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Name Strings
Greg Roth, NVIDIA Corporation (groth 'at'
Koji Ashida, NVIDIA Corporation
Gregory Prisament, NVIDIA Corporation
Greg Roth, NVIDIA Corporation
James Helferty, NVIDIA Corporation
Antoine Chauveau, NVIDIA Corporation
Last Modified Date: September 27, 2013
NVIDIA Revision: 7.0
OpenGL ES Extension #93
Written against the OpenGL ES 2.0 Specification.
NV_draw_buffers affects this extension.
Unextended OpenGL ES 2.0 only supports using ReadPixels to read from
the default color buffer of the currently-bound framebuffer.
However, it is useful for debugging to be able to read from
non-default color buffers. Particularly, when the NV_draw_buffers
extension is supported, each framebuffer may contain multiple color
buffers. This extension provides a mechanism to select which color
buffer to read from.
This document describes two extensions to allow an implementation to
support a subset of the total functionality.
The NV_read_buffer extension adds the command ReadBufferNV, which is
used to select which color buffer of the currently-bound framebuffer
to use as the source for subsequent calls to ReadPixels,
CopyTexImage2D, and CopyTexSubImage2D. If the system-provided
framebuffer is bound, then ReadBufferNV accepts value BACK. If a
user-created FBO is bound, then ReadBufferNV accepts COLOR_ATTACHMENT0.
Additionally, if the NV_draw_buffers extension is supported,
ReadBufferNV accepts COLOR_ATTACHMENTn_NV (n is 0 to 15).
The NV_read_buffer_front extension requires NV_read_buffer and adds
the ability to select the system-provided FRONT color buffer as the
source for read operations when the system-provided framebuffer is
bound and contains both a front and back buffer.
New Procedures and Functions
void ReadBufferNV(GLenum mode)
New Tokens
Accepted by the <pname> parameter of GetIntegerv:
Changes to Chapter 4 of the OpenGL ES 2.0 Specification
(Per-Fragment Operations and the Framebuffer)
Section 4.3.1 (Reading Pixels), subsection "Obtaining Pixels from
the Framebuffer" add:
For color formats, the read buffer from which values are obtained
is one of the color buffers; the selection of color buffer for the
bound framebuffer object is controlled with ReadBufferNV.
The command
void ReadBufferNV(enum src);
takes a symbolic constant as argument. <src> must be FRONT, BACK,
index of the color attachment point. Otherwise, an INVALID_ENUM
error is generated. Further, the acceptable values for <src> depend
on whether the GL is using the default framebuffer (i.e.
FRAMEBUFFER_BINDING is zero), or a framebuffer object (i.e.
FRAMEBUFFER_BINDING is non-zero) and whether the default framebuffer
is single or double buffered. For more information about framebuffer
objects, see section 4.4.
If the object bound to FRAMEBUFFER_BINDING is not framebuffer
complete (as defined in section 4.4.5), then ReadPixels generates
the error INVALID_FRAMEBUFFER_OPERATION. If ReadBufferNV is supplied
with a constant that is neither legal for the default framebuffer,
nor legal for a framebuffer object, then the error INVALID_ENUM
When FRAMEBUFFER_BINDING is zero, i.e. the default framebuffer,
<src> must be FRONT, BACK or NONE. If the requested buffer is
missing, the error INVALID_OPERATION is generated. If there is a default
framebuffer associated with the context, the initial setting for
READ_BUFFER_NV is BACK, otherwise it is NONE.
When the GL is using a framebuffer object, <src> must be NONE,
COLOR_ATTACHMENT0, or COLOR_ATTACHMENTi_NV, where <i> is the index
of the color attachment point. Specifying COLOR_ATTACHMENT0 or
COLOR_ATTACHMENTi_NV enables reading from the image attached to the
framebuffer at COLOR_ATTACHMENTi_NV. For framebuffer objects, the
initial setting for READ_BUFFER_NV is COLOR_ATTACHMENT0.
ReadPixels generates an INVALID_OPERATION error if it attempts to
select a color buffer while READ_BUFFER_NV is NONE or if the GL is
using a framebuffer object (i.e., READ_FRAMEBUFFER_BINDING is non-zero)
and the read buffer selects an attachment that has no image attached.
