Trapezoids??

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alfkil
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Trapezoids??
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??