How to Draw a Line at Non Integer Pixel Coordinates on an Image
A long time ago I wrote an article on drawing lines with the Wu-Line algorithm. This can be used to draw lines from non-integer coordinates. The effect is very nice to see when animated. You can still find it in the Wayback Machine:



Wu Anti-aliased Lines from The good-looking textured light-sourced bouncy fun smart and stretchy page

There's also some pseudo-code to get you started.