Section 4.3.2 (Pixel Draw/Read State) Replace first paragraph:
The state required for pixel operations consists of the parameters
that are set with PixelStore. This state has been summarized in
tables 3.1. Additional state includes an integer indicating the
current setting of ReadBufferNV. State set with PixelStore is GL
client state.
Dependencies on NV_read_buffer_front:
If NV_read_buffer_front is not supported, add to the third paragraph
describing ReadBufferNV:
If <src> is FRONT, the error INVALID_ENUM is generated.
Dependencies on NV_draw_buffers:
If NV_draw_buffers is not supported, change all references to
"COLOR_ATTACHMENTi_NV, where <i> is the index of the color attachment
New State
Add Table 6.X Framebuffer (State per framebuffer object):
State Type Get Command Initial Value Description
--------------- ---- ------------ ------------- -----------
READ_BUFFER_NV Z10* GetIntegerv see 4.2.1 Read source buffer
1. Should we use ReadBufferNV to specify whether ReadPixels reads
from the window system provided framebuffer or FBO?
No. The switching is automatic with FBO binding. The read buffer
state belongs to the rendering surface, so switching the rendering
surface automatically switches which read buffer to use.
This is consistent with the behavior of OpenGL 2.0 with the
ARB_framebuffer_object extension and unextended OpenGL 3.0.
2. Should we have FRONT/BACK, LEFT/RIGHT buffer enums for <mode>
parameter of ReadBufferNV to be used with window system provided
OpenGL ES 2.0 does not support stereo framebuffers, so for now we
only support FRONT and BACK.
3. Why separate NV_read_buffer and NV_read_buffer_front?
SUGGESTION: Some platforms, such as those with a compositing
window system, may be unable to read from the front buffer.
However, we would like to allow these platforms to read from any
of the buffers drawn to using the NV_draw_buffers extension.
4. Should this extension allow reading from depth and stencil buffers?
While originally part of this document, support for reading from
depth and stencil buffers has been moved to the
NV_read_depth_stencil extension. It is clearer to devote one
document to the re-introduction of ReadBuffer, and a separate
document to legalizing new format and type combinations for
5. Should ReadBufferNV() pass if READ_BUFFER points to a non-
existent buffer?
Early drivers followed the precedent set by Issue 55 of the
EXT_framebuffer_object spec; ReadBufferNV() would cause an error if
a FBO was bound and the requested buffer did not exist.
OpenGL ES 3.0 and OpenGL 4.3 allow it to pass.
RESOLVED: Behavior should match OpenGL ES 3.0. Application developers
are cautioned that early Tegra drivers may exhibit the previous
6. What should happen if COLOR_ATTACHMENT0, the default ReadBufferNV
is not bound and ReadBufferNV() gets called on this attachment?
Behavior matches the resolution of Issue 5.
7. Version 6 of this specification isn't compatible with OpenGL ES 3.0.
For contexts without a back buffer, this extension makes FRONT the
default read buffer. ES 3.0 instead calls it BACK.
How can this be harmonized?
RESOLVED: Update the specification to match ES 3.0 behavior. This
introduces a backwards incompatibility, but few applications are
expected to be affected. In the EGL ecosystem where ES 2.0 is
prevalent, only pixmaps have no backbuffer and their usage remains
Revision History
Rev. Date Author Changes
---- -------- --------- -------------------------------------
7 09/27/13 achauveau Harmonize BACK vs. FRONT selection
with GLES 3.0.
6 07/11/13 jhelferty Changes in behavior to match GLES 3.0
5 06/07/11 groth Responded to feedback. Clarified
non-FBO behavior and state ownership.
added a few issues.
4 06/01/11 groth Mostly rewrote spec edits to better
match the spec and more clearly
describe behavior.
3 03/22/09 gprisament Split depth & stencil reading into
separate document
Inline dependencies on NV_draw_buffers
Re-wrote overview section.
2 07/03/08 kashida Change to depend on
1 06/10/07 kashida First revision.