Greedy coloring

This example demonstrates coloring of regions with minimal number of colors, while ensuring that neighbouring regions have different colors. See also https://en.wikipedia.org/wiki/Four_color_theorem.

The coloring is made using a greedy algorithm, so the number of colors is not necessarily minimal.

def greedy_coloring():
        """
        Shows how to change colors of regions.
        """

        # Create image
        img = pi.newimage(ImageDataType.UINT8, 200, 200, 1)

        # Plot some overlapping spheres.
        # Each of the spheres has different color
        pi.sphere(img, [100, 100, 0], 50, 7)
        pi.sphere(img, [150, 100, 0], 50, 10)
        pi.sphere(img, [50, 100, 0], 50, 15)
        pi.sphere(img, [100, 50, 0], 50, 20)
        pi.sphere(img, [150, 150, 0], 50, 25)

        # Save the initial image
        pi.writeraw(img, output_file("before_coloring"))

        # Minimize number of colors in the image, still making
        # sure that all neighbouring spheres have different color.
        pi.greedycoloring(img)

        # Save the result
        pi.writeraw(img, output_file("after_coloring"))

Input image for greedy coloring. Pixel values are converted to colors using random look-up table, background is black.

Output image from greedy coloring. Pixel values are converted to colors using random look-up table, background is black.