glintro - Man Page
glIntro(3G) OpenGL Reference glIntro(3G)
NAME
glIntro - Introduction to OpenGL
OVERVIEW
OpenGL is a high-performance 3D-oriented renderer that supersedes IrisGL.
It is supported on all SGI graphics adaptors except for the G, GT and
GTX. A number of other workstation and personal computer vendors also
support OpenGL.
See glXIntro for a short example program and compilation instructions.
OPENGL EXTENSIONS
SGI has implemented the following extensions to OpenGL. Note that the
set of supported extensions varies from machine to machine; see below for
more information.
EXT_abgr
extends the list of host-memory color formats. Specifically, it
provides a reverse-order alternative to image format RGBA. The ABGR
component order matches the cpack Iris GL format on big-endian
machines. For more information see glDrawPixels, glGetTexImage,
glReadPixels, glTexImage1D, and glTexImage2D.
EXT_blend_color
allows a constant to be used as a factor in the blending equation.
A typical use is to blend two RGB images. Without the constant
blend factor, one image must have an alpha channel with each pixel
set to the desired blend factor. For more information see
glBlendColorEXT, glBlendFunc, glGetBooleanv, glGetIntegerv,
glGetFloatv, and glGetDoublev.
EXT_blend_logic_op
defines an additional blending equation for glBlendEquationEXT.
This equation is a simple logical combination of the source and
destination colors. For more information see glGetBooleanv,
glGetIntegerv, glGetFloatv, and glGetDoublev.
EXT_blend_minmax
allows the blend equation to be changed using glBlendEquationEXT and
introduces two new blend equations, one to produce the minimum color
components of the source and destination colors and one to produce
the maximum. For more information see glGetBooleanv, glGetIntegerv,
glGetFloatv, and glGetDoublev.
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glIntro(3G) OpenGL Reference glIntro(3G)
EXT_blend_subtract
defines two additional blending equations for use with
glBlendEquationEXT. These new equations are similar to the default
blending equation, but produce the difference of its terms, rather
than the sum. Image differences are useful in many image processing
applications. For more information see glGetBooleanv, glGetIntegerv,
glGetFloatv, and glGetDoublev.
SGIX_clipmap
introduces new filtering and memory management techniques for
handling extraordinarily large textures. Clipmaps provide many of
the features of mipmaps, while using a small fraction of the texture
memory required for mipmaps of equivalent size. They are especially
useful for rendering terrain and roaming over large images.
Clipmaps are supported on InfiniteReality systems. For more
information, see glTexParameter.
SGI_color_matrix
adds a 4x4 matrix stack and matrix multiplication to the pixel
transfer path. The color matrix operates on RGBA pixel components.
It can be used to reorder or duplicate color components, and to
implement simple color-space conversions. For more information, see
glGetBooleanv, glGetIntegerv, glGetFloatv, glGetDoublev, and
glPixelTransfer.
SGI_color_table
defines a new RGBA-format color lookup table mechanism, and several
new lookup tables in the OpenGL pixel path. The new lookup tables
are treated as one-dimensional images with internal formats, like
texture images and convolution filter images. This allows the
tables to operate on a subset of the components of passing pixels.
(For example, a table with internal format GL_ALPHA modifies only
the A component of each passing pixel, leaving the R, G, and B
components untouched.) A small subset of this extension is
supported on RealityEngine, RealityEngine2, and VTX systems; because
of this, the extension is not listed in the extensions string
returned by glGetString. The full extension is supported on all
other systems. For more information, see glColorTableSGI,
glColorTableParameterSGI, and glGetColorTableParameterSGI.
