You cannot sell renders made with Terragen Classic. You cannot reverse-engineer the software. However, downloading the original freeware .exe from a reputable archive is generally considered acceptable software preservation.

Creating reflective, adjustable water planes to simulate lakes, oceans, and rivers.

Generate landscapes from 2D heightmaps using simple "raise and lower" tools.

On the Terragen Classic download page, you will see options for different operating systems.

While Planetside Software has moved on to Terragen 4, they often keep an archive of older versions available.

While Planetside Software has transitioned its focus to Terragen 4 , the "Classic" version remains available for enthusiasts:

offers foundational concepts on heightfields and procedural terrain that apply to both classic and modern versions. Planetside Software how to render skyboxes