Problem 852. Index of neighbor pixel with steepest gradient

Unlike in various applications, where the gradient of a two dimensional matrix is calculated in x and y direction, the gradient of a digital elevation model (DEM) is usually returned as the steepest gradient. The steepest gradient is the largest downward slope of a pixel to one of its eight neighbors.
In this problem, your task will be to return the linear index of the steepest neighbor for each pixel in a gridded DEM. Pixels that don't have downward neighbors should receive the index value zero.
An example should help. The DEM is
dem = [1 5 9; ...
4 5 6; ...
8 7 3];
The result should be
IX = [0 1 4; ...
1 1 9; ...
2 9 0];
The results may not be unique, but the test cases have been built so that this is not a problem. The spatial resolution of the dem is dx=1 and dy=1. Note that the diagonal distance is hypot(dx,dy).
Solve

Solution Stats

40.51% Correct | 59.49% Incorrect
Last Solution submitted on Dec 07, 2025

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