These files are actually scripts that define how a material should be constructed by layering a number of textures and applying various effects. It tells the graphics pipeline what to do if it encounters a triangle of that material. The scripts are written in a very simple language specific to defining shaders. The scripts are parsed by the engine on start-up and stored as an internal data structure. The graphics engine selects the data structure corresponding to the material of the triangles it’s about to render and that data structure drives the processing that is done to those triangles. For some materials a single texture pass is sufficient, some materials may require eight passes and lots of effects.

Not all surfaces need to be have a shader defined for it. By default, surfaces of models are rendered using a single texture and surfaces of the map are rendered using a single colour texture plus a lightmap texture.

Let’s have a look at a portion of one of those shader files.

This is only one block in the file, some of the files contain lots of these blocks, some only contain one. In other words, a single shader file can define one or more shaders.

The first line gives the shader name. Each shader has to have a unique name. In this case the name is “textures/battleship2a/shipdoor2a_trans”, but a name like “foobar” is fine too. You will notice that the name in this case looks like a file path, and that’s no coincidence. A lot of the shaders have names that correspond to the file path of a texture image file. Model files provide a material name for surfaces. If these are matched by a shader name, then that shader will be used. Otherwise the CoD engine will look for a JPG or a TGA file matching that name and use that. If these aren’t found either, your level designer or model builder goofed. You’ll see warnings in the console and the surface will look weird.

Let’s further pick apart this shader script. After the name comes a block within curly brackets, this is the definition of the shader. After the block, you can define another shader, e.g.

Name1
{
	shader 1
}

Name2
{
	shader 2
}

The block starts with some general keywords, then come one or more sub-blocks delimited by curly brackets. Each of those corresponds to a shader stage. These are applied in order. Each stage typically refers to a different texture file. You can compare stages with the concept of layers in PhotoShop.

In a simple implementation each of the stages corresponds to a render pass over the geometry. A clever implementation will try to combine multiple stages into one multi-texture pass.

The script structure is therefore:

Name1
{
	top-level settings
	{
		stage 1
	}
	{
		stage 2
	}
	{
		stage 3
	}
}

Lines at either the top level or the stage level start with a keyword and have some parameters following on.

I am not going to go in depth about Q3A shaders, as explaining it all would take far too long. I will however give you this link which will take you to the most recent version of the Q3A Shader Manual. Keep in mind that while CoD and CoD:UO are based on the Q3A engine, there are some differences in the shaders syntax that will become apparant as you delve deeper into this.