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# GLKit Effects Constants

## Overview

GLKit effects implement shaders to perform their calculations. These shaders require vertex data to be provided by your application. All of the effects provided by GLKit use a consistent set of indices to reference vertex data provided by your application. When your application enables a vertex attribute array by calling the `glEnableVertexAttribArray` function, or specifies the vertex attribute pointer by calling the `glVertexAttribPointer` function, it uses one of these constants to specify the `index` parameter.

## Topics

### Constants

[`GLKVertexAttrib`](/documentation/GLKit/GLKVertexAttrib)

Values used as indices in OpenGL code to associate vertex data with an attribute in a named shader effect.


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