*GFXprim* uses a Cartesian coordinate system. The pixel *(0, 0)* lies at the upper
left corner of the bitmap and coordinates increase to the right and to the
bottom. The last valid pixel, that still lies in the bitmap of size *w* and
*h*, occupies coordinate *(w-1,h-1)*. The precise position of the point with
integer coordinates is in the middle of the pixel (and the border is equidistant
between the pixel centers i.e. +/-0.5).

Figure 1. GFXprim coordinate system and gp_line_xyxy(5, 3, 9, 3)

The width and height, when passed as an argument, are interpreted as the number of
pixels to draw (rather than the length from one pixel center to another pixel
center). Which means that for a rectangle of size *(w,h)* starting at *(x,y)*
the first pixel to draw would be *(x,y)* and last one would be *(x+w-1,y+h-1)*.
In the degenerate case, when *w* or *h* is less than or equal to zero, no action is taken.

Figure 2. gp_rectangle_xywh(2, 1, 8, 4)