property AlphaEdgeFeathering: Integer;
If the layer has an alpha channel then edges of the image will be "feathered." Feathering softens the edges of an image by applying alpha transparency at a rate that is proportional to the distance to the edge (specifically, adding a gradient in the alpha channel).
The value (0 to 255) indicates the depth in pixels of the feather effect.
Default: 0 (no edge feathering)
// Apply alpha edge feathering to all selected layers
for i := 0 to ImageEnView1.LayersCount - 1 do
if ImageEnView1.Layers[ I ].Selected then
ImageEnView1.Layers[ I ].AlphaEdgeFeathering := 5;