Figure 2.

The watershed segmentation of a density map, D₀, is a set of regions, R₀, with regions corresponding to points positioned at every local density maxima, M₀. Density maps are shown using iso-surfaces, regions are drawn using smooth surfaces that enclose contained voxels, and points of local density maxima are drawn using spheres. Grouping by scale-space filtering moves the points in M₀ by steepest ascent to local density maxima in D₁, yielding new points M₁. When two or more points in M₁ coincide, the corresponding regions are grouped, as shown in the images on the right, producing R₁. R₃ results after two more smoothing and grouping steps, in which each region corresponds to a single protein. The illustrated density map was generated from PDB:1xck at 10Å resolution.