foreground
How much of the icon canvas the foreground may fill, as a ratio in 0 < s <= 1.
The mark keeps its aspect ratio, is centred, and fits inside a box that is this fraction of the canvas on each axis. Transparent padding inside your PNG counts as part of the image. The background always fills the whole canvas, and each platform's own mask is applied afterwards.
Unset, every renderer keeps the ratio it has always used, so existing icons do not move.
Throws
Gradle
at task time when not in 0.0 < s <= 1.0.