Greedy coloring
===============

This example demonstrates :ref:`coloring <greedycoloring>` 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"))
		
		
		
.. figure:: figures/greedy_coloring_input.png
   :scale: 100 %
   :alt: Input image

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


.. figure:: figures/greedy_coloring_output.png
   :scale: 100 %
   :alt: Output image

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