The simplest example of combining render elements to match the Terragen beauty pass, which is sufficient for many projects, is this: tgRgb = tgSurfRgb + tgAtmoRgb + tgCloudRgb
See attached: CmpRndElm_01_tgSurf.jpg
Note that each of these render elements are blended or merged together in an "additive" way, and that the end result matches the Terragen beauty pass. Sometimes you'll want to adjust or fine-tune your image, so each of the three "Rgb" render elements above can be recreated by combining other render elements. The newly combined elements then replace the "Rgb" render element in the comp.
There are two methods for recreating the "tgSurfRgb'' element. The first gives you control over direct and indirect lighting. The second gives you control over the diffuse and specular as well.
For control over direct and indirect lighting on a surface use: tgSurfRgb = tgSurfDirect + tgSurfIndirect.
For control over the diffuse and specular lighting on a surface choose this: tgSurfRgb = tgSurfDirectDiff + tgSurfDirectSpec + tgSurfIndirectDiff + tgSurfIndirectSpec
Using either of these methods results in matching the Terragen beauty pass, and adds the ability to "dial in" the amount of surface lighting you want.
Likewise the atmosphere "tgAtmoRgb" render element can also be recreated by combining the tgAtmoDirect and tgAtmoIndirect render elements, which will give you control over the atmosphere's direct and indirect lighting: tgAtmoRgb = tgAtmoDirect + tgAtmoIndirect
And finally, the clouds "tgCloudRgb" render element can be recreated in a similar fashion, by combining the "tgCloudDirect" and "tgCloudIndirect" render elements: tgCloudRgb = tgCloudDirect + tgCloudIndirect
See CmpRndElm_06_ReplaceAll.jpg
We're working on a Wiki document for all this and we'll cover the tgAlpha render elements in that.