Written explanations
The three things that go wrong after you save
The tool explains itself as you use it. These do not: they are for the moment afterwards, when the PNG is on your disk and something about it is not what you expected.
Why any of this is written down
Almost nobody arrives here wanting to understand alpha channels. They want a shoe on transparency, or a person without the kitchen behind them, and the tool gives them that in about four decisions. The trouble starts one step later — in the slide deck, the marketplace listing, the email attachment — where a file that looked perfect on the checkerboard turns up boxed in white, or fringed in the green of a hedge that is no longer in the picture.
Every one of those moments has a mechanical cause, and none of them is a bug you can report. They are consequences of how transparency is stored, how a boundary is estimated and what a scanner does to a sheet of paper. A tooltip cannot hold that; a thousand words can. So the three pieces below take one cause each, say what is physically happening, and finish with the settings or the habits that get you a better result next time.
The three
Why a transparent PNG can still arrive white
You saved it, you opened it, and there is a white rectangle around your dog. Nothing is broken — something downstream of this site decided what to paint behind the fourth channel, and this is the list of which programs keep it, which flatten it on arrival, and which destroy it outright.
Read this one first if the file looks fine here and wrong everywhere else.
Hair, fur and every other soft edge
A boundary is not a line. It is a strip a few pixels wide in which every pixel is partly subject and partly whatever stood behind it, and the whole argument about cutout quality happens inside that strip. What widening it buys, what it drags along with it, and the four kinds of edge no number on this page rescues.
Read this one if the outline is right but the strands are gone, or present and tinted.
Getting a signature off a scan
Ink on paper is not an object standing in front of a background, and a subject model looks for the wrong thing entirely — it finds the sheet, keeps the sheet, and hands back a rectangle. The measurement that works instead is against the paper itself, and it is six steps rather than one.
Read this one before scanning anything, because half of it is about the scan.
Where colour fringe comes from
A green rim round a portrait shot in a garden is not a mistake in the outline. Those pixels really were part lawn, and the file still carries the lawn's share of the colour. This is the arithmetic of a mixed pixel, how unmixing takes the old background back out, and what the edge-tint number on the Save panel measures.
Read this one if the edge is in the right place and the wrong colour.
Exam and ID photo and signature sizes
Forms reject a signature two kilobytes too heavy or a photo ten pixels too wide. The IBPS, SSC, UPSC, PAN and passport figures in one table, each with the source it came from and the day it was checked — and why the notification in front of you overrules all of them.
Read this one before an upload with a size limit.
A white background marketplaces accept
Nearly white is not white. A product photo on 250, 250, 250 fails a check that wants 255, 255, 255, and JPEG compression can move a pure background off pure. How the saved file is decoded and counted here, how much of the frame a product should fill, and what is only common practice.
Read this one before listing products on a marketplace.
What you will not find in here
No comparisons, no rankings and no claims about how many seconds anything takes. There is also nothing about the operations this site deliberately does not perform — putting a new scene behind the subject you extracted, painting an unwanted object out of the middle of a frame, trimming to passport dimensions, squeezing a file under a kilobyte cap. Those are real jobs and they are somebody’s, but writing about them here would be writing about software you cannot use on this page.
What you will find is the failure cases stated plainly, including the ones with no fix: brown hair against a brown bookcase, a wine glass that comes back opaque, lace whose holes fill in, a blurred arm with no edge to find. A person who knows where a method stops working gets a better result than a person who finds out halfway through.
When you are ready to cut something, start on the front page, and the three preset routes in the bar above start it from a different position rather than doing a different thing.