A shader is software which runs on a graphics card to determine how an object should be drawn. Source uses shaders for everything in the 3D world.
Shaders are manipulated with parameters stored in material files. While the most common are quite simple, very complex ones exist to handle effects like real-time shadowing, lighting and refraction.
If a shader is missing, on a material or model, the surface will appear as a white wireframe. Pink and black squares are not the product of amissing shader, but instead are generated when a material itself is missing.
There are two variations of shaders, Pixel shaders and Vertex shaders, each of which performs a different task in the rendering pipeline. Shaders form a replacement for the fixed function pipeline and allow developers greater control over rendering output by providing the ability to modify pixels and vertices dynamically. The SDK includes many existing shaders.
There are currently three main shader languages: High Level Shader Language (HLSL), C for Graphics (Cg) and OpenGL Shading Language (GLSL). The Source engine uses HLSL based shaders. However Cg is so similar that most Cg shaders can be quickly and easily ported to HLSL.
A shader model defines how advanced shading techniques are allowed to get on a graphics card. This prevents older graphics cards from being physically able to recognize newer shading techniques.
Modern versions of Source support Shader Model 2.0 (including Pixel Shader 2.0b) and Shader Model 3.0.
When creating shaders for newer graphics cards, it's important to remember to support those with older cards, or you will quickly limit the specs of your game to only a select few. Older cards can require so called "shader fallbacks" to be specified, where a backup shader (using an older shader model) will be used if the newer shader fails.
If you want to learn more about the detailed specs of different Shader Models, read the Wikipedia article.
For information on authoring shaders for use in the Source engine, please see Shader Authoring.
Vertex shaders are applied for each vertex run on a programmable pipeline. Its most basic goal is to transform geometry into screenspace coordinates so that the Pixel shader can rasterize an image. Vertex shaders can modify these position coordinates to perform mesh deformation. They can also receive additional information from the mesh, including normals, tangents, and texture coordinates. The vertex shader then writes to output registers; the written values are then interpolated across the vertices in the pixel shaders. Vertex shaders cannot create vertices.
A heavily commented example vertex shader, ready for use in Source is provided below.
Example vertex shader
This is a pass through shader - in so far as it makes no major modification to the vertex data, instead just passing the data through to the pixel shader stage.
Pixel shaders are applied for each pixel rendered to the screen. A pixel shader expects input from interpolated vertex values, which it then uses to rasterize the image. Pixel shaders can produce a huge range of effects involving the color of individual pixels such as refraction, per-pixel lighting or reflection.
A heavily commented example pixel shader, ready for use in Source is provided below.
Example pixel shader
The pixel shader below is intended for use as a post-process shader and creates a grayscale effect.
Applications of shaders in Source
The Source engine provides for two separate forms of shaders, Postprocess and Per-Object, the majority of the effects and materials used within the Source engine rely heavily on their Pixel shader components.
A Postprocess shader is typically a Pixel shader that works on a quad rendered across the entire screen. The quad is textured with a copy of the frame buffer, the Pixel shader can then alter and modify the rendered output to create a variety of effects, such as basic color modification to more advanced processes such as motion blur and bloom.
The Source SDK provides an example of this form of shader in the postprocess files (
Advanced Postprocess shaders, such as the bloom and motion blur shaders included with source, may also need to use custom Render Targets. For more information on integrating a Postprocess shader with a mod, see Custom Postprocessing Effects
A Per-Object shader in the Source engine is used on any object with the shader referenced in the relevant Valve Material (.vmt) file, such as a model or piece of brushwork. A Per-Object shader could be used to create a refractive material, modify a models vertices dynamically or other advanced rendering effects.
The Source SDK provides an example of a Per-Object shader in the lightmap files (