Gimp's median blur filter can smooth rough edges. Yes, I am aware it does mild damage to the subject or object's outline but it is far better then crude roughness. The reason I like this is because it is very quick to do.

Using Gimp's water pixel filter on the top layer, follwoed up by the selection wand to remove backgrounds from an image. This is sometimes superior to matting levin and open source machine learning. Depending on the scenario. I used another free and open source program (Pixelitor) to make the background.