2010 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 + 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 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/.