OpenGL: Create a sky box?

Question

I'm new to OpenGL. I'm using JOGL.

I would like to create a sky for my world that I can texture with clouds or stars. I'm not sure what the best way to do this is. My first instinct is to make a really big sphere with quadric orientation GLU_INSIDE, and texture that. Is there a better way?

Acinus

Here one jogl cube map sample – Stoeber

Answer 1

Yes.

Making a really big sphere has two major problems. First, you may encounter problems with clipping. The sky may disappear if it is outside of your far clipping distance. Additionally, objects that enter your sky box from a distance will visually pass through a very solid wall. Second, you are wasting a lot of polygons(and a lot of pain) for a very simple effect.

Most people actually use a small cube(Hence the name "Sky box"). You need to render the cube in the pre-pass with depth testing turned off. Thus, all objects will render on top of the cube regardless of their actual distance to you. Just make sure that the length of a side is greater than twice your near clipping distance, and you should be fine.

Bannerol

Answer 2

A skybox is a pretty good way to go. You'll want to use a cube map for this. Basically, you render a cube around the camera and map a texture onto the inside of each face of the cube. I believe OpenGL may include this in its fixed function pipeline, but in case you're taking the shader approach (fixed function is deprecated anyway), you'll want to use cube map samplers (samplerCUBE in Cg, not sure about GLSL). When drawing the cube map, you also want to remove translation from the modelview matrix but keep the rotation (this causes the skybox to "follow" the camera but allows you to look around at different parts of the sky).

The best thing to do is actually draw the cube map after drawing all opaque objects. This may seem strange because by default the sky will block other objects, but you use the following trick (if using shaders) to avoid this: when writing the final output position in the vertex shader, instead of writing out .xyzw, write .xyww. This will force the sky to the far plane which causes it to be behind everything. The advantage to this is that there is absolutely 0 overdraw!

Typecase

+1 - Good point about the overdraw. In general, this answer is more complete than mine. – Bannerol

Answer 3

Spheres are nice to handle as they easily avoid distortions, corners etc. , which may be visible in some situations. Another possibility is a cylinder.

For a really high quality sky you can make a sky lighting simulation, setting the sphere colors depending on the time (=> sun position!) and direction, and add some clouds as 3D objects between the sky sphere and the view position.

Antimissile