Cramming for an exam...

 I have a task to complete for Halo: Raptor. It's potentially enormous and will require me to stretch my skills far beyond their current limits. Not only do I need to improve the quality of my output, but I also need to radically change my process for improved efficiency and portability.

Giant terrains...how do you get them to render? One strategy is to separate a terrain into layers rendered as a composite or faked in a single pass using alpha cards.

I'll try that last one first. Create a mountain range render, then place that render on a card that conforms to your camera. This tricks the eye into believing the mountains are a natural background in the scene, not a fake one. If done right.

Mountain Alpha Cards

Actually, this could be a new product for Superhive...

Goes away and makes an attempt...

It works, but...

The workflow is good, but there are a few caveats. Firstly, Blender has a full compositor, and doing this as a compositing task offers a lot more control. However, that comes with a more hands-on approach with the compositor, and you need a bit more planning.

Here's what I did:

Background Terrain Cards

1. Render a very big mountain range, up to 8K if possible. The size of the card will be determined by your scene requirements. If your mountains are a little more than a misty silhouette, you only need to worry about a jaggy edge. Starting out big means you can downsize your image during an optimisation pass, which is easier than finding out that your image started out too small.

2. In the Render tab, go to the Film panel and turn on Transparent. This turns the background of your render transparent.

Save the output as a file that stores the alpha channel, such as a PNG (RGBA). 

10,000 x 2000 (rendered at x1.5) 17MB file

2. I optionally took the output to Affinity Photo and added a mask/transparency gradient on the bottom edge. This lets the card sit above a terrain with a softer edge. Often, the mountains will sit behind a foreground, so this isn't necessarily useful. You can maybe do this in Blender using Texture Paint with a falloff curve, but I knew how to do this in ten seconds in an external editor.

Bottom edge edited.

3. Create the card object on which you will project your mountain image. Don't use a plane! Instead, create a cylinder with a good number of faces. Make it huge. The one in this example has a 2000m radius and is 500m tall. Delete both the top and bottom faces, then delete enough segments so that your card image will match its aspect ratio. Don't forget to select faces in flip the normals so that they point inwards.

4. Create a material for the card. Plug the image's Alpha channel into your shader's Alpha input. In this example I added an extra node to allow me to increase the card's decrease brightness. Play around with your material setup to help make the results suitable for your scene.

5. You may need to fix the card's UV map to make it render correctly. It should be one of the easier UV fixing tasks you do, as the card's UV should just be a contiguous set of faces forming a long rectangle. Play with the UV so that your mountains have the correct aspect ratio, not too tall or stretched too long.

Add a foreground terrain.

6. That's all the setup done. Next, you need to play around with everything to get it working as well as possible. The first thing you need to account for is lighting. You need your sunlight source in your scene to match with the light falling on the background card.

Projecting onto the cylinder means that, if done correctly, you can turn the camera and maintain the illusion.