GLAPI/glDrawArraysInstancedBaseInstance

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glDrawArraysInstancedBaseInstance
Core in version 4.4
Core since version 4.2
Core ARB extension ARB_base_instance

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

Function Definition

 void glDrawArraysInstancedBaseInstance(GLenum mode​, GLint first​, GLsizei count​, GLsizei primcount​, GLuint baseinstance​);
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: \left \lfloor \frac{gl\_InstanceID}{divisor} + baseInstance \right \rfloor. 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

glBindVertexArray, glDrawArrays, glDrawArraysIndirect, glDrawArraysInstanced, glMultiDrawArrays, glMultiDrawArraysIndirect

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/.