    the coordinate [0,0] always refers to the <em>lower left pixel</em> of a
    bitmap, while coordinate [width-1, rows-1] to its <em>upper right
    pixel</em>.</p>

    <p>In the vectorial case, point coordinates are expressed in floating
    units, like (1.25, -2.3). Such a position doesn't refer to a given
    pixel, but simply to an immaterial point in the 2D plane.</p>

    <p>The pixels themselves are indeed <em>square boxes</em> of the 2D
    plane, whose centers lie in half pixel coordinates.  For