3D Skybox

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3D skybox geometry can be indistinguishable from normal level geometry. Seamless transitions from normal level geometry and 3D skybox are very possible, and used in many of the official Counter-Strike: Source maps.

Note:A 3D skybox is not a replacement for a 2D skybox. The 2D skybox is always in the background of the 3D skybox. To edit or change the standard 2D skybox, see Skybox (2D).

Benefits

Standard skyboxes are simple 2D images, mapped onto a cube so they surround the level. 3D skyboxes are full 3D geometry, and properly parallax as the player moves through the level. Since they are rendered in real time, they scale with the video card resolution and also support real-time shader effects not possible in 2D skyboxes.

The main benefit of using a 3D skybox is that it allows the level designer to make the map look much larger than is possible with the standard world geometry. The largest map that can be made without a 3D skybox is 32768 units on each side. Using a 3D skybox, the map can be extended to 16 times that amount.

Maps of this size are possible because 3D skyboxes are created in the Hammer editor at 1/16 the scale of standard maps. At run time, the engine scales them back up to match the scale of the rest of the map. Not only does this save space in the editor views, it also is much less expensive to compile and render.

Due to their scale, 3D skyboxes have much lower lightmap and texture resolution. This makes 3D skybox geometry less expensive performance-wise than standard world geometry.

3D skyboxes can be used to overcome draw distance limitations, if a low-detail skybox identical to the main map is created. A mod team wanting to do this would have to develop code to ease the transition between map versions.

Basic characteristics

3D Skyboxes have the following characteristics:

• Are constructed (by default) in 1/16 scale in the Hammer editor and then rendered at 16 times size in the engine to match the world geometry.
• Support materials with shader effects such as normal mapping, water, environment mapping, proxies, etc.
• Are non-interactive – players and other world entities cannot enter 3D skyboxes. The 3D skybox is just a visual extension of the map extents.
• Can be constructed with brushes, displacements, static and dynamic props, lights, brush entities, and point entities (such as env_sprite).
• Any models that are placed in the 3D skybox must be authored at 1/16 scale. Standard models (props) are not scaled properly for 3D skybox. Special 1/16 scale versions must be used. For Half-Life 2, these props can be found in the models/props_skybox directory.
• Should not contain info_player_start, NPCs or monsters.
• Have lower lightmap and texture resolution because of the scale at which they are created.
• Must have their own lighting sources, although a light_environment in the non-skybox section will also be used for the 3D skybox lighting.
• Use the sky_camera entity to control how they line up with the non-skybox world geometry.
• 3D skyboxes have their own fog parameters, adjustable in the sky_camera entity.
• Geometry in the 3D skybox is not occluded or culled like the rest of the level geometry. Adding lots of detailed brush geometry or models to the 3D skybox, especially with translucent materials, can severely affect performance.
• A map must have a light_environment in it, otherwise models in the 3d skybox will be lit incorrectly. A light_environment in the non-skybox part of the map negates the need for one in the 3D skybox.

Construction of 3D skyboxes

• A note for Portal developers: Ok, from all the people I have talked to and tests I have done, it seems that the Portal game engine is incapable of properly portraying a 3d skybox when you are looking through a portal. For those of us who want to use a 3d skybox in portal, my personal opinion is that it's fine to do... the gameplay isn't particularly changed by the image difference on the other side of a portal, except in some extreme cases, and perhaps there will one day be a patch (or maybe even a new engine!) to fix the issue. Regardless, I feel it's important that this be on the "3d skybox" page, as I assume there are other mappers out there that had this problem and couldn't find a solution anywhere. --RigorMortis 19:49, 2 Jan 2008 (PST)

The most efficient way of creating a 3D skybox that matches or "lines up" with the your current map is to use some of the geometry in the main part of the level. Follow these steps after you've created your main level geometry:

1. Add a sky_camera entity at the world origin (coordinates 0, 0, 0), the center of the map grid. The sky_camera is a reference point used by the renderer to align the 3D skybox with the main part of the map. In other words, it acts as a marker telling the renderer how the 3D skybox's origin and the world's origin relate to one another. You can think of this process as being similar to selecting all the 3D skybox geometry and then aligning the sky_camera entity in that selection with the world's origin (0,0,0). The geometry is translated back to that position in the world. This is the same concept used when the skybox is rendered in the engine.
2. Select some distinct parts of the level that you can use a guideline for scale and position in the 3D skybox. Common elements to select for this purpose would be some of the larger structures, as well as the walls, cliffs, hills, etc. that define the edges of the map. Make sure you also have the sky_camera you created selected as well.
   Note:It is important not to select any model entities, only brushes. Models cannot be used as reference geometry, because models cannot be scaled down in the Hammer editor.
3. Choose Copy from the Edit menu, or hit CTRL-C.
4. Choose Paste Special from the Edit menu. Set the Number of copies to paste to 1, and all other values to 0. Hit OK. This makes a duplicate of the geometry you had selected.
5. Leaving geometry selected, choose Transform from the Tools menu. Select the radio button next to Scale. Type in a value of .0625 for X, Y and Z. This is the decimal equivalent to 1/16 – the scale of the 3D skybox. Click OK.
   Note:If you want textures on the geometry to scale accordingly, make sure you have texture lock enabled.
6. You now have a 1/16 scale version of your reference geometry. Now drag (move) it to another part of the map where you wish build your 3D skybox. It doesn't matter where it is as long as it's not touching any part of the main level geometry area.
7. Don't forget to resize textures on all visible faces (if you forgot the texture lock. 0.01625 is optimal for the 1/16 scale of a skybox). This will allow areas of some skyboxes (like oceans and long expanses of brushes to be seamless.
   You can also use the Scaling Texture Lock button before you transform the geometry, which is right next to the Texture Lock button on the toolbar.
8. Delete the sky_camera entity from the normal world map area you started from. Important: only the sky_camera entity in the 3D skybox should remain in the map.
9. Build your 3D skybox geometry around the reference geometry, using it as a guide. You can build skybox geometry that meets the reference geometry seamlessly. You can use brush and displacement geometry. Models can also be placed, but since the 3D skybox is at 1/16 scale, any models must also be at 1/16 scale. The model hl2\models\props_skybox\coast01.mdl is a sample model in 1/16 scale. Try positioning the 3D view camera near the height of the player. This will give you a good idea of how the 3D skybox will look when it's rendered in the engine.
10. Add a hollow cube of brushes around your reference geometry and assign the tools\toolsskybox material to it. The standard 2D cubic skybox will appear on these surfaces. The 3D skybox area must be sealed with these brushes.
11. When you're done, delete or hide all of the reference geometry except for the sky_camera entity. You may find that it work best to add the reference geometry to its own visgroup so that it can be toggled on and off or re-placed. Just make sure you turn it off before you save and compile the level, or it will be compiled into the 3D skybox.
12. Your original map still needs the tools\toolsskybox material wherever you want to see the sky, however it will now show the 3D skybox as well as the 2D skybox (which is what it did previously).

Testing

If you properly sealed your 3D skybox, you can now compile the map and check out your new 3D skybox in the engine.

Examples

• See sdk_content\hl2\mapsrc\sdk_3d_skybox.vmf for an example of a 3D skybox.
• An example of extending draw distances with 3D skyboxes (commented)
• 3D Skybox Video Tutorial
• HL2 sample VMF for 3D Skybox water/world