cleanEdge is a pixel art upscaling algorithm designed for rotating sprites. Its goal is to improve the jumbled pixels and broken lines and edges that result from basic nearest neighbor rotation. The way this works is by upscaling the art in a way that prioritizes those lines and edges.
While cleanEdge is designed for sprite rotations, I've taken care to ensure the results when scaling also look nice. You can switch between these two results at the top.

Click and drag/scroll on the frame to rotate/scale the image, or enter the values manually.
Tip: Hold Shift while scrolling to make fine adjustments.

option descriptions:

• toggle: enables/disables the algorithm to compare results to basic nearest neighbor rotation/scaling.
• highest color: the color used in overlap priority calculations. E.g. if highest color is pure white, brighter colors (those closer to white) will overlap darker colors when applicable.
  If your art has outlines, matching this setting to that color usually provides better results.
• line width: how far each pixel's color will spread into its neighbors, where applicable. A value of ≈ 0.707 will make slopes line up along the diagonal (effectively cancelling out the influence of highest color)
• slopes: in scale mode, toggles 2:1 grade slopes between pixels. If disabled, only 45 degree angles will be used. (this is always enabled in rotation mode)

This version of cleanEdge is in the form of a shader, you can find the source code here!
It's MIT licensed, feel free to use it for any of your projects :)
(if you do, credit is appreciated!)