The FellWalker Algorithm
FellWalker is a new algorithm for identifying clumps, implemented by CUPID. It steps through all pixels in the input array with value above a specified threshold, and for each such pixel walks up a line of steepest gradient starting at the pixel until a local maximum is reached. A search is then made for a higher pixel value in a small neighbourhood centred on the peak. If a higher value is found, the algorithm jumps to that pixel and continues up-hill. If at any point a walk reaches a pixel that has already been assigned to a clump, then all pixels on the walk so far are assigned to the same clump, and the walk is terminated at that point. Otherwise, the walk continues until a maximum is found that is higher than all pixels in its neighbourhood. The maximum is then designated as a clump peak, and all pixels traversed in the up-hill walk are assigned to the clump.