Basically a shader processes the information stored in textures and performs mathematical operations on them to produce the image that you see on your screen.
Shaders can be divided into pixel shaders and vertex shaders. Vertex shaders alter the position of the vertices of the mesh, e.g. the deformed wrecks after a unit has died. Pixel shaders define the color of the surfaces. When people speak about „shaders“, most of the time they mean pixel shaders.
The shaders for Forged Alliance are written in HLSL (high level shading language) and the code is stored in .fx files in the effects.nx2 archive in the gamedata folder. Mesh.fx contains the shaders for the buildings and units in the game.
When starting the game, the shaders are compiled and stored in a cache folder in C:\Users\Your_Username\Appdata\Local\Gas Powered Games\Supreme Commander Forged Alliance. When the shader files are edited, the files in the cache need to be deleted to trigger a recompile at game start.
Shaders are no rocket science, but they involve vector algebra. You should be familiar with the concept of vectors and know what a dot product and normal vecors are to understand the shader code.
The shader code is evaluated on all points of the surface with the specified shader. To produce variation on the surfaces, textures are used to control the shaders result. Each evaluation uses the color of the texure at this particular point of the object. Most Pixelshaders in this game return an RGB-color and an alpha value. The alpha value is used by the engine to determine how much bloom is added when the color is bright.
Every unit has a diffuse texture (also called albedo) that basically stores the color of the surface, and a normal map that fakes geometric details. There is also a specular texture (sometimes referred to as teamcolspec) that is used to store four greyscale textures in a single RGBA texture:
In HLSL the channels of a texture can be accessed like so:
exampletexture.r returns the red channel of the texture as a float value between 0 and 1.
exampletexture.rgb returns the red green and blue channel as a vector with three float values.
The environment map is basically a picture of the sky. In other games it is sometimes called the skybox. It is used to make shiny materials look believable. Note that the environment map can be different from the sky texture that you see in the game, if the mapmaker was not paying attention.
Every map has a sunlight and an ambientlight (the light that comes from the sky from all directions). There are no dynamic lights in supreme commander.
The best way to understand the shaders is to look at the code and then to alter a line and see how the changes look in game. You can specify a different shader for each fidelity setting, so if you copy paste the shader and make single edits you can compare the canges ingame by switching between the fidelity settings. (the console command Graphics_fidelity comes in handy here. 0 is low, 1 is medium and 2 is high). Also look at the textures of a unit to see where which texture is applied.
The names of the factions in the shader files do not always match the usual faction name. Insect refers to Cybran, UnitFalloff is used for Seraphim and NormalMapped is for UEF. A trailing PS indicates a pixel shader, a trailing VS indicates a vertex shader.