It’s seven steps in total and is prone to being inaccurate; plus, the radius of
the corners can’t be changed on the fly. Applying levels can also be a bit
fiddly. One advantage is that different levels settings can be used to obtain
different degrees of antialiasing, from incredibly soft to completely aliased.
- Create a new layer
- Draw a rectangular selection
- Enter quick mask (Q)
- Gaussian blur by half the radius that you’d like for the rounded
corners. (For example, a 10-pixel radius would need a 5-pixel blur.) - Apply Levels (Command + L), and use about 118 for the black point
and 137 for the white point on the input levels - Exit quick mask (Q)
- Fill selection
On the positive side, this blur method can be used to quickly create some
interesting and organic shapes that would be difficult to draw by hand.