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.
Questions about the edge
- What is a matte?
- The flat colour that goes under the whole frame before the picture is composited on top of it. It is not a background image and it is not behind a subject you cut out. It is one colour, under everything, and it is what the transparent pixels become.
- Which matte removes the halo?
- The one nearest the colour the artwork was cut out against, which is what the sampled swatch works out for you. A rim appears when the old background colour in the part-covered pixels argues with the new one under them. Match the two and there is nothing left to argue about. The swatch is greyed out on files with no part-covered pixels, because then there is no old colour recorded anywhere.
- Should I leave the edge repair on?
- Leave it on for anything cut out of a coloured panel. It reads the band between three and ninety-seven percent coverage and replaces the colour there. On a photograph with no alpha the band is empty, so it runs over nothing and changes nothing.
- Does the repair change pixels that were fully opaque?
- No. It only touches the band, and a pixel at full coverage is outside it. Press Z and the magnifier shows the worst edge in the image at four hundred percent, with the repair off on the left and on on the right.
- My PNG is transparent but the tool says the swatch changes nothing.
- Then the file has no alpha channel and never did. A checkerboard in the program you exported from is that program drawing its own canvas, not a property of the file. The intake note under the picture says which colour type the PNG actually declares.
Read next
- Choosing a matte colourThe same mark on four mattes against four destinations.
- Why a PNG turns blackThe other end of the same decision, when the default goes the other way.
- The converterSame engine, swatch row folded, for a file you already know the answer for.