OK, I found out one thing: There are "smoothing" and "non smoothing" buttons but....
it's always blured (smoothed) when exported to another format, like PNG or TIFF.
For JPG it's not a problem, but a PNG image should be optimized for solid color.
The "smoothing" and "non smoothing" buttons are also disabled with the brush. So I must work only with the pencil, but the size of the pencil is limited to the pre-defined shapes.