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Name Strings
Jason Green, TransGaming
Gavriel State, TransGaming
Rob Barris, Blizzard
Mark Krenek, Aspyr
Ryan Gordon, Destineer
Nicholas Vining, Destineer
Mark Kilgard, NVIDIA (mjk 'at'
Implemented by NVIDIA, October 2008
Last Modified Date: October 28, 2008
Version: 5
This extension is written against the OpenGL 2.1 Specification but
can apply to OpenGL 1.1 and up.
This extension interacts with EXT_vertex_array.
This extension interacts with EXT_secondary_color.
This extension interacts with NV_vertex_program.
This extension interacts with ARB_vertex_program.
This extension interacts with ARB_vertex_shader.
This extension provides a single new component format for vertex
arrays to read 4-component unsigned byte vertex attributes with a
BGRA component ordering.
OpenGL expects vertex arrays containing 4 unsigned bytes per
element to be in the RGBA, STRQ, or XYZW order (reading components
left-to-right in their lower address to higher address order).
Essentially the order the components appear in memory is the order
the components appear in the resulting vertex attribute vector.
However Direct3D has color (diffuse and specular) vertex arrays
containing 4 unsigned bytes per element that are in a BGRA order
(again reading components left-to-right in their lower address
to higher address order). Direct3D calls this "ARGB" reading the
components in the opposite order (reading components left-to-right
in their higher address to lower address order). This ordering is
generalized in the DirectX 10 by the DXGI_FORMAT_B8G8R8A8_UNORM
For an OpenGL application to source color data from a vertex
buffer formatted for Direct3D's color array format conventions,
the application is forced to either:
1. Rely on a vertex program or shader to swizzle the color components
from the BGRA to conventional RGBA order.
2. Re-order the color data components in the vertex buffer from
Direct3D's native BGRA order to OpenGL's native RGBA order.
Neither option is entirely satisfactory.
Option 1 means vertex shaders have to be re-written to source colors
differently. If the same vertex shader is used with vertex arrays
configured to source the color as 4 floating-point color components,
the swizzle for BGRA colors stored as 4 unsigned bytes is no longer
appropriate. The shader's swizzling of colors becomes dependent on
the type and number of color components. Ideally the vertex shader
should be independent from the format and component ordering of the
data it sources.
Option 2 is expensive because vertex buffers may have to be
reformatted prior to use. OpenGL treats the memory for vertex arrays
(whether client-side memory or buffer objects) as essentially untyped
memory and vertex arrays can be stored separately, interleaved,
or even interwoven (where multiple arrays overlap with differing
strides and formats).
Rather than force a re-ordering of either vertex array components
in memory or a vertex array format-dependent re-ordering of vertex
shader inputs, OpenGL can simply provide a vertex array format that
matches the Direct3D color component ordering.
This approach mimics that of the EXT_bgra extension for pixel and
texel formats except for vertex instead of image data.
New Procedures and Functions
New Tokens
Accepted by the <size> parameter of ColorPointer,
SecondaryColorPointer, and VertexAttribPointer:
BGRA 0x80E1
Additions to Chapter 2 of the OpenGL 2.1 Specification (OpenGL Operation)
-- Section 2.8 "Vertex Arrays" (page 24)
Replace the sentence beginning "size, when present, ..." with:
"size, when present, indicates the number of values per vertex that
are stored in the array as well as their component ordering."
Add these final sentences to the end of the same paragraph:
"The error INVALID_VALUE is generated if size is BGRA and type is
not UNSIGNED_BYTE. The error INVALID_VALUE is generated by
VertexAttribPointer if size is BGRA and normalized is FALSE."
Edit Table 2.4 "Vertex array sizes (values per vertex) and data types"
as follows:
* Rename the "Sizes" column to "Sizes and component ordering".
* Add "BGRA" to the "Sizes and component order" column of the
following rows: ColorPointer, SecondaryColorPointer, and
These are the commands capable of accepting normalized
coordinates and accepting 4 for "size" plus the special case of
SecondaryColorPointer command.
* Add the following sentence to the end of the table caption:
"If the size parameter is BGRA, the vertex array values are always
normalized irrespective of the Normalized column."
"The one, two, three, or four values in an array that correspond to
a single vertex comprise an array element. When the BGRA token is
specified for size, it indicates four values. The values within
each array element are stored sequentially in memory. However if
the size is specified with BGRA, the first, second, third, and fourth
values of each array element are read from the third, second, first,
and fourth values in memory respectively."
Additions to Chapter 3 of the OpenGL 2.1 Specification (Rasterization)
Additions to Chapter 4 of the OpenGL 2.1 Specification (Per-Fragment
Operations and the Frame Buffer)
Additions to Chapter 5 of the OpenGL 2.1 Specification (Special
Additions to Chapter 6 of the OpenGL 2.1 Specification (State and
State Requests)
Additions to the AGL/GLX/WGL Specifications
Additions to the OpenGL Shading Language
GLX Protocol
The error INVALID_VALUE is generated when ColorPointer,
SecondaryColorPointer, or VertexAttribPointer is
called with size set to BGRA and type is not UNSIGNED_BYTE.
The error INVALID_VALUE is generated when VertexAttribPointer is
called with size set to BGRA and normalized is FALSE.
