NAME
    WritePixel -- Change the pen num of one specific pixel in a
                 specified rastport.

SYNOPSIS
    error = WritePixel(  rp, x,  y)
     d0                 a1 D0  D1

    LONG WritePixel( struct rastport *, short, short );

FUNCTION
    Changes the pen number of the selected pixel in the specified
    rastport to that currently specified by pena, the primary
    drawing pen. Obeys minterms in rastport.

INPUTS
    rp    - a pointer to the rastport structure
    (x,y) - point within the rastport at which the selected
            pixel is located.

RESULT
    error = 0 if pixel succesfully changed
          = -1 if (x,y) is outside the rastport

BUGS

SEE ALSO
    readpixel() graphics/rastport.h