Well first of all you will need a few programs to make this work, but once you have everything its a pretty easy process to make quick and easy terrain in Hammer.
- Terragen - Download
- Yes, there is a licensed version and a free version. The free version has some restrictions, but will work fine for what you will be using it for.
- TGPGuiLib - Download
- Just needed to get the next tool to work correctly
- For Export Only - Download
- This is what you will need to export your terrain file as a .bmp image for the next tool to be able to read and make displacements for you
- DispGen - Download
- This is the tool that will make your displacements and save a .vmf file for you to open and edit to your liking in Hammer.
Now, open up Terragen, and a window will be there labeled Landscape. This is really all you will be dealing with, other then maybe the 3d view to get an idea of how it will look. Click on the 'Generate Terrain' button and another window appears. Things may look a bit confusing so I'm going to attempt to give a basic description for each option. However, you can find more information about this here.
The last two features, I recommend leaving them default. So click Generate Terrain till you get something you like. You can also click on View/Sculpt and make some crude changes with the sculpt tool as well, its pretty simple to use, but hard to get anything too detailed with it.
Once you have a terrain you wish to work with, I recommend saving it as the Terragen terrain file, just in case. Now, when you are ready to export it, click on Accessories and select F.E.O Windows BMP. Just leave everything in the next window at default settings. NOTICE: If this option is not available, you have not correctly installed For Export Only!
You can kinda preview how the terrain looks while still in Terragen. Just in the 'Rendering Control' box, uncheck 'Fixed Height Above Surface'. Then change your altitude so you are above the terrain to see everything. Then click the 3D button on the left to get a preview. You can move around by holding down the left mouse button to look and 'WASD' keys to move around.
Now then, open up DispGen. You can mess with the brush generation parameters. If you even have a general knowledge of Hammer you should be able to figure it out. Where it says 'Heightmap File', click the button with three dots, and select your .BMP that you exported from Terragen. You can also click 'Run Smoothing' if you so desire.
Pick a material and maybe an Alpha Map if you have one (I'm not going into alpha maps in this tutorial). Lastly, under VMF output, click the button with three dots and name a new map to save your newly created terrain in. Just click on Generate, and you should have a .vmf file with terrain all created for you and ready to edit.
Have fun with this. Hopefully, I'll see some maps with badass terrain ;) This can also be used in any Source game really. If you have any questions, just send me a PM, and I'll try to get back to you ASAP.