The swatch row is open and the edge repair is on. Change the colour and watch the rim.
Why this page is different from the front one
Three controls start in a different place. The picker is open rather than folded away, the edge repair is on before the first preview is drawn, and the warning about transparency being permanent is printed whether or not the file turns out to have much of it. Everything else is the same converter, and the JPG it writes is byte for byte the one the front page would write from the same settings.
The reason for a separate route is that people arrive here already knowing the problem. They have converted the file once, opened it, and found a white box where a logo used to float, or a dark rim that was not there before. That is not a question about how to convert a file. It is a question about one decision inside the conversion, and the controls for that decision should not be behind a disclosure.
Reading a rim
- A pale rim on a dark matte, or a dark rim on a pale one, usually means the artwork was cut out against the opposite. The sampled swatch proposes the colour it was cut out against; try that first.
- A rim that is grey on every matte, and gets worse the lighter the matte, usually means the edge pixels were written already multiplied by their own coverage. The repair detects that case and divides it back out rather than borrowing.
- A rim only along one side of a shape is a drop shadow, not a fringe. A shadow is real transparency doing its job, and flattening it onto the wrong colour turns it into a smudge. There is no repair for that; there is a matte that matches the destination.
- No rim at any magnification means the edges are hard. Palette PNGs are usually this way: transparency is on or off per pixel, with nothing in between for a matte to show through.
What the magnifier is showing
The tile it picks is measured, not chosen: the square with the most part-covered pixels in the whole image. At four hundred percent, with smoothing off, each pixel is a block of sixteen and a two-pixel band becomes eight blocks wide. At one hundred percent a repaired edge and an unrepaired one are the same picture, which is exactly why a before and after at one hundred percent proves nothing.
Underneath the two tiles is a sentence saying what was found and what was done with it. That sentence is the tile's equivalent rather than its caption: a magnified crop is of no use to somebody using a screen reader, and the fact it carries has to be available in words or it is not available.
