The Alpha Filter option that is available from the Layer menu allows applying any filter to just the alpha channel of you layer.
Select the Alpha Filter menu
- From the Layers menu
- Select the Alpha Filter menu item.
Choose a Filter
- Select a filter
- Adjust the parameters
- Notice the softer edges. The advantage of using the Alpha Filter to soften the edges of an object on a layer is that the color data for the object is not blurred since only the alpha channel is changed..