# How do I create a horizon?

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?

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I agree it looks distinctly unnatural. BTW how is you camera going to work? Do you need a full skybox or just a backdrop and do you want to generate the sky using noise or is sampling from a texture fine? If its just a backdrop (ala a 2.5D game) you could sample from something like this images.bibliocad.com/library/image/00010000/9000/… which would be tileable is its a panoramic. – ClassicThunder Feb 11 '12 at 4:28
Some kind of transformation to the Y coordinate (or Z, if that's your "upwards" coordinate) might do something. For example, y' = 1 − (1 − y)^2. The idea being to compress the Y coordinate around y = 0, and keep it the same or expand it near y = 1. – John Calsbeek Feb 11 '12 at 5:12
@Calsbeek this would be a great answer! :) – Maik Semder Feb 11 '12 at 10:21

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 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.

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This answer skirts the issue - I want to know how to make the texture I generated appear as the earth's sky dome. – bobobobo Jun 5 '12 at 19:14

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.

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

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