Via Hacker News: You may know that you can reverse a string by first reversing the order of the words in the string, then reversing the characters in each word. (So hello world becomes world hello, and then we recursively reverse each word to get dlrow olleh.) Well, it turns out you can do the same kind of thing in two dimensions: to rotate an image by 90 degrees, you can just rotate the order of the quadrants, and then recursively rotate each quadrant.

This is neat because it provides a way to losslessly rotate a non-square image by 90 degrees without changing its aspect ratio: a portrait remains a portrait, and a landscape remains a landscape, even after rotation. I say “losslessly” because all the pixels are still there: no resampling occurs. The rotation algorithm simply shuffles the pixels around into its best approximation of a rotated image.

Check it out on your own sample image below:

The Javascript/Canvas code embedded in the iframe above is also downloadable here.

See also:

• “Image rotation by three affine transformations” (2021-11-26)