As well as writing HLSL shader programs, you can also write raw OpenGL Shading Language (GLSL) shaderA program that runs on the GPU. More info
See in Glossary programs in Unity. This is only supported on OpenGL Core and OpenGL ES platforms.
Note: This is not recommended or needed as part of a normal workflow; Unity cross-compiles your HLSL into optimized GLSL when needed. The use of raw GLSL is only recommended for testing, or for some special use cases.
GLSL program snippets are written between GLSLPROGRAM and ENDGLSL keywords.
In GLSL, all shader function entry points have to be called main(). When Unity loads the GLSL shader, it loads the source once for the vertex program, with the VERTEX preprocessor define, and once more for the fragment program, with the FRAGMENT preprocessor define. So the way to separate vertex and fragment program parts in GLSL snippet is to surround them with #ifdef VERTEX .. #endif and #ifdef FRAGMENT .. #endif. Each GLSL snippet must contain both a vertex program and a fragment program.
Standard include files match those provided for Cg/HLSL shaders; they just have a .glslinc extension:
UnityCG.glslinc
Vertex shader inputs come from predefined GLSL variables (gl_Vertex, gl_MultiTexCoord0, …) or are user defined attributes. Usually only the tangent vector needs a user defined attribute:
attribute vec4 Tangent;
Data from vertex to fragment programs is passed through varying variables, for example:
varying vec3 lightDir; // vertex shader computes this, fragment shader uses this
Unity does some preprocessing during Shader compilation; for example, texture2D/texture2DProj functions may be replaced to texture/textureProj, based on graphics API (GlES3, GLCore). Some extensions don’t support new convention, most notably GL_OES_EGL_image_external.
If you want to sample external textures in GLSL shaders, use textureExternal/textureProjExternal calls instead of texture2D/texture2DProj.
Example:
gl_FragData[0] = textureExternal(_MainTex, uv);