I think you're (or me ) mixing regions and tiles. I think you're talking about regions. From the help: xLeft -xl <int> set X sub-region left pixel boundary of the final image xRight -xr <int> set X sub-region right pixel boundary of the final image yLow -yl <int> set Y sub-region low pixel boundary of the final image yHigh -yh <int> set Y sub-region high pixel boundary of the final image