NAME
    ReadPixel -- read the pen number value of the pixel at a
                 specified x,y location within a certain RastPort.

SYNOPSIS
    penno = ReadPixel( rp,    x,    y )
      d0               a1  d0:16 d1:16

    LONG ReadPixel( struct RastPort *, SHORT, SHORT );

FUNCTION
    Combine the bits from each of the bit-planes used to describe
    a particular RastPort into the pen number selector which that
    bit combination normally forms for the system hardware selection
    of pixel color.

INPUTS
    rp -  pointer to a RastPort structure
    (x,y) a point in the RastPort

RESULT
    penno - the pen number of the pixel at (x,y) is returned.
            -1 is returned if the pixel cannot be read for some reason.

BUGS

SEE ALSO
    WritePixel()    graphics/rastport.h