Do not assign drainage basins to invalid pixels #161
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edge_count is currently equal to the number of pixels, but that might change in the future.
The placeholder edges from src -> -1 denote sinks, outlets, and invalid pixels (i.e. NaNs). By initializing drainage basins when tgt < 0, we create drainage basins at the sinks and outlets but also at the invalid pixels. This commit changes this behavior to create a new basin only upon reaching a valid edge src->tgt where tgt does not yet belong to a drainage basin.
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This PR fixes a problem with drainage basins which would lead to pixels that are not part of the flow network such as NaNs in the DEM being assigned to their own drainage basin.
The problem was that sinks, outlets and invalid pixels are all assigned placeholder edges by
flow_routing_d8_carvethat point from them to -1. We were using this placeholder edge to detect when to create a new drainage basin. This works for the sinks and outlets, but also creates a drainage basin for the invalid pixels. This PR switches this behavior to 1. skip these placeholder edges and 2. test whether the target pixel has not yet been assigned to a drainage basin. This matches the behavior of the MATLAB implementation, leaving invalid pixels with a 0 label for their drainage basin.This is part of some changes I am planning to the representation of flow networks in libtopotoolbox that should eventually remove those placeholder edges.