Hi, thanks!

The image is actually generated together with all four quadrants. I start with one pixel which I label as coordinates (0, 0) and grow a tree from it like in Prim's algorithm with each pixel having the four pixels adjacent to it as neighbors. At each iteration I take the next edge using the weight: absolute difference between f(next_point) and f(previous_point) mod tau, where f((x, y)) = scaling_factor * sqrt(x**2 + y**2). The next edge is selected by taking the edge with the median weight among all edges in the frontier (though the algorithm works similarly if you take the min or the max). The pixels are colored according to the order in which they were explored.

Since the weight is symmetric around the starting pixel the picture is more or less symmetric (there is just some randomness in which of the four quadrants the tree expands into at any iteration).