alter recipe 36 to emulate Truman Show poster effect

The following patch uses a downsampled image as a position 'mask' for a 20x20 version of recipe 36. I'd like to take this further to emulate the Truman Show poster effect(jpg attached).

Possible way to do this:

— downsample image X to 20 x 20
— sort the 400 'pixels' according to luminance
— use these to reconstruct the image by replacing each position in the 20x20 greyscale map with a section of the original image that has an appropriate/approximate luma value

Any help with figuring out how to sort, store, and recall the luma values would be appreciated!