1. Reduce source image to 13 colors of the NES palette.
2. Divide image into 8x8 squares.
3. For each tile, reduce to 4 colors (3 if black is not present).
While doing so, keep track of the # of unique palette combinations & the tile indices they were originally found at.
If this number ever exceeds 5:
- Combine the first 4 tiles into a 16x16 block.
- Count the total # of colors.
- Reduce to 1 fewer.
- Split back into tiles (make sure they retain the original indices)
And restart the loop.