Dependencies on EXT_vertex_array
This extension's additional behavior for ColorPointer apply to
ColorPointerEXT too.
Dependencies on EXT_secondary_color
This extension's additional behavior for SecondaryColorPointer
applies to SecondaryColorPointerEXT too.
Dependencies on NV_vertex_program
This extension's additional behavior for VertexAttribPointer applies
to VertexAttribPointerNV too.
Dependencies on ARB_vertex_program and ARB_vertex_shader
This extension's additional behavior for VertexAttribPointer applies
to VertexAttribPointerARB too.
New State
Change tables 6.6 and 6.7 to fix Type column by making the type
a k-valued integer where k is 3, 2, and 5 for color, secondary,
and vertex attrib arrays respectively.
(table 6.6, "Vertex Array Data", p. 271)
Get Value Type Get Command Initial Value Description Sec Attribute
--------- ---- ----------- ------------- --------------------- --- ---------
COLOR_ARRAY_SIZE Z3 GetIntegerv 4 Color components 2.8 vertex-array
per vertex
SECONDARY_COLOR_ARRAY_SIZE Z2 GetIntegerv 3 Secondary color 2.8 vertex-array
components per vertex
(table 6.7, "Vertex Array Data (cont.)", p. 272)
Get Value Type Get Command Initial Value Description Sec Attribute
--------- ---- ----------- ------------- --------------------- --- ---------
VERTEX_ATTRIB_ARRAY_SIZE Z5 GetIntegerv 16+ x Z5 Vertex attrib array 2.8 vertex-array
New Implementation Dependent State
1. What should this extension be called?
RESOLVED: EXT_vertex_array_bgra
Because the extension adds a new "vertex array" format with the
"bgra" component ordering and assuming a normalized representation
as BGRA with UNSIGNED_BYTE would provide for image data.
2. How should the vertex array API be changed?
RESOLVED: Allow BGRA as a valid token for the size field of
vertex array specification commands. While the size parameter
is of type GLint, the valid sizes are in the 1 to 4 range so
the GL_BGRA token is easily distinguished from the existing
small integer values.
This mimics the way OpenGL 1.1 changed the "components" field
of glTexImage1D and glTexImage2D to specify an internalformat
rather than simply a number of components from 1 to 4.
This approach has the advantage of not adding any new commands
or tokens since the GL_BGRA token already exists.
3. What vertex array specification commands should accept the new
BGRA vertex array format for their size parameter?
RESOLVED: All vertex array specification commands which accept 4
as a valid size plus the glSecondaryColorPointer command because
Direct3D allows a BGRA-ordered secondary color (its specular
vertex attribute) to be specified.
Because DirectX 10 makes its formats highly orthogonal and it
allows a corresponding format (DXGI_FORMAT_B8G8R8A8_UNORM),
the BGRA vertex array format should be similarly general.
4. Should a 4-component BGRA secondary color be allowed?
DirectX 9 supports a 4-component secondary color. All four
components should be available to a vertex shader or vertex
program. Fixed-function operation without lighting enabled
should pass through the secondary color alpha component. However
fixed-function lighting specifies its output secondary color alpha
is always output as 1 and ignores the input secondary color alpha.
5. So should 4 be allowed as a valid size parameter for
RESOLVED: Not for this extension. We leave it to another
extension or core revision could extend the secondary color to
4 components.
6. Should the BGRA vertex array format only work with the
This is consistent with Direct3D's use of the D3DCOLOR type which
is defined with a BGRA ordering and 8-bit normalized components.
7. How should the BGRA vertex array format's normalized behavior
interact with the "normalized" parameter of glVertexAttribPointer?
RESOLVED: Since BGRA implies normalization, it should be an
error to request BGRA with the normalized parameter set to false.
This ensures the following statement in the caption of table 2.4
remains true: "For generic vertex attributes, fixed-point data
are normalized if and only if the VertexAttribPointer normalized
flag is set."
8. Should the glVertexAttribIPointerEXT command accept GL_BGRA for
its size parameter?
RESOLVED: No. Because the BGRA vertex array format implies
normalized component values, that is inconsistent with providing
integer vertex attributes.
9. Should this apply to glVertexPointer?
RESOLVED: No. glVertexPointer doesn't support GL_UNSIGNED_BYTE
as a type so GL_BGRA doesn't make sense.
10. To what vertex arrays should this extension apply?
RESOLVED: Just primary color (glColorPointer), secondary
color (glSecondaryColorPointer), and generic vertex attribs
The rationale is these are the formats that take 4 component
attributes that can be normalized (making a special exception to
treat the secondary color array as having 4 components for the
purpose of BGRA support to match Direct3D). Texture coordinate
sets and vertex positions are not normalized. Normals are
normalized but only 4 coordinates.
11. What Direct3D 9 functionality provides BGRA vertex arrays?
You can specify BGRA vertex arrays with either Flexible Vertex
Formats (FVFs) or a Direct3D 9 Vertex Declaration.
The Vertex Declaration declaration data type is
Revision History
Rev. Date Author Changes
---- -------- --------- ----------------------------------------
1 1/11/07 mjk Initial version
2 1/31/07 mjk review comments; issue 9
3 7/16/08 mjk Removed TexCoordPointer behavior
Added issues 10 and 11
4 10/7/08 mjk Implemented now
5 10/28/08 mjk Provide state tabe updates