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Adding a 3D skybox can be a powerful addition to the visual treatment of a level. The 3D skybox is extra area constructed by the level designer that is outside the bounds of the gameplay portion of the map. When the map is loaded, Source enlarges the objects in the 3D skbox area and places them outside the boundaries of the current level, between the skybox and the player. This is used to give the appearance of a much wider world out side of your level, without being too expensive on the budget due to the (by default) 1:16 scale. 3D skyboxes are non-interactive – the player and other entities cannot move into the 3D skybox space.
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.
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.
3D Skyboxes have the following characteristics:
- Are constructed 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
- Can not contain prop_physics and other physics entities, info_player_start, NPCs or monsters.
- Have lower lightmap and texture resolution because of the scale at which they are created.
- Must have they their own lighting sources. For example, the light_environment in the non-skybox section will not affect 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.
- The 3D skybox MUST have a
light_environmentin it, otherwise models will be lit incorrectly. For best results use an exact copy of the one used in the main part of the map (note that a
light_environmentin the 3D skybox negates the need for one in the main map).
Construction of 3D skyboxes
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:
- 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.
- 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.
- Choose Copy from the Edit menu, or hit CTRL-C.
- 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.
- 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.
- . 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.
- 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.
- 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.mdlis 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.
- Add a hollow cube of brushes around your reference geometry and assign the
tools\toolsskyboxmaterial to it. The standard 2D cubic skybox will appear on these surfaces. The 3D skybox area must be sealed with these brushes.
- Add a light_environment to the 3D skybox. You may want to copy and paste the one from the main part of the level, so the lighting matches.
- 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 it’s 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.
- Your original map still needs the
tools\toolsskyboxmaterial 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).
If you properly sealed your 3D skybox, you can now compile the map and check out your new 3D skybox in the engine.
sdk_content\hl2\mapsrc\sdk_3d_skybox.vmffor an example of a 3D skybox
- An example of extending draw distances with 3D skyboxes (commented)
|Skies and environment maps||Skybox (2D) • Skybox (3D) • HDR Skies • Skybox with Terragen • Skybox with Terragen - Advanced • List of skies|
|Terrain and displacement mapping||Displacements • Holes in displacements • Digital Elevation Models • Creating custom terrain with Worldmachine|