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# docs.GL

OpenGL 4
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# glDrawElements

## Name

glDrawElements — render primitives from array data

## C Specification

`void glDrawElements(` | GLenum mode,
| GLsizei count,
| GLenum type,
| const GLvoid * indices`)`;

## 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`, and `GL_TRIANGLES` are accepted.

`count`

  Specifies the number of elements to be rendered.

`type`

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

`indices`

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

## Description

`glDrawElements` specifies multiple geometric primitives with very few subroutine calls. Instead of calling a GL function to pass each vertex attribute, you can use glVertexAttribPointer to prespecify separate arrays of vertex attributes and use them to construct a sequence of primitives with a single call to `glDrawElements`.

When `glDrawElements` is called, it uses `count` sequential elements from an enabled array, starting at `indices` to construct a sequence of geometric primitives. `mode` specifies what kind of primitives are constructed and how the array elements construct these primitives. If more than one array is enabled, each is used.

To enable and disable a generic vertex attribute array, call glEnableVertexAttribArray and glDisableVertexAttribArray.

## Notes

If the current program object, as set by glUseProgram, is invalid, rendering results are undefined. However, no error is generated for this case.

## Errors

`GL_INVALID_ENUM` is generated if `mode` is not an accepted value.

`GL_INVALID_ENUM` is generated if `type` is not `GL_UNSIGNED_BYTE` or `GL_UNSIGNED_SHORT`.

`GL_INVALID_VALUE` is generated if `count` is negative.

`GL_INVALID_FRAMEBUFFER_OPERATION` is generated if the currently bound framebuffer is not framebuffer complete (i.e. the return value from glCheckFramebufferStatus is not `GL_FRAMEBUFFER_COMPLETE`).

## Examples

Render an indexed vertex array (not loaded into OpenGL) using texture UV and normal vertex attributes.

    glEnableVertexAttribArray(texcoord_attrib_index); // Attribute indexes were received from calls to glGetAttribLocation, or passed into glBindAttribLocation.
    glEnableVertexAttribArray(normal_attrib_index);
    glEnableVertexAttribArray(position_attrib_index);

    glVertexAttribPointer(texcoord_attrib_index, 2, GL_FLOAT, false, 0, texcoords_data); // texcoords_data is a float*, 2 per vertex, representing UV coordinates.
    glVertexAttribPointer(normal_attrib_index, 3, GL_FLOAT, false, 0, normals_data); // normals_data is a float*, 3 per vertex, representing normal vectors.
    glVertexAttribPointer(position_attrib_index, 3, GL_FLOAT, false, 0, vertex_data); // vertex_data is a float*, 3 per vertex, representing the position of each vertex

    // num_vertices is the number of verts in your vertex_data.
    // index_data is an array of unsigned int offsets into vertex_data.
    glDrawElements(GL_TRIANGLES, num_vertices, GL_UNSIGNED_INT, index_data);

    glDisableVertexAttribArray(position_attrib_index);
    glDisableVertexAttribArray(texcoord_attrib_index);
    glDisableVertexAttribArray(normal_attrib_index);

Render an indexed buffer object using texture UV and normal vertex attributes.

    glBindBuffer(GL_ARRAY_BUFFER, vertex_buffer); // vertex_buffer is retrieved from glGenBuffers
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, index_buffer); // index_buffer is retrieved from glGenBuffers

    glEnableVertexAttribArray(texcoord_attrib_index); // Attribute indexes were received from calls to glGetAttribLocation, or passed into glBindAttribLocation.
    glEnableVertexAttribArray(normal_attrib_index);
    glEnableVertexAttribArray(position_attrib_index);

    // vertex_stride is the size of bytes of each vertex in the buffer object
    // vertex_position_offset and kin are the offset in bytes of the position data
    // in each vertex. For example if your vertex structure is
    // [ position, texcoord, normal ] then position vertex_position_offset will
    // have offset 0, vertex_texcoord_offset is 12 (position is 3 * sizeof(float)
    // bytes large, and texcoord comes just after) and vertex_normal_offset is
    // 20 = 5 * sizeof(float).
    GLintptr vertex_texcoord_offset = 3 * sizeof(float);
    GLintptr vertex_normal_offset = 5 * sizeof(float);
    GLintptr vertex_position_offset = 0 * sizeof(float);

    glVertexAttribPointer(texcoord_attrib_index, 2, GL_FLOAT, false, vertex_stride, (GLvoid*)vertex_texcoord_offset);
    glVertexAttribPointer(normal_attrib_index, 3, GL_FLOAT, false, vertex_stride, (GLvoid*)vertex_normal_offset);
    glVertexAttribPointer(position_attrib_index, 3, GL_FLOAT, false, vertex_stride, (GLvoid*)vertex_position_offset);

    // num_vertices is the number of verts in your vertex_data.
    // index_data is an array of unsigned int offsets into vertex_data.
    glDrawElements(GL_TRIANGLES, num_vertices, GL_UNSIGNED_INT, NULL);

    glDisableVertexAttribArray(position_attrib_index);
    glDisableVertexAttribArray(texcoord_attrib_index);
    glDisableVertexAttribArray(normal_attrib_index);

## Tutorials

open.gl - The Graphics Pipeline
opengl-tutorial.org - Tutorial 13 : Normal Mapping

## See Also

glCheckFramebufferStatus, glDisableVertexAttribArray, glDrawArrays, glEnableVertexAttribArray, glUseProgram, glVertexAttribPointer

## Copyright

Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see https://web.archive.org/web/20171022161616/http://oss.sgi.com/projects/FreeB/.

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