United Arab Emirates vs Yemen

Both were measured as horizontal bands.

Learn to tell them apart →

Both flags are drawn at 64×48 and cut into squares 8 across. Each square is averaged to one colour, and the score is the worst single square — so two flags that agree everywhere except one corner are as far apart as that corner, which is the right answer to a question about whether anybody could tell them apart. 0 is the same picture; the number is the worst single tile, out of 255.

What it cannot see. Both are stretched to the same box, so the shape of the flag and how wide it is flown are gone — that is why Indonesia and Monaco, which differ in nothing else, are not measured as identical but as their reds. And a square is an eighth of the flag across, so a coat of arms, a star or a line of writing is averaged into the field behind it rather than counted as a difference.