In using TerraGen, I have learned a few tricks. One that took me for a loop was the ground shadows. The solution is actualy quite simple. Render the image without any figures or props and shadows turned off, but the ground turned on. export this image and use the ground to create a mask which you use to select from the TerraGen source background. Set the ground texture to the newly created TerraGen source, and make sure that all of the lights are white. Then slowly play with the ambient field on the ground texture until it matches (or in my case closely matches) the background. Then add the figures/props and adjust the primary light source to match where the sun should be in the TerraGen background.