A skybox is the color information that the engine uses to clear the screen. For those that have used something like Processing, it’s like the “background( )” used at the top of a “draw( )” loop. Its purpose is to overwrite what was drawn on the previous frame, so that the current frame will not have remnants from the former. It is drawn first, as opposed to filled later so that transparent objects without opaque geometry behind it will blend properly.
A shader for a skybox is essentially the same as any other screen space shader, and can be made unlit. Because Unity does not use any geometry for the sky box by default, skyboxes utilizing a non-screen space approach require a custom geometry to which a material shader could be applied.
A fully procedural skybox has the advantage that it can require no texture sampling and therefore can remain pixel perfect at every resolution. A procedural sky box attempting to create Earth-like atmospheric effects can leverage the same game state parameters that other shaders have access to, such as camera look direction and world light (highest priority directional light) direction. By utilizing the inverse of the world light direction, the light direction can be made to look as though its coming from a source on the skybox. Similarly, the dot product of the source light and the apparent path to the zenith can be used to blend between the appropriate atmospheric hues. Also, because each rasterized pixel for a perspective camera is computed using a projected “look angle” from the camera’s transform, glows and other faux-scattering effects can be applied based on each pixel’s relative angular distance from the direction of the “sun”.
As an alternative to strict realism, a 4-way blend approach (example of math pictured below) can be used to strictly control the color at sunrise (SR), high noon (HN), sunset (SS), and midnight (MN). In the approach, below, the dot product of the world space light direction and orthogonal zenith angles are used to deterministically assign skybox color based on the “sun’s” apparent position in the sky. This approach assumes that the sun travels a roughly equatorial path over the game world, but testing shows that an offset of up to 30-degrees has little visible impact to the output colors. The reason for this set up is that it provides a meaningful 0-1 output node for each reference bearing and no contribution beyond 90-degrees to either side. This creates a sort of 4-way LERP. The inclusion of the power operator to each output allows the user to define the characteristic of the blend characteristic for values in between the poles.
Applying a custom shaded skybox to a camera is only slightly different than applying a post-processing style screen space shader effect to a camera. A material must be made to reference the skybox shader. Next the camera’s clear flag should be set to skybox. By default, Unity’s procedural skybox will be used. However, a skybox component can be added to the camera’s game object to allow an alternative skybox material to be used in its place.
Though the skybox may seem like a special case within the Unity development environment, understanding it’s function, mechanisms, affordances, and implementation makes it much more straightforward to understand and is a great opportunity to inject a bespoke aesthetic to any project.