There is one more special kind of texture map: the cube map,
which is introduced in Panda3D version 1.1. A cube map is similar to
a 3-D texture, in that it requires 3-D texture
coordinates (u, v, w); also, a cube map is stored on disk as a
sequence of ordinary 2-D images.
But unlike a 3-D texture, which is defined by stacking up an arbitrary
number 2-D images like pancakes to fill up a volume, a cube map is
always defined with exactly six 2-D images, which are folded together
to make a cube.
The six images of a cube map are numbered from 0 to 5, and each image
corresponds to one particular face of the cube:
|image 0||The +u (or +x) face (right)|
|image 1||The -u (or -x) face (left)|
|image 2||The +v (or +y) face (forward)|
|image 3||The -v (or -y) face (back)|
|image 4||The +w (or +z) face (up)|
|image 5||The -w (or -z) face (down)|
By +x face, we mean the face of the cube farthest along the
positive X axis. In Panda3D's default Z-up coordinate system, this is
the right face. Similarly, the -x face is the face
farthest along the negative X axis, or the left face, and so on
for the Y and Z faces. Since the coordinates of a texture map are
called (u, v, w) instead of (x, y, z), it is technically
more correct to call these the +u and -u faces, though
it is often easier to think of them as +x and -x.
The faces are laid out according to the following diagram:
Imagine that you cut out the above diagram and folded it into a cube.
You'd end up with something like this:
Note that, when you hold the cube so that the axis indications for
each face are in the appropriate direction (as in the picture above),
several of the faces are upside-down or sideways. That's because of
the way the graphics card manufacturers decided to lay out the cube
map faces (and also because of Panda3D's default coordinate system).
But in fact, it doesn't matter which way the faces are oriented, as
long as you always generate your cube map images the same way.
In some sense, a cube map is a kind of surface texture, like an
ordinary 2-D texture. But in other sense, it is also volumetric like
a 3-D texture: every point within the 3-D texture coordinate space is
colored according to the face of the cube it comes closest to. A sphere model with the cube map applied to it would pick up the same six faces:
Note that, while a 3-D texture assigns a different pixel in the
texture to every point within a volume, a cube map assigns a
different pixel in the texture to every direction from the
You can load a cube map from a series of six image files, very similar to the way you load a 3-D texture:
tex = loader.loadCubeMap('cubemap_#.png')
As with a 3-D texture, the hash mark ("#") in the filename will be
filled in with the image sequence number, which in the case of a cube
map will be a digit from 0 to 5. The above example, then, will load
the six images "cubemap_0.png", "cubemap_1.png", "cubemap_2.png",
"cubemap_3.png", "cubemap_4.png", and "cubemap_5.png", and assemble
them into one cube map.