TImageEnView.MakeSelectionFeather

procedure MakeSelectionFeather(Radius: Double);

Modifies the current selection by applying a blur effect to make soften the borders.
Radius specifies the feathering intensity (amount of blur).

Note: If SelectionMaskDepth is 1 bit, it will be promoted to 8 bit.

Demos\ImageEditing\SoftSelections\SoftSel.dpr

ImageEnView1.SelectionMaskDepth := 8;
ImageEnView1.Select( 10, 10, 100, 100 );
ImageEnView1.MakeSelectionFeather( 4 );