http://wiki.polycount.net/CubeMap
A cubemap is six images that are mapped onto a cube, creating a 360° panorama. Cubemaps are supported natively in graphics hardware so they're really quick to transform and render, because of this there's a unified method for rendering them in games.
Contents
1. Cube Map
3. 3ds Max
1. Metal Bump9
4. DxTex
5. Maya
6. Photoshop
4. Links
5. Software
A cubemap in horizontal-cross format. |
A cubemap in vertical-cross format. |
A cubemap using a horizontal layout. |
The cube is usually centered on the current viewpoint or camera. This means, when used as a backdrop or skybox it always appears infinitely distant. If you turn on wireframe mode in any 3d engine using a cubemap skybox, you'll see the geometry of the box. In the end it's just a regular mesh object.
Reflections & Lighting
When used for reflections, the reflection cubemap is usually centered on the mesh object. The viewing angle bounces off the mesh surface at an angle opposite to the surface normals, so it points to particular parts of the cube map.
Cubemaps can also be used for soft ambient-occlusion-like environment lighting, by using a diffusely convolved cubemap.
Creating Cubemaps
Angular Distortion
A cube map has to be created with some angular distortion in order to display a seamless panoramic image.
A cube map is always displayed from a viewpoint in the center of a cube mesh. This means the center of each cube face is closer to you than the edges and corners are. To counteract this, a cube map needs angular distortion.
This distortion becomes more obvious when you view it at any other angle, other than from the center of a cube.
(distortion images needed) |
Cube Map Formats
The bitmaps for each of the six sides need to be square, each needs to be powers-of-two in size (128, 256, 512, etc.), and all six images need to be the same resolution.
Usually for games you store the bitmaps all in one file using DDS or another compressed format, though some editors prefer them stored as six separate bitmaps (then compress them internally).
3ds Max
There are two main ways to create cubemaps in 3ds Max:
3ds Max can display cubemaps in real-time if you use the DDS cubemap format and a DirectX Shader material.
Metal Bump9
3ds Max will render it directly into DDS cubemap format, but it's limited to 256x256, and you can't set the DDS compression (it uses ARGB).
Reflect/Refract Map
The six rendered bitmaps will automatically be loaded into the Reflect/Refect map, or you can convert them into a DDS cubemap with Dxtex or another tool.
Warning: The render cannot be canceled once the object is picked, so save your Max file before rendering, in case Max freezes!
DxTex
Microsoft's DxTex offers a way to create a single DDS cube bitmap from six bitmaps. DxTex is automatically installed when you install the DirectX SDK. Once installed it will be found in the folder (SDK root)\Utilities\Bin\x86 or x64. Use the x86 version unless you're running a 64bit OS. There's also a DDS plugin for Photoshop in the SDK, it expects a 1x6 layout in Photoshop:
The DDS format does not actually store the images in a horizontal layout... each image is stored as a separate layer or slice in the DDS file. The DxTex plugin merely lays them out horizontally to make the cubemap easier to edit in Photoshop.
How to compile a DDS cubemap in DxTex:
Maya
The easiest way to render a Maya scene into a cubemap is to use six cameras, then stitch the six images together into a cubemap.
Workflow tips can be found in the Polycount thread [Maya 2009] how to render an environment map?.
Photoshop
The NVIDIA DDS plugin for Photoshop is one way to create DDS cubemaps. It expects you to lay out the faces of the cubemap horizontally: +X -X +Y -Y +Z -Z. In 3ds Max terms this is: RT, LF, UP, DN, BK, FR.
The DDS format does not actually store the images in a horizontal layout... each image is stored as a separate layer or slice in the DDS file. The Nvidia plugin merely lays them out horizontally to make the cubemap easier to edit in Photoshop.
Cubemap Seams
Cubemaps can show seams along the edges of the cube if they aren't mapped properly.
When a cubemap is rendered automatically in graphics hardware by a game engine, it will be seamless because it uses the Clamp uv address mode.
However if an artist manually creates a cube mesh and maps the six bitmaps onto it, usually this will cause seams from texture filtering. By default most materials use the Wrap uv address mode, which allows a texture to tile across a mesh. With a cubemap however this causes a sliver of the opposite side of each texture to be filtered into each edge. If your material has the option to use Clamp instead of Wrap, this will remove the seams.
There are two solutions for this:
Seams on the edges of a cubemap. |
Seams are solved by adding borders to each face texture. |
Links
Software