2011 Khronos Group
glDrawElementsInstancedBaseInstance 3G
glDrawElementsInstancedBaseInstance draw multiple instances of a set of elements with offset applied to instanced attributes

## C Specification

void `glDrawElementsInstancedBaseInstance` GLenum mode GLsizei count GLenum type const void * indices GLsizei primcount GLuitn baseinstance

## Parameters

mode

  Specifies what kind of primitives to render. Symbolic constants `GL_POINTS`, `GL_LINE_STRIP`, `GL_LINE_LOOP`, `GL_LINES`, `GL_LINE_STRIP_ADJACENCY`, `GL_LINES_ADJACENCY`, `GL_TRIANGLE_STRIP`, `GL_TRIANGLE_FAN`, `GL_TRIANGLES`, `GL_TRIANGLE_STRIP_ADJACENCY`, `GL_TRIANGLES_ADJACENCY` and `GL_PATCHES` are accepted.

count

  Specifies the number of elements to be rendered.

type

  Specifies the type of the values in indices. Must be one of `GL_UNSIGNED_BYTE`, `GL_UNSIGNED_SHORT`, or `GL_UNSIGNED_INT`.

indices

  Specifies a pointer to the location where the indices are stored.

primcount

  Specifies the number of instances of the specified range of indices to be rendered.

baseinstance

  Specifies the base instance for use in fetching instanced vertex attributes.

## Description

`glDrawElementsInstancedBaseInstance` behaves identically to glDrawElements except that primcount instances of the set of elements are executed and the value of the internal counter instanceID advances for each iteration. instanceID is an internal 32-bit integer counter that may be read by a vertex shader as `gl_InstanceID`.

`glDrawElementsInstancedBaseInstance` has the same effect as:

        if (mode, count, or type is invalid )
            generate appropriate error
        else {
            for (int i = 0; i < primcount ; i++) {
                instanceID = i;
                glDrawElements(mode, count, type, indices);
            }
            instanceID = 0;
        }

Specific vertex attributes may be classified as instanced through the use of glVertexAttribDivisor. Instanced vertex attributes supply per-instance vertex data to the vertex shader. The index of the vertex fetched from the enabled instanced vertex attribute arrays is calculated as gl _ InstanceID divisor + baseInstance . Note that baseinstance does not affect the shader-visible value of `gl_InstanceID`.

## Notes

`glDrawElementsInstancedBaseInstance` is available only if the GL version is 4.2 or greater.

`GL_LINE_STRIP_ADJACENCY`, `GL_LINES_ADJACENCY`, `GL_TRIANGLE_STRIP_ADJACENCY` and `GL_TRIANGLES_ADJACENCY` are available only if the GL version is 3.2 or greater.

## Errors

`GL_INVALID_ENUM` is generated if mode is not one of `GL_POINTS`, `GL_LINE_STRIP`, `GL_LINE_LOOP`, `GL_LINES`, `GL_TRIANGLE_STRIP`, `GL_TRIANGLE_FAN`, or `GL_TRIANGLES`.

`GL_INVALID_VALUE` is generated if count or primcount are negative.

`GL_INVALID_OPERATION` is generated if a geometry shader is active and mode is incompatible with the input primitive type of the geometry shader in the currently installed program object.

`GL_INVALID_OPERATION` is generated if a non-zero buffer object name is bound to an enabled array and the buffer object's data store is currently mapped.

## See Also

glDrawElements, glDrawArraysInstanced

## Copyright

Copyright 2011 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. http://opencontent.org/openpub/.