Feather Edges

border fade or soften existing alpha · rect / ellipse · batch

Two related jobs. Border fade takes a rectangular image and makes its outer edge fade to transparent, so it can sit on any background without a visible boundary — the vignette-into-nothing look. Softening existing transparency blurs the alpha channel of an image that is already cut out, which is the fix for a hard, obviously-selected edge.

Border fade: rectangle or ellipse

Rectangle measures each pixel's distance to the nearest of the four edges and fades anything within the radius. The corners get a soft rounded look automatically because they are close to two edges at once. Ellipse instead fades based on distance from the centre relative to an inscribed ellipse, which gives the classic oval photographic vignette — the corners go entirely, not just softly.

The smooth curve uses a smoothstep so the fade starts and ends gently; linear is a straight ramp, which has a faint visible boundary where it begins. Smooth is almost always the better choice — linear is there for when you want the transition itself to be detectable, such as a deliberate graduated mask.

Softening an existing cut-out

In alpha mode the radius is a blur applied to the alpha channel only, which turns a one-pixel hard edge into a gradient several pixels wide. That is enough to stop a cut-out object looking pasted. Two or three pixels is right for a normal composite; more than about eight starts to look like the object is out of focus.

Blurring alpha alone grows the shape slightly, because half of the new gradient extends outward into what was transparent — and that outward half is filled with whatever colour was hiding behind the transparency, often a fringe of the original background. Shrinking first pulls the alpha edge inward by a few pixels before the blur, so the softened edge ends up sitting where the hard edge was and the fringe is cut off. If a cut-out develops a pale halo after feathering, a shrink of two or three is the fix.

FAQ