In Unity, you write shader programs using the HLSL programming language.
Unity originally used the Cg language, hence the name of some of Unity’s shader keywords (CGPROGRAM) and file extensions (.cginc). Unity no longer uses Cg, but these keywords and file extensions are still supported. Note that all shader program code must be valid HLSL, even if it uses the Cg-related keywords and file extensions.
The only difference between using HLSLPROGRAM and CGPROGRAM is in the files that Unity automatically includes when it compiles the shader program. This is for backwards compatibility reasons. For more information, see Built-in shader include files.
The HLSL language itself has two syntaxes: a “legacy” DX9-style syntax, and a more modern DX10+ style syntax.
The difference is mostly in how texture sampling functions work:
sampler2D, tex2D() and similar functions. This syntax works on all platforms.Texture2D, SamplerState and .Sample() functions. Some forms of this syntax do not work on OpenGL platforms, due to how textures and samplers are not different objects in OpenGL.In Unity, you can avoid problems with HLSL syntax platform support by using predefined macros to declare and sample textures. Unity expands these macros to the most appropriate syntax, depending on the platform that the shader is being compiled for.
Different platforms use different shader compilers for shader program compilation as follows:
You can use predefined shader macros to identify which compiler Unity is using, for instance to use HLSL syntax only supported by one compiler, or to work around a compiler bug.
Shader compilation involves several steps. One of the first steps is preprocessing the shader source. By default, Unity uses the platform compiler’s preprocessor to perform this step; however, you can choose to override this and use Unity’s Caching Shader Preprocessor to perform preprocesing. The Caching Shader Preprocessor is up to 25% faster than the default preprocessors used by platform compilers.
The Caching Shader Preprocessor caches intermediate preprocessing data to speed up shader import and compilation time. The Editor reuses this cached data, and only needs to parse include files when their contents change. This makes compiling multiple variants of the same shader more efficient. Enabling the Caching Shader Preprocessor has the most noticeable effect when shaders within a project use a large set of common include files.
Note that the Caching Shader Preprocessor is experimental; it is still in active development. You can provide feedback on this experimental feature in the Unity forum.
You can enable or disable the Caching Shader Preprocessor using the Caching Shader Preprocessor (Experimental) checkbox in the Shader Compilation section of the Editor settings window. You can also enable or disable this feature in a C# script, using the EditorSettings.cachingShaderPreprocessor API.