Hi Chris, glad to hear you're enjoying Terragen. Welcome to the community!
It's pretty easy to convert procedurals to heightfield, you just need to be aware of the consequences which are that you lose any information that can't be represented in the 2D/planar heightfield, i.e. overhangs, and that the resolution of the resulting heightfield is finite, unlike the procedurals that you're generating it from. So you can potentially lose some terrain features and some detail doing this. You can add back in procedural detail over the top of your heightfield terrain later however and possibly regain some of what you lost. This in fact suggests the best practice for how to approach this which would be to put your heightfield conversion and operations in the top or middle of your terrain network, where you've created the major terrain features, but are not yet creating fine details; this way you don't lose real detail and can effectively add on more detail afterward without wasting processing time on features that get lost.
In any case, to convert procedural to Heightfield, use a Heightfield Generate node and feed the output of your procedural terrain network into the Shader input for the Heightfield Generate. Then set the size and resolution in the Heightfield Generate: "Number of Points" is the heightfield resolution in pixels, higher values give more detail and a more faithful representation of your procedurals, but also use more memory and take longer to calculate; "Size in Meters" sets the area of coverage, since a heightfield terrain is finite, unlike procedurals, you need to set how large an area will be converted to heightfield in your scene, and this setting works together with "Number of Points" to define the resolution-per-area of the resulting heightfield. You can see the bounding box of the heightfield in the 3D preview to work out the size you need. Once you've sorted these settings out to your liking, just hit Generate and it will process the Shader inputs to create a resulting Heightfield. You can then run Erosion and other Heightfield operators on it. You can also apply procedurals further down the shader network to add in additional detail or effects that can't be represented in a heightfield (e.g. overhangs).
- Oshyan