EXT_convolution
adds 1- or 2-dimensional convolution operations to the pixel
transfer process. Pixel drawing, reading, and copying, as well as
texture image definition, are candidates for convolution. The
convolution kernels are themselves treated as 1- and 2-dimensional
images, which can be loaded from application memory or from the
framebuffer. A subset of this extension is supported on
RealityEngine, RealityEngine2, and VTX systems; the full extension
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glIntro(3G) OpenGL Reference glIntro(3G)
is supported on all other systems. For more information see
glConvolutionFilter1DEXT, glConvolutionFilter2DEXT,
glCopyConvolutionFilter1DEXT, glCopyConvolutionFilter2DEXT,
glGetConvolutionFilterEXT, glSeparableFilter2DEXT,
glGetSeparableFilterEXT, glConvolutionParameteriEXT,
glConvolutionParameterivEXT, glConvolutionParameterfEXT,
glConvolutionParameterfvEXT, glGetConvolutionParameterivEXT,
glGetConvolutionParameterfvEXT, glEnable, glDisable, glIsEnabled,
glGetBooleanv, glGetIntegerv, glGetFloatv, glGetDoublev,
glPixelTransferi, and glPixelTransferf.
EXT_copy_texture
provides the ability to copy pixels directly from the framebuffer
into texture memory. At present only a small subset of this
extension has been implemented on RealityEngine, RealityEngine2, and
VTX systems, so the extension name is not listed in the extensions
string returned by glGetString. It is fully supported on all other
systems. For more information, see glCopyTexImage1DEXT,
glCopyTexImage2DEXT, glCopyTexSubImage1DEXT, glCopyTexSubImage2DEXT
and glCopyTexSubImage3DEXT.
SGIS_detail_texture
introduces texture magnification filters that blend between the
level 0 image and a separately defined "detail" image. This detail
blending can be enabled for all color channels, for the alpha
channel only, or for the red, green, and blue channels only. It is
available only for 2D textures. Supported on RealityEngine,
RealityEngine2, and VTX systems and on InfiniteReality systems. For
more information see glDetailTexFuncSGIS, glGetDetailTexFuncSGIS,
glTexImage2D, glTexParameterf, glTexParameterfv, glTexParameteri,
glTexParameteriv, glTexSubImage2DEXT, glGetTexImage,
glGetTexLevelParameterfv, glGetTexLevelParameteriv,
glGetTexParameterfv, glGetTexParameteriv, glGetBooleanv,
glGetIntegerv, glGetFloatv, and glGetDoublev.
SGIS_fog_func
Standard OpenGL defines three fog modes: GL_LINEAR, GL_EXP
(exponential), and GL_EXP2 (exponential squared). Visual simulation
systems can benefit from more sophisticated atmospheric effects.
This extension provides the ability to define a custom fog blending
function by specifying a set of control points that will be
interpolated by the function. Supported on InfiniteReality systems.
For more information, see glFogFuncSGIS, glFog, and glGet.
SGIX_fog_offset
In highly-fogged environments, emissive objects (like simulated
automobile headlights or runway landing lights) can appear
unrealistically dim. This extension brightens fogged objects by
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glIntro(3G) OpenGL Reference glIntro(3G)
offsetting the Z value used in fog computations. Supported on
InfiniteReality systems. For more information, see glEnable, glFog,
and glGet.
EXT_histogram
defines pixel operations that count occurrences of specific color
component values (histogram) and track the minimum and maximum color
component values (minmax). An optional mode allows pixel data to be
discarded after the histogram and/or minmax operations are
completed. Otherwise the pixel data continue on to the next
operation unaffected. For more information see glHistogramEXT,
glResetHistogramEXT, glGetHistogramEXT,
glGetHistogramParameterivEXT, glGetHistogramParameterfvEXT,
glMinmaxEXT, glResetMinmaxEXT, glGetMinmaxEXT,
glGetMinmaxParameterivEXT, glGetMinmaxParameterfvEXT, glEnable,
glDisable, glIsEnabled, glGetBooleanv, glGetIntegerv, glGetFloatv,
and glGetDoublev.
