# Vertex Shader/Defined Inputs

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Vertex Shaders have the following built-in input variables.

```
in int gl_VertexID;
in int gl_InstanceID;
in int gl_DrawID; //Requires GLSL 4.60 or ARB_shader_draw_parameters
in int gl_BaseVertex; //Requires GLSL 4.60 or ARB_shader_draw_parameters
in int gl_BaseInstance; //Requires GLSL 4.60 or ARB_shader_draw_parameters
```

- gl_VertexID
- the index of the vertex currently being processed. When using non-indexed rendering, it is the effective index of the current vertex (the number of vertices processed + the first value). For indexed rendering, it is the index used to fetch this vertex from the buffer.

- gl_InstanceID
- the index of the current instance when doing some form of instanced rendering. The instance count
*always*starts at 0, even when using base instance calls. When not using instanced rendering, this value will be 0.

**Warning:**This value

*follow the baseInstance provided by some instanced rendering functions. gl_InstanceID always falls on the half-open range [0, instancecount). If you have GLSL 4.60, you may use gl_BaseInstance to compute the proper instance index.*

**does not**- gl_DrawID
- the index of the drawing command within multi-draw rendering commands (including indirect multi-draw commands). The first draw command has an ID of 0, increasing by one as the renderer passes through drawing commands.
- This value will always be a Dynamically Uniform Expression.
- gl_BaseVertex
- the value of the baseVertex parameter of the rendering command. If the rendering command did not include that parameter, the value of this input will be 0.
- gl_BaseInstance
- the value of the baseInstance parameter of the instanced rendering command. If the rendering command did not include this parameter, the value of this input will be 0.