You are required to register your copy of Terragen if it is for commercial use, as specified in the License Agreement presented to you when you install or first use Terragen. If you only intend to use Terragen personally, on a non-profit basis, registration is optional and you may continue to use the unregistered version of Terragen free of charge.Registered users will also have access to priority email support, and will be able to render images larger than 1280 x 960, terrains larger than 513 x 513 and enhanced anti-aliasing modes.