SGIX_interlace
modifies the behavior of glDrawPixels, glCopyPixels, glTexImage2D,
glTexSubImage2DEXT, glCopyTexImage2DEXT and glCopyTexSubImage2DEXT,
such that when GL_INTERLACE_SGIX is enabled the source image is
considered to be a field of an "interlaced" frame. That is, the
effective source image has height equal to twice the actual height
and every other row contains "transparent" pixels that do not affect
the corresponding destination pixels in the target image. For
example:
glEnable(GL_INTERLACE_SGIX);
set current raster position to (xr,yr)
glDrawPixels(width, height, GL_RGBA, GL_UNSIGNED_BYTE, I0);
set raster position to (xr,yr+zoomy)
glDrawPixels(width, height, GL_RGBA, GL_UNSIGNED_BYTE, I1);
is equivalent to
glDisable( GL_INTERLACE_SGIX);
set current raster position to (xr,yr)
glDrawPixels(width, 2xheight, GL_RGBA, GL_UNSIGNED_BYTE, I2);
where pixel rows (0,2,4,...) of I2 are from image I0, and rows
(1,3,5,...) are from image I1.
This is particularly useful for assembling consecutive interlaced
video format fields to into a complete frame in either the
framebuffer or in texture memory. Supported on RealityEngine,
RealityEngine2, and VTX systems and on InfiniteReality systems.
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glIntro(3G) OpenGL Reference glIntro(3G)
SGIS_multisample
provides a mechanism to antialias all primitives. The technique is
to sample all primitives multiple times at different locations
within each pixel (rather than just the pixel center). The color
sample values are resolved to a single, displayable color each time
a pixel is updated, so the antialiasing appears to be automatic at
the application level. Supported on RealityEngine, RealityEngine2,
and VTX systems and on InfiniteReality systems. For more
information see glSampleMaskSGIS, glSamplePatternSGIS,
glTagSampleBufferSGIX, glXChooseVisual, glXGetConfig, glEnable,
glDisable, glIsEnabled, glPushAttrib, glGetBooleanv, glGetDoublev,
glGetIntegerv, and glGetFloatv.
EXT_packed_pixels
provides support for packed pixels in host memory. A packed pixel
is represented entirely by one unsigned byte, one unsigned short, or
one unsigned integer. The fields with the packed pixel are not
proper machine types, but the pixel as a whole is. Thus the pixel
storage modes, and their unpacking counterparts, all work correctly
with packed pixels. This extension is not supported on
RealityEngine, RealityEngine2, and VTX systems. For more information
see glDrawPixels, glReadPixels, glTexImage1D, glTexImage2D,
glTexImage3DEXT, glTexSubImage1DEXT, glTexSubImage2DEXT,
glTexSubImage3DEXT, glConvolutionFilter1DEXT,
glConvolutionFilter2DEXT, glConvolutionFilter3DEXT,
glSeparableFilter2DEXT, glSeparableFilter3DEXT, glColorTableSGI,
glTexImage4DSGIS, and glTexSubImage4DSGIS.
EXT_polygon_offset
allows depth values of fragments to be displaced so that lines (or
points) and polygons that lie in the same plane can be rendered
without interaction -- the lines are rendered either completely in
front of or behind the polygons (depending on the sign of the offset
factor). It also allows multiple coplanar polygons to be rendered
without interaction, if different offset factors are used for each
polygon. For more information see glPolygonOffsetEXT, glEnable,
glDisable, glIsEnabled, glGetBooleanv, glGetIntegerv, glGetFloatv,
and glGetDoublev.
SGIX_reference_plane
allows a group of coplanar primitives to be rendered without depth-
buffering artifacts. This is accomplished by generating the depth
values for all the primitives from a single ``reference plane''
rather than from the primitives themselves. This ensures that all
the primitives in the group have exactly the same depth value at any
given sample point, no matter what imprecision may exist in the
original specifications of the primitives or in the GL's coordinate
transformation process. SGIX_reference_plane is useful for
generating hidden-line drawings, for applying decals to polygons,
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glIntro(3G) OpenGL Reference glIntro(3G)
and for multipass rendering techniques. Supported on
InfiniteReality systems. For more information, see
glReferencePlaneSGIX.
SGIX_shadow
provides support for rendering shadows using shadow maps. First the
application renders the scene from the point of view of the light
source, and copies the resulting depth buffer to a texture with
internal format GL_DEPTH_COMPONENT. Next the application renders
the scene from the normal viewpoint. Then the application enables
the texture parameter GL_TEXTURE_COMPARE_SGIX, sets the texture
comparison operator and texture matrix appropriately, and re-renders
the scene with 2D texturing enabled. During this final rendering
pass, the depth value generated by iterating the r texture
coordinate is compared with the shadow map stored in texture memory,
and the results of the comparison indicate whether the pixel being
textured is in shadow. The filtered result of the shadow
comparisons can be blended with the pixel to darken it. Supported
on InfiniteReality systems. For more information, see
glTexParameter.
SGIX_shadow_ambient
controls the filtered texture value generated in shadowed regions
(see SGIX_shadow). In effect, this changes the ambient lighting in
shadows. Supported on InfiniteReality systems. For more
information, see glTexParameter.
SGIS_sharpen_texture
introduces texture magnification filters that sharpen the resulting
image by extrapolating from the level 1 image to the level 0 image.
Sharpening can be enabled for all color channels, for the alpha
channel only, or for the red, green, and blue channels only.
Supported on RealityEngine, RealityEngine2, and VTX systems and on
InfiniteReality systems. For more information see
glSharpenTexFuncSGIS, glGetSharpenTexFuncSGIS, glTexParameteri,
glTexParameterf, glTexParameteriv, glTexParameterfv,
glGetTexParameteriv, and glGetTexParameterfv.
EXT_subtexture
allows a contiguous portion of an already-existing texture image to
be redefined without affecting the remaining portion of the image or
any of the other state that describes the texture. There are three
new calls: glTexSubImage1DEXT, glTexSubImage2DEXT, and
glTexSubImage3DEXT. A subset of this extension is available on
RealityEngine, RealityEngine2, and VTX systems, and the full
extension is available on all other systems. Refer to the man pages
for more details.
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glIntro(3G) OpenGL Reference glIntro(3G)
EXT_texture
provides support for a variety of resolutions of color components in
texture images. That is, instead of treating a retained image as
having 1, 2, 3, or 4 components, it is treated as though it had a
specific format, such as GL_LUMINANCE_ALPHA, or just GL_ALPHA. This
extension also defines a robust method for applications to determine
what combinations of texture dimensions and resolutions are
supported by an implementation and it introduces a new texture
environment: GL_REPLACE_EXT. For more information see glTexImage1D,
glTexImage2D, glGetTexLevelParameteriv, glGetTexLevelParameterfv,
glTexEnvf, glTexEnvi, glTexEnvfv, and glTexEnviv.
EXT_texture3D
supports 3-dimensional texture mapping. It also defines the in-
memory formats for 3D images, and adds pixel storage modes to
support them. For more information see glTexImage3DEXT,
glPixelStore, glEnable, glDisable, glIsEnabled, glTexParameteriv,
glTexParameterfv, glGetBooleanv, glGetIntegerv, glGetFloatv,
glGetDoublevEnable, glGetTexImage, glGetTexLevelParameteriv,
glGetTexLevelParameterfv, glGetTexParameteriv, and
glGetTexParameterfv.
SGIX_texture_add_env
defines a new texture environment function which scales the texture
value by the constant texture color and then adds a bias color.
Supported only on InfiniteReality systems. For more information see
glTexEnv.
SGI_texture_color_table
adds a color lookup table to the texture mapping process. For more
information, see glColorTableSGI.
SGIS_texture_edge_clamp
The GL normally clamps texture coordinates to the range [0,1]. This
can cause the texture sampling filter to straddle the edge of the
texture image, taking half its sample values from within the texture
image, and the other half from the texture border. Sometimes this
is undesirable. SGIS_texture_edge_clamp defines a new texture
clamping method that ensures all sample values fall within the
texture image. For more information see glTexParameter.
SGIS_texture_filter4
allows 1D and 2D textures to be filtered using an application-
defined symmetric and separable filter with four samples per
dimension. In the most common 2D case, the filter is bicubic. This
filtering can yield better-quality images than mipmapping, and is
often used in image processing applications. Supported on
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glIntro(3G) OpenGL Reference glIntro(3G)
InfiniteReality systems. For more information see glTexParameter,
glGetTexParameter, glTexFilterFuncSGIS, and glGetTexFilterFuncSGIS.
SGIS_texture_lod
provides mechanisms that reduce the number of mipmap levels required
for mipmapped texturing. This allows a large texture to be loaded
and used initially at low resolution, and to increase the resolution
gradually as time passes or as more mipmap levels become available.
Supported on InfiniteReality systems. For more information, see
glTexParameter.
EXT_texture_object
supports named texture objects whose contents and parameters may be
changed after they are defined. (Contrast this with textures in
display lists, which cannot be modified after the display lists are
created.) For machines with special texture memories,
EXT_texture_object also provides simple texture memory management.
For more information, see glGenTexturesEXT, glDeleteTexturesEXT,
glBindTextureEXT, glPrioritizeTexturesEXT, glAreTexturesResidentEXT,
and glIsTextureEXT.
SGIX_texture_scale_bias
adds scale, bias, and clamp operations to the texture pipeline.
These operations are applied to the filtered result of a texture
lookup, before that result is used in the texture environment
equations and before the texture color lookup table of
SGI_texture_color_table, if that extension exists. Not supported on
RealityEngine, RealityEngine2, and VTX systems or on High Impact and
Maximum Impact systems. For more information, see glTexParameterfv,
glTexParameteriv, and glGetTexParameter.
SGIS_texture_select
adds new texture internal formats beyond those defined by
EXT_texture. The purpose of these new formats is to reorganize the
components of a texture into groups of components. This allows
better utilization of texture memory by subdividing the internal
representation of a texel into 1, 2, or 4 smaller texels.
For example on InfiniteReality systems, an 8-bit luminance texture
would normally use a full 16 bits of texture memory for each texel,
but if the application chooses the GL_DUAL_LUMINANCE8_SGIS internal
format, then two 8-bit luminance textures can be packed into the
same space. When such a texture is active, the application must
select which of the two packed textures will be used for drawing.
Supported on InfiniteReality systems and on High Impact and Maximum
Impact systems. For more information, see glTexImage1D,
glTexImage2D, glTexImage3DEXT, glTexParameterfv, glTexParameteriv,
and glGetTexParameter.
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glIntro(3G) OpenGL Reference glIntro(3G)
EXT_vertex_array
adds the ability to specify multiple geometric primitives with very
few subroutine calls. Instead of calling an OpenGL procedure to pass
each individual vertex, normal, or color, separate arrays of
vertexes, normals, and colors are prespecified, and are used to
define a sequence of primitives (all of the same type) when a single
call is made to glDrawArraysEXT. A stride mechanism is provided so
that an application can choose to keep all vertex data staggered in
a single array, or sparsely in separate arrays. Single-array storage
generally will provide better performance.
This extension also supports the rendering of individual array
elements, each specified as an index into the enabled arrays.
For more information see glArrayElementEXT, glDrawArraysEXT,
glVertexPointerEXT, glNormalPointerEXT, glColorPointerEXT,
glIndexPointerEXT, glTexCoordPointerEXT, glEdgeFlagPointerEXT, and
glGetPointervEXT.
USING EXTENSIONS
Procedure names and tokens for OpenGL extensions are suffixed with EXT or
with a vendor-specfic acronym. EXT is used for extensions that have been
reviewed and will be supported by more than one OpenGL vendor.
SGI also supports some vendor-specific extensions. Procedure names and
tokens for the SGI-specific extensions are suffixed with SGI, SGIS or
SGIX. ``SGI'' is used for extensions that will be available across the
product line (although the support for all machines might not be released
simultaneously). ``SGIS'' is used for extensions that will be available
on a subset of SGI platforms. ``SGIX'' extensions are experimental; in
future releