HLSL
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Source uses the C-style HLSL (High Level Shading Language) for shaders. Shaders written in other languages, such as GLSL or CG can be quickly and easily ported to HLSL. For example, ToGL converts from GLSL to HLSL on the fly.
Direct3D Versions and Shader Models
Direct3D 9 shaders can be designed using any of the following: Shader Model 1, 2 and 3. It should be noted that there are versions 2.0a and 2.0b which are optimized for specific vendor graphics cards.
Direct3D 10 shaders can only be designed on Shader Model 4.
Direct3D 11 shaders can be designed on Shader Model 5.
Direct3D 12 shaders can be designed on Shader Model 5 (DX Feature Level 11.0), 6 and later.
