The samples are regularly spaced. Here's the weighted filter used in this rendering:

1/81 2/81 3/81 2/81 1/81
2/81 4/81 6/81 4/81 2/81
3/81 6/81 9/81 6/81 3/81
2/81 4/81 6/81 4/81 2/81
1/81 2/81 3/81 2/81 1/81

This filter allows samples taken from neighboring pixels to influence the color of the current pixel. The result is softer profiles and edges.