I don't understand why the author didn't just do his canvas drawing in a resolution independent (or quasi-independent) way. Just choose a unit relative to the s

I don't understand why the author didn't just do his canvas drawing in a resolution independent (or quasi-independent) way. Just choose a unit relative to the size of the canvas, and do everything in those units, transforming your coordinates to fit the width of the canvas element, which is resized to fit the device.