The three things that go wrong after you save

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.

• 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.

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.