10-05-2019, 01:47 PM
Subtract from selection would work, if you had done a Select All first. But Select All - Current Selection can be done with Select>Invert.
However, this is not what you should do here. On the edge of your shape, the pixels are partially transparent. If you use that as a selection, the pixels on the edge will be partially selected, which means they are going to be partially painted, and you will have a bit of transparency remaining. What you really want is bucket fill a fulll layer under your shape, or use the "Behind" mode of the paint tool without a selection.
However, this is not what you should do here. On the edge of your shape, the pixels are partially transparent. If you use that as a selection, the pixels on the edge will be partially selected, which means they are going to be partially painted, and you will have a bit of transparency remaining. What you really want is bucket fill a fulll layer under your shape, or use the "Behind" mode of the paint tool without a selection.