Growing curves from a distribution

This should be very close to the example image after assigning region colors to the image. 

Left: stipples: The maximum cost is 2500 and the curve is created using random strategy.

Right: stipples: The maximum cost is 1000 and the curve is created using random strategy.

Next steps:

1. experiment different curves.

2. find a good color scheme.

3. wisely choose a better curve type for each stipple.

4. find a way to resolve the issue in weak magnitude of the vector field since those areas with weak direction could create annoying patterns.

5. increase the quality of a small number of curves.
6. assign stroke textures.
7. evaluate the quality in terms of structural preservation, tone matching, or aesthetic appearance.
8. avoid isolated curves.
9. add a variety of curve types.
10. different from the old AT paper, it seems that the similar curves should be avoided in this application.