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Name Strings
Daniel Koch, TransGaming (daniel 'at'
Brian Salomon
Daniel Koch
Implemented in ANGLE ES2
Last Modified Date: February 22, 2011
Author Revision: 22
OpenGL ES Extension #110
OpenGL 1.5 or OpenGL ES 1.0 are required.
Some of the functionality of this extension is not supported
when implemented against OpenGL ES.
EXT_texture_rg interacts with this extension.
The extension is written against the OpenGL 3.2 Specification
(Core Profile).
This extension introduces a mechanism to allow reversing the order
in which image rows are written into a pack destination. This
effectively allows an application to flip the results of a ReadPixels
in the y direction operation without having to render upside down.
The coordinate system of OpenGL is vertically reversed in comparison to a
number of other graphics systems such as native windowing APIs. Applications
that perform ReadPixels may have to either render to an intermediate color
buffer before calling ReadPixels or perform a flip in software after
ReadPixels. In some systems the GL can perform the row reversal during
ReadPixels without incurring additional cost.
IP Status
No known IP claims.
New Procedures and Functions
New Types
New Tokens
Accepted by the <pname> parameter of PixelStore{if}, GetIntegerv(),
GetBooleanv(), and GetFloatv():
Additions to Chapter 3 of the OpenGL 3.2 Specification (Rasterization)
In Section 4.3.1 (Reading Pixels) add a row to table 4.7:
| Parameter Name | Type | Initial Value | Valid Range |
In Section 4.3.1 (Reading Pixels) modify the second paragraph of subsection
"Placement in Pixel Pack Buffer or Client Memory" to read:
When PACK_REVERSE_ROW_ORDER_ANGLE is FALSE groups of elements are placed
in memory just as they are taken from memory when transferring pixel
rectangles to the GL. That is, the ith group of the jth row
(corresponding to the ith pixel in the jth row) is placed in memory just
where the ith group of the jth row would be taken from when transferring
pixels. See Unpacking under section 3.7.2. The only difference is that
the storage mode parameters whose names begin with PACK_ are used
instead of those whose names begin with UNPACK_. If the format is RED,
GREEN, BLUE, or ALPHA, only the corresponding single element is written.
Likewise if the format is RG, RGB, or BGR, only the corresponding two or
three elements are written. Otherwise all the elements of each group are
written. When PACK_REVERSE_ROW_ORDER_ANGLE is TRUE the order of the rows
of elements is reversed before the data is packed. That is, the element
corresponding to pixel (x, y + height - 1) becomes the first element
packed, followed by (x + 1, y + height - 1), etc. Otherwise, pixel data
is packed in the same manner as when PACK_REVERSE_ROW_ORDER_ANGLE is
Additions to Chapter 6 of the OpenGL 3.2 Specification (State and State Requests)
In Section 6.1.4 add the following sentence to the fifth paragraph
(beginning with "For three-dimensional and two-dimensional array
When PACK_REVERSE_ROW_ORDER_ANGLE is TRUE the order of rows within
each image are reversed without reordering the images themselves.
Dependencies on OpenGL ES
If implemented for OpenGL ES, this extension behaves as specified, except:
-Delete all references to formats RED, GREEN, BLUE, RG, and BGR.
-The language about image order in Section 6.1.4 does not apply as OpenGL ES
does not have GetTexImage.
Dependencies on EXT_texture_rg
If EXT_texture_rg is present reinsert language about formats RED and RG
into the OpenGL ES 2.0 specification.
New State
Get Value Type Get Command Value Description Sec.
--------- ---- ----------- ------- ----------- ----
PACK_REVERSE_ROW_ORDER_ANGLE B GetIntegerv FALSE Pixel pack row order reversal 4.3.1
New Implementation Dependent State
Sample Code
/* Allocate space to hold the pixel data */
const GLvoid* pixels = malloc(width * height * 4);
/* Bind the framebuffer object to be read */
glBindFramebuffer(GL_READ_FRAMEBUFFER, framebuffer);
/* Enable row order reversal */
/* The pixel data stored in pixels will be in top-down order, ready for
* use with a windowing system API that expects this order.
glReadPixels(x, y, width, height, GL_RGBA, GL_UNSIGNED_BYTE, pixels);
Revision History
Revision 1, 2011/11/22 (Brian Salomon)
- First version
Revision 2, 2012/02/22 (dgkoch)
- prepare for publishing