def buildShadowMaskImage(dqfile,detnum,extnum,maskname,bitvalue=None,binned=1):
    """ Builds mask image from WFPC2 shadow calibrations.
      detnum - string value for 'DETECTOR' detector
    """
    # insure detnum is a string
    if type(detnum) != type(''):
        detnum = repr(detnum)

    _funcroot = '_func_Shadow_WF'

    # build template shadow mask's filename

    # If an old version of the maskfile was present, remove it and rebuild it.
    if fileutil.findFile(maskname):
        fileutil.removeFile(maskname)

    _use_inmask = not fileutil.findFile(dqfile) or bitvalue is None

    # Check for existance of input .c1h file for use in making inmask file
    if _use_inmask:
        #_mask = 'wfpc2_inmask'+detnum+'.fits'
        _mask = maskname
        # Check to see if file exists...
        if not fileutil.findFile(_mask):
        # If not, create the file.
        # This takes a long time to run, so it should be done
        # only when absolutely necessary...
            try:
                _funcx = _funcroot+detnum+'x'
                _funcy = _funcroot+detnum+'y'

                _xarr = np.clip(np.fromfunction(eval(_funcx),(800,800)),0.0,1.0).astype(np.uint8)
                _yarr = np.clip(np.fromfunction(eval(_funcy),(800,800)),0.0,1.0).astype(np.uint8)
                maskarr = _xarr * _yarr

                if binned !=1:
                    bmaskarr = maskarr[::2,::2]
                    bmaskarr *= maskarr[1::2,::2]
                    bmaskarr *= maskarr[::2,1::2]
                    bmaskarr *= maskarr[1::2,1::2]
                    maskarr = bmaskarr.copy()
                    del bmaskarr

                #Write out the mask file as simple FITS file
                fmask = fits.open(_mask, mode='append', memmap=False)
                maskhdu = fits.PrimaryHDU(data=maskarr)
                fmask.append(maskhdu)

                #Close files
                fmask.close()
                del fmask
            except:
                return None

    else:
        #
        # Build full mask based on .c1h and shadow mask
        #
        fdq = fileutil.openImage(dqfile, mode='readonly', memmap=False)
        try:
            # Read in DQ array from .c1h and from shadow mask files
            dqarr = fdq[int(extnum)].data
            #maskarr = fsmask[0].data

            # Build mask array from DQ array
            dqmaskarr = buildMask(dqarr,bitvalue)

            #Write out the mask file as simple FITS file
            fdqmask = fits.open(maskname, mode='append', memmap=False)
            maskhdu = fits.PrimaryHDU(data=dqmaskarr)
            fdqmask.append(maskhdu)

            #Close files
            fdqmask.close()
            del fdqmask
            fdq.close()
            del fdq

        except:
            fdq.close()
            del fdq
            # Safeguard against leaving behind an incomplete file
            if fileutil.findFile(maskname):
                os.remove(maskname)
            _errstr = "\nWarning: Problem creating DQMASK file for "+rootname+".\n"
            #raise IOError, _errstr
            print(_errstr)
            return None


    # Return the name of the mask image written out
    return maskname
Builds mask image from WFPC2 shadow calibrations.
      detnum - string value for 'DETECTOR' detector