glBeginTransformFeedback - OpenGL ES 3.2 Reference Pages

## Name

glBeginTransformFeedback — start transform feedback operation

## C Specification

`void glBeginTransformFeedback(` | GLenum primitiveMode`)`;

`void glEndTransformFeedback(` | void`)`;

## Parameters for `glBeginTransformFeedback`

`primitiveMode`

  Specify the output type of the primitives that will be recorded into the buffer objects that are bound for transform feedback.

## Description

Transform feedback mode captures the values of varying variables written by the vertex shader (or, if active, the geometry shader). Transform feedback is said to be active after a call to `glBeginTransformFeedback` until a subsequent call to `glEndTransformFeedback`. Transform feedback commands must be paired. An implicit glResumeTransformFeedback is performed by `glEndTransformFeedback` if the transform feedback is paused. Transform feedback is restricted to non-indexed `GL_POINTS`, `GL_LINES`, and `GL_TRIANGLES`.

If no geometry shader is present, while transform feedback is active the `mode` parameter to glDrawArrays and other drawing commands must match those specified in the following table:

Transform Feedback `primitiveMode` | Allowed Render Primitive `modes`
--- | ---
`GL_POINTS` | `GL_POINTS`
`GL_LINES` | `GL_LINES`, `GL_LINE_LOOP`, `GL_LINE_STRIP`, `GL_LINES_ADJACENCY`, `GL_LINE_STRIP_ADJACENCY`
`GL_TRIANGLES` | `GL_TRIANGLES`, `GL_TRIANGLE_STRIP`, `GL_TRIANGLE_FAN`, `GL_TRIANGLES_ADJACENCY`, `GL_TRIANGLE_STRIP_ADJACENCY`

If a geometry shader is present, the output primitive type from the geometry shader must match those provided in the following table:

Transform Feedback `primitiveMode` | Allowed Geometry Shader Output Primitive Type
--- | ---
`GL_POINTS` | `points`
`GL_LINES` | `line_strip`
`GL_TRIANGLES` | `triangle_strip`

## Notes

Geometry shaders, and the `GL_TRIANGLES_ADJACENCY`, `GL_TRIANGLE_STRIP_ADJACENCY`, `GL_LINES_ADJACENCY` and `GL_LINE_STRIP_ADJACENCY` primtive modes are available only if the GL ES version is 3.2 or greater.

## Errors

`GL_INVALID_OPERATION` is generated if `glBeginTransformFeedback` is executed while transform feedback is active.

`GL_INVALID_ENUM` is generated by `glBeginTransformFeedback` if `primitiveMode` is not one of `GL_POINTS`, `GL_LINES`, or `GL_TRIANGLES`.

`GL_INVALID_OPERATION` is generated if `glEndTransformFeedback` is executed while transform feedback is not active.

`GL_INVALID_OPERATION` is generated by glDrawArrays and other drawing commands if transform feedback is active and `mode` is not compatible with `primitiveMode` as described above.

`GL_INVALID_OPERATION` is generated by `glBeginTransformFeedback` if any binding point used in transform feedback mode does not have a buffer object bound. In interleaved mode, only the first buffer object binding point is ever written to.

`GL_INVALID_OPERATION` is generated by `glBeginTransformFeedback` if no binding points would be used, either because no program object is active of because the active program object has specified no varying variables to record.

## API Version Support

| OpenGL ES API Version
--- | ---
Function Name | 2.0 | 3.0 | 3.1 | 3.2
`glBeginTransformFeedback` | - | ✔ | ✔ | ✔
`glEndTransformFeedback` | - | ✔ | ✔ | ✔

## See Also

glPauseTransformFeedback, glResumeTransformFeedback

## Copyright

Copyright © 2010-2015 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/.