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Name
MESA_pack_invert
Name Strings
GL_MESA_pack_invert
Contact
Brian Paul, Tungsten Graphics, Inc. (brian 'at' tungstengraphics.com)
Keith Whitwell, Tungsten Graphics, Inc. (keith 'at' tungstengraphics.com)
Status
Shipping (Mesa 4.0.4 and later)
Version
1.0
Number
TBD
Dependencies
OpenGL 1.0 or later is required
This extensions is written against the OpenGL 1.4 Specification.
Overview
This extension adds a new pixel storage parameter to indicate that
images are to be packed in top-to-bottom order instead of OpenGL's
conventional bottom-to-top order. Only pixel packing can be
inverted (i.e. for glReadPixels, glGetTexImage, glGetConvolutionFilter,
etc).
Almost all known image file formats store images in top-to-bottom
order. As it is, OpenGL reads images from the frame buffer in
bottom-to-top order. Thus, images usually have to be inverted before
writing them to a file with image I/O libraries. This extension
allows images to be read such that inverting isn't needed.
IP Status
None
Issues
1. Should we also defined UNPACK_INVERT_MESA for glDrawPixels, etc?
Resolved: No, we're only concerned with pixel packing. There are other
solutions for inverting images when using glDrawPixels (negative Y pixel
zoom) or glTexImage (invert the vertex T coordinates). It would be easy
enough to define a complementary extension for pixel packing in the
future if needed.
New Procedures and Functions
None
New Tokens
Accepted by the <pname> parameter of PixelStorei and PixelStoref
and the <pname> parameter of GetIntegerv, GetFloatv, GetDoublev
and GetBooleanv:
PACK_INVERT_MESA 0x8758
Additions to Chapter 2 of the OpenGL 1.4 Specification (OpenGL Operation)
None
Additions to Chapter 3 of the OpenGL 1.4 Specification (Rasterization)
None
Additions to Chapter 4 of the OpenGL 1.4 Specification (Per-Fragment
Operations and the Frame Buffer)
Add the following entry to table 4.4 (PixelStore parameters) on page 182:
Parameter Name Type Initial Value Valid Range
---------------------------------------------------------
PACK_INVERT_MESA boolean FALSE TRUE/FALSE
In the section labeled "Placement in Client Memory" on page 184
insert the following text into the paragraph before the sentence
that starts with "If the format is RED, GREEN, BLUE...":
"The parameter PACK_INVERT_MESA controls whether the image is packed
in bottom-to-top order (the default) or top-to-bottom order. Equation
3.8 is modified as follows:
... the first element of the Nth row is indicated by
p + Nk, if PACK_INVERT_MESA is false
p + k * (H - 1) - Nk, if PACK_INVERT_MESA is true, where H is the
image height
"
Additions to Chapter 5 of the OpenGL 1.4 Specification (Special Functions)
None
Additions to Chapter 6 of the OpenGL 1.4 Specification (State and
State Requests)
None
Additions to Appendix A of the OpenGL 1.4 Specification (Invariance)
None
Additions to the AGL/GLX/WGL Specifications
None
GLX Protocol
None
Errors
None
New State
Add the following entry to table 6.20 (Pixels) on page 235:
Get Value Type Get Cmd Initial Value Description Sec Attribute
--------------------------------------------------------------------------------------------------
PACK_INVERT_MESA boolean GetBoolean FALSE Value of PACK_INVERT_MESA 4.3.2 pixel-store
Revision History
21 September 2002 - Initial draft
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