What this tutorial does. 
Before: sharp, jagged, edges:
After: cleaner, smoother, edges:
...
1. Open a render.
2. In GIMP.... Layer -> transparency -> alpha to selection
3. Select -> shrink. Select a value of 1 to 5 pixels. A value of 1 - 2 pixels will be the correct setting most of the time. But, I only started doing this yesterday so I can't really say, broz.
4. Ctrl + I (invert selection)
5. Filters -> blur -> gaussian blur. A value of 1 to 3 pixels seems to work best, but see #3 for why this may not necessarily be the case.
6. The end~
Enjoy.
Before: sharp, jagged, edges:
After: cleaner, smoother, edges:
...
1. Open a render.
2. In GIMP.... Layer -> transparency -> alpha to selection
3. Select -> shrink. Select a value of 1 to 5 pixels. A value of 1 - 2 pixels will be the correct setting most of the time. But, I only started doing this yesterday so I can't really say, broz.
4. Ctrl + I (invert selection)
5. Filters -> blur -> gaussian blur. A value of 1 to 3 pixels seems to work best, but see #3 for why this may not necessarily be the case.
6. The end~
Enjoy.