it is comparing numbers, not recognising objects.
It works beautifully on a logo on white, a screenshot, a scanned signature, a product shot taken against a seamless backdrop — and, because each step compares a pixel with its neighbour rather than with the starting point, on smooth gradients too: a sky, a wall with a shadow falling across it, a studio backdrop with a vignette. It has two failure modes worth knowing. First, it only removes background connected to the edge: the enclosed gap inside the letter "O" or between an arm and a torso stays filled, because the flood never reaches it. Second, anything in your subject that matches the background survives or vanishes with it — a white shirt on a white background will be eaten through. The tolerance slider is the dial between those two failures: too low leaves a halo of background, too high starts eating the subject.
The two modes answer different questions
This is the distinction that should decide which one you pick, and it is not "easy background" versus "hard background". Take a photo of a bird perched on a branch against the sky. Fast mode removes the sky and leaves the branch: it follows colour spreading in from the edge, and the branch is a different colour, so it stays. AI mode removes the sky and the branch: it never looks for background at all — it looks for the subject, decides the bird is it, and discards everything else.
So the question is not which one is more accurate. It is whether you are asking "take the background out" or "give me only the subject". A product photographed on a table: fast mode keeps the table, AI mode hands you the product alone. Neither is wrong, and picking the wrong one is the most common way to end up with a cut-out missing something you wanted. Technically, AI mode runs a segmentation model — a neural network that decides pixel by pixel what belongs to the subject — which is what lets it cope with strands of hair, fur, the gap between fingers or a translucent edge.
The cost is real and worth stating plainly. The model and its runtime are roughly 50 MB, downloaded from a third-party CDN the first time you use the mode, then kept in your browser cache. On a fast connection that is a few seconds; on mobile data it is a meaningful chunk of an allowance. Processing then takes a few seconds per image and is noticeably slower on older phones. Everything after the download happens locally — the model comes to your image, not the other way round.
Why the result is always a PNG
Transparency needs an alpha channel, and JPG does not have one. Save a cut-out as JPG and the transparent area collapses to a solid colour, which defeats the entire exercise. Both modes therefore output PNG, and the preview shows a chequerboard behind each result so you can see the transparency rather than guess at it. If you need a smaller file for the web afterwards, convert the PNG to WebP — it keeps the alpha channel.
Getting a clean cut
If you control how the photo is taken, you control most of the result. Put the subject well clear of the background so the two are not lit alike; avoid a backdrop that shares colours with the subject; keep the lighting even to stop shadows creeping across the backdrop; and shoot at the highest resolution you can, because a cut edge that looks rough at 500 pixels wide looks fine at 2000. For fast mode specifically, a plain sheet of paper or a wall is worth more than any slider.