To draw something like a sky, we have to draw some triangles in the background. Such a structure is often spherical, cubical or cylindrical and is called skybox or skydome. In this case, we are using a spherical mesh.
The scene (houses, trees, etc.) should be drawn - if possible - at first. Since drawing the sky is consuming much performance, we avoid drawing pixels that are hidden by othe objects by drawing the scene first.
« back