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# docs.GL

GLSL 4

# gl_ViewportIndex

## Name

gl_ViewportIndex — contains the index of the viewport to be used in viewport transformation and scissoring

## Declaration

    // In geometry shaders

`out int gl_ViewportIndex ;`

    // In fragment shaders

`in int gl_ViewportIndex ;`

## Description

In the geometry language, `gl_ViewportIndex` is used to specify the index of the viewport to which the next primitive emitted from the shader should be drawn. Primitives generated by the geometry shader will undergo viewport transformation and scissor testing using the viewport transformation and scissor rectangle selected by the value of `gl_ViewportIndex`. The viewport index used will come from one of the vertices in the primitive being shaded. However, which vertex the viewport index comes from is implementation-defined, and so it is recommended to use the same viewport index for all vertices of a primitive. If a geometry shader does not assign a value to `gl_ViewportIndex`, viewport transform and scissor rectangle zero will be used. If a geometry shader statically assigns a value to `gl_ViewportIndex` and there is a path through the shader that does not assign a value to `gl_ViewportIndex`, the value of `gl_ViewportIndex` is undefined for executions of the shader that take that path.

In the fragment language, `gl_ViewportIndex` will have the same value that was written to the output variable `gl_ViewportIndex` in the geometry stage. If the geometry stage does not dynamically assign to `gl_ViewportIndex`, the value of `gl_ViewportIndex` in the fragment shader will be undefined. If the geometry stage makes no static assignment to `gl_ViewportIndex`, the fragment stage will read zero. Otherwise, the fragment stage will read the same value written by the geometry stage, even if that value is out of range. If a fragment shader contains a static access to `gl_ViewportIndex`, it will count against the implementation defined limit for the maximum number of inputs to the fragment stage.

## Version Support

| OpenGL Shading Language Version
--- | ---
Function Name | 1.10 | 1.20 | 1.30 | 1.40 | 1.50 | 3.30 | 4.00 | 4.10 | 4.20 | 4.30 | 4.40 | 4.50
gl_ViewportIndex (geometry stage) | - | - | - | - | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔
gl_ViewportIndex (fragment stage) | - | - | - | - | - | - | - | - | - | ✔ | ✔ | ✔

## See Also

gl_Layer

## Copyright

Copyright © 2011-2014 Khronos Group. This material may be distributed subject to the terms and conditions set forth in the Open Publication License, v 1.0, 8 June 1999. https://opencontent.org/openpub/.

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