1. The Sky Texture
2. Adding a 3D Skybox

Every map must have some sort of physical boundary (otherwise, there would be leaks), but if there seems to be nothing outside of the boundary, it would look strange and unrealistic. We don't live in a world where there is just a void outside of our immediate surroundings. To make our maps more realistic, we must build a 3D skybox.

Placement and Scale of 3D Skybox

The 3D skybox is new in the Source engine. The original Counter-Strike did not support 3D skybox. You build the 3D skybox the same way you map, but there are some differences.

First of all, the objects in the skybox seem far away because they follow the rule of parallex. This means that when you walk around in the map, they seem to shift slightly in position relative to the perspective of the player. You should have some sort of physical barrier (i.e. a wall, a cliff, a large building) separating the boundary of the map and the skybox. Otherwise, the skybox would look very strange.

You can place anything in the skybox, except for spawn points or weapons. The players can never jump into the skybox area, so you obviously cannot put hostage rescue points or bombsites in the skybox. Brushes, light entities, models can all be used in the skybox--but with a big difference.

By default, everything inside the skybox are magnified (scaled up) sixteen times in-game. So when you build your skybox, you have to make everything 1/16 in scale. It's kind of like working with doll houses. In addition, the relative distance of objects in the skybox are scaled 16 times too. So if two buildings are 4 units apart in Hammer, they will be 64 units apart in-game. This is very important!

The most difficult concept to grasp for the skybox is that it's not built directly outside of your map. Instead, Hammer uses a sky_camera entity so that you can build the skybox in a different location in the Hammer editor. Personally, I think the sky_camera entity is one of the most poorly named entity in Hammer. It has nothing to do with a camera. It links the skybox with the map and determines the relative position of the objects in the skybox versus the map.

Building the Skybox

The only way to truly understand how skybox works is to build one, so let's get to it. Open up firstmap again. In a different place in the Hammer grid, place a sky_camera entity. Somewhere inside our map, place another sky_camera entity. These two sky_camera entities act as the link between the map and the skybox objects.