if ( $backdrop ) {              # not 1st?
        my $mask = $_-> copy;
        my @mask_plt = map NC( 255, 255, 255 ), $_-> getcolors;
        my $trans_idx = $_-> tags( name => 'gif_trans_index' );
        my $left      = $_-> tags( name => 'gif_left' );
        my $top       = $_-> tags( name => 'gif_top' );

        $mask_plt[ $trans_idx ] = NC( 0, 0, 0 ) 
            if defined $trans_idx;
        $mask-> setcolors( colors => \@mask_plt );
        $backdrop-> compose( src => $_, mask => $mask, 
                             tx => $left, ty => $top );
    }
    else {
        $backdrop = $_-> to_rgb8
    }