What a halo is, and how it is taken out
A cutout made against a white page often keeps a one- or two-pixel ring of that page. Those pixels are part subject and part white, but the file marks them as more opaque than they are. Pulling the boundary in (halo shrink) hides the ring by removing it; rebuilding from colour repairs it, because the amount of white in each pixel says how much of it was really subject. The two are on the Edge panel and can be used together.
The alpha view opens first on this page, because that is where a halo is easiest to see: white is kept, black is gone, grey is part-way. The X-ray lens (L) magnifies it under the pointer.
- A halo of a busy background. If the cutout was made on a photo rather than a flat colour, there is no single colour to take out. Halo shrink and the take-away brush still work.
- Detail lost before. Strands of hair the first tool cut off are not in the file, and nothing here can bring them back.
Questions
Why does my cutout have a white outline on dark backgrounds?
It was made on a white background and its edge pixels kept part of that white, with too much opacity. 'Matte against: white' works out how much of each edge pixel is subject from how much white is in it, sets the transparency to match, and takes the white back out.
The halo is dark, not white.
Choose Black under 'Matte against' on the Edge panel. The page guesses white or black from the edge itself when the file opens, and says which it chose.
The fix ate into fine detail.
Lower the halo shrink, or paint the detail back with the Restore brush on the Refine panel: it paints the opened file's own transparency back under the brush and nowhere else.
What does the edge-tint number mean?
On the Save panel it is the average colour distance between part-way edge pixels and the solid subject next to them, measured in the saved file. A halo reads high; a clean edge reads low.
My file has no transparency.
Then there is nothing to repair, and the page says so and cuts it with the guided selection instead.
Related
- Make a picture transparentThe same studio, with the file-format question answered on the way out.
- Remove white backgroundA colour key on white with exact unmixing, so anti-aliased edges become soft transparency instead of a grey ring.
- Where colour fringe comes fromWhy an edge keeps the old background's colour, how unmixing takes it back out, and how to read the edge-tint number.
- Cut out an image by handPolygon, freehand lasso or a magnetic edge that snaps to the photo, then the edge band finds the real boundary.
