How do I create a horizon?

Question

I added a sky to my raytracer that gets sampled when a ray completely misses any object.

What I'm doing here is if a ray completely misses anything in the scene, it returns the Perlin noise function at (ray.direction.x, ray.direction.y, ray.direction.z) - in other words it "samples a sphere around the origin".

But it "looks wrong" (because the sky looks like it's facing you).

How can I improve the sky and make it look more like the earth's horizon?

Or is this more or less what it should look like, and I should simply add more geometry on the floor?

Answer 1

Alternatively you could use a cube map with a sky texture instead of using perlin noise. And I also think that will make your scene look better due to having more detail and variation in the environment mapping. That's exactly what I did when I implemented my raytracer.

Another reason I recommend this, is that the environment does not need to be only clouds. It can have mountains, it can be an indoor scene, can be a scene from mars. Using a cube map the only thing you need to do is swap the texture and you're done.

So the first step will be to find a cube map that fits your needs. It could be stored as six separate textures, or as a single texture like the one below. Also check this link for more info on how to create your own:

Then on your code, I'd recommend creating a CubeMap class to encapsulate the sampling calculations. This class should know how to load the cube map texture, and given a ray it should be able to know in which face and in what position to sample. In my implementation I found it easier to store each of the six faces in separate texture objects, and branch based on the direction of the ray:

class CubeMap
{
    public void Load(string path);
    public Color GetSample(Ray ray);
    private Texture[6] faces;
}

In case you don't have a Texture class yet, just create your own. It can be as simple as a two dimensional array of colors. So, just as a reference, and with no guarantee on the efficiency or soundness of this implementation, here's the code I used on my project - CubeMap and CubeFace.

Answer 2

You might try using 2D Perlin with (ray.dir.x/ray.dir.z, ray.dir.y/ray.dir.z). The projects the direction vector onto the Z=1 plane. It should give you the effect of an infinite (flat) sky.

Answer 3

You might want to check Iñigo Quilez's (iq) Dynamic Clouds article out -- It looks like something that might fit your ticket.