As a result of technical advancements in graphics cards, some areas of 3D graphics programming have become quite complex. To simplify the process, new features were added to graphics cards, including the ability to modify their rendering pipelines using vertex and pixel shaders.
In the beginning, vertex and pixel shaders were programmed at a very low level with only the assembly language of the graphics processing unit. Although using the assembly language gave the programmer complete control over code and flexibility, it was fairly hard to use. A portable, higher level language for programming the GPU was needed, so Cg was created to overcome these problems and make shader development easier.
Some of the benefits of using Cg over assembly are:
High level code is easier to learn, program, read, and understand than assembly code.
Cg code is portable to a wide range of hardware and platforms, unlike assembly code, which usually depends on hardware and the platforms it's written for.
The Cg compiler can optimize code and do lower level tasks automatically, which are hard to do and error prone in assembly.
Cg has six basic data types, some of them are the same as in C, others are especially added for GPU programming, these types are:
float - a 32bit floating point number
half - a 16bit floating point number
int - a 32bit integer
fixed - a 12bit fixed point number
bool - a boolean variable
sampler* - represents a texture object
Cg also features vector and matrix data types that are based on the basic data types, such as float3 and float4x4. Such data types are quite common when dealing with 3D graphics programming. Cg also has struct and array data types, which work in a similar way to their C equivalents.
Cg supports a wide range of operators, including the common arithmetic operators from C, the equivalent arithmetic operators for vector and matrix data types, and the common logical operators.
Functions and control structures
Cg shares the basic control structures with C, like if/else, while, and for. It also has a similar way of defining functions.
The standard Cg library
As in C, Cg features a set of functions for common tasks in GPU programming. Some of the functions have equivalents in C, like the mathematical functions abs and sin, while others are specialized in GPU programming tasks, like the texture mapping functions tex1D and tex2D.
The Cg runtime library
Cg programs are merely vertex and pixel shaders, and they need supporting programs that handle the rest of the rendering process. Cg can be used with two APIs: OpenGL or DirectX. Each has its own set of Cg functions to communicate with the Cg program, like setting the current Cg shader, passing parameters, and such tasks.
In addition to being able to compile Cg source to assembly code, the Cg runtime also has the ability to compile shaders during execution of the supporting program. This allows the runtime to compile the shader using the latest optimizations available for hardware that the program is currently executing on. However, this technique requires that the source code for the shader be available in plain text to the compiler, allowing the user of the program to access the source-code for the shader. Some developers view this as a major drawback of this technique.
To avoid exposing the source code of the shader, and still maintain some of the hardware specific optimizations, the concept of profiles was developed. Shaders can be compiled to suit different graphics hardware platforms (according to profiles). When executing the supporting program, the best/most optimized shader is loaded according to its profile. For instance there might be a profile for a graphics card that supports complex pixel shaders, and another profile for one that supports only minimal pixel shaders. By creating a pixel shader for each of these profiles a supporting program enlarges the number of supported hardware platforms without sacrificing picture quality on powerful systems.