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Since a signed 2-bit integer can only represent -1, 0, or 1, it is
tempting to simply to convert it directly to a float. This maps it
onto the correct range of [-1.0, 1.0]. However, it gives different
values compared to the usual equation:
(2.0 * 1.0 + 1.0) * (1.0 / 3.0) = +1.0 (same)
(2.0 * 0.0 + 1.0) * (1.0 / 3.0) = +0.33333333... (different)
(2.0 * -1.0 + 1.0) * (1.0 / 3.0) = -0.33333333... (different)
According to the GL_ARB_vertex_type_2_10_10_10_rev extension, signed
normalization is performed using equation 2.2 from the GL 3.2
specification, which is:
f = (2c + 1)/(2^b - 1). (2.2)
Comments below that equation state: "In general, this representation is
used for signed normalized fixed-point parameters in GL commands, such
as vertex attribute values." Which is what we're doing here.
The 3.2 specification goes on to declare an alternate formula:
f = max{c/(2^(b-1) - 1), -1.0} (2.3)
which is closer to the existing code, and maps the end points to exactly
-1.0 and 1.0. Comments below the equation state: "In general, this
representation is used for signed normalized fixed-point texture or
framebuffer values." Which is *not* what we're doing here.
It then states: "Everywhere that signed normalized fixed-point
values are converted, the equation used is specified." This is the real
clincher: the extension explicitly specifies that we must use equation
2.2, not 2.3. So we need to do (2x + 1) / 3.
This matches the behavior expected by oglconform's packed-vertex test,
and is correct for desktop GL (pre-4.2). It's not correct for ES 3.0,
but a future patch will correct that.
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Marek Olšák <maraeo@gmail.com>
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File: docs/README.WIN32 Last updated: 23 April 2011 Quick Start ----- ----- Windows drivers are build with SCons. Makefiles or Visual Studio projects are no longer shipped or supported. Run scons osmesa mesagdi to build classic mesa Windows GDI drivers; or scons libgl-gdi to build gallium based GDI driver. This will work both with MSVS or Mingw. Windows Drivers ------- ------- At this time, only the gallium GDI driver is known to work. Source code also exists in the tree for other drivers in src/mesa/drivers/windows, but the status of this code is unknown. General ------- After building, you can copy the above DLL files to a place in your PATH such as $SystemRoot/SYSTEM32. If you don't like putting things in a system directory, place them in the same directory as the executable(s). Be careful about accidentially overwriting files of the same name in the SYSTEM32 directory. The DLL files are built so that the external entry points use the stdcall calling convention. Static LIB files are not built. The LIB files that are built with are the linker import files associated with the DLL files. The si-glu sources are used to build the GLU libs. This was done mainly to get the better tessellator code. If you have a Windows-related build problem or question, please post to the mesa-dev or mesa-users list.