A DESCRIPTION OF THE REQUEST :
The xrender pipeline uses a bresenham function as "defined in the book" to emulate lines by drawing a series of rectangles, as many Xorg drivers are not able to accelerate diagonal lines.

The function however does not take into account that rectangles can span more than 1 pixel in a diretcion, so for almost horizontal/verical lines a lot more rectangles than really required are emmited.

JUSTIFICATION :
to improve performance of diagonal lines with the xrender backend