2011-2013
Khronos Group
glDrawArraysInstancedBaseInstance
3G

# glDrawArraysInstancedBaseInstance

draw multiple instances of a range of elements with offset applied to instanced attributes

## C Specification

                    void glDrawArraysInstancedBaseInstance
                    GLenum mode
                    GLint first
                    GLsizei count
                    GLsizei primcount
                    GLuint baseinstance

## Parameters

`mode`

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

`first`

  Specifies the starting index in the enabled arrays.

`count`

  Specifies the number of indices to be rendered.

`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

`glDrawArraysInstancedBaseInstance` behaves identically to glDrawArrays except that `primcount` instances of the range 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`.

`glDrawArraysInstancedBaseInstance` has the same effect as:

        if ( mode or count is invalid )
            generate appropriate error
        else {
            for (int i = 0; i < primcount ; i++) {
                instanceID = i;
                glDrawArrays(mode, first, count);
            }
            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 &plus; baseInstance . Note that `baseinstance` does not affect the shader-visible value of `gl_InstanceID`.

## Errors

`GL_INVALID_ENUM` is generated if `mode` is not one of the accepted values.

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

`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

glDrawArrays, glDrawElementsInstanced

## Copyright

Copyright 2011-2013 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/.