The Qt X11 renderer uses trapezoids to render with XRenderCompositeTrapezoids. I am trying to use the code to render with CompositeTags, but all I get is this mishmash of triangles.

XRender defines a trapezoid as

{
A,B,C,D
top, bottom
}

...where A,B,C and D are points (x,y) and top and bottom are floats.

Now my question is this: What role does the top and bottom values play?? As far as I can see, it is enough with four points to define a trapezoid, so why are the top and bottom values needed??