A power of two is 2 taken to an exponent. In order:
1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, et cetera.
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Sixth, make sure textures are exactly 4 times the original size of the tile. In most cases,
you should end up with a power of 2 texture (256x256, 512x512, 1024x512, et cetera). However,
if the original art has a weird resolution, then you should use the original ×4.