## calculate the Mandelbrot values for half the line
        my @colors;
        for my $x ( 0 .. $halfX - 1  ) {
            push( @colors, rgb2n( (0)x3 ) ), next if clip( $x, $y );
            my $m = mandelbrot( ( $x - $halfX ) / $halfX, ( $y - $halfY ) / $halfY );
            my $c = mapColors( $m );
            push @colors, $m == 0 ? 0 : $c;
        }
        ## compress pixels to count/color pairs (line segments)
        my( $c, @lineSegs ) = 1;
        for my $x ( 0 .. $halfX - 2 ) {
            ++$c, next if $colors[ $x ] == $colors[ $x + 1 ];
            push @lineSegs, [ $c, $colors[ $x ] ];
            $c = 1;
        }
        push @lineSegs, [ $c, $colors[ - 1] ];
        
        ## Queue back the line segments for half the line.
        $Qout->enqueue( [ $y, \@lineSegs ] );