ANGLE_pack_reverse_row_order

GL_ANGLE_pack_reverse_row_order

Daniel Koch, TransGaming (daniel 'at' transgaming.com)

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.

No known IP claims.

None

None

Accepted by the <pname> parameter of PixelStore{if}, GetIntegerv(),
GetBooleanv(), and GetFloatv():

    PACK_REVERSE_ROW_ORDER_ANGLE    0x93A4

In Section 4.3.1 (Reading Pixels) add a row to table 4.7:

    +------------------------------+---------+---------------+-------------+
    | Parameter Name               | Type    | Initial Value | Valid Range |
    +------------------------------+---------+---------------+-------------+
    | PACK_REVERSE_ROW_ORDER_ANGLE | boolean | FALSE         | TRUE/FALSE  |
    +------------------------------+---------+---------------+-------------+

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
    FALSE.

In Section 6.1.4 add the following sentence to the fifth paragraph
(beginning with "For three-dimensional and two-dimensional array
textures..."):
    When PACK_REVERSE_ROW_ORDER_ANGLE is TRUE the order of rows within
    each image are reversed without reordering the images themselves.

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.

If EXT_texture_rg is present reinsert language about formats RED and RG
into the OpenGL ES 2.0 specification.

None

None

None

/* 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 */
glPixelStore(GL_PACK_REVERSE_ROW_ORDER_ANGLE, TRUE);

/* 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 1, 2011/11/22 (Brian Salomon)
  - First version
Revision 2, 2012/02/22 (dgkoch)
  - prepare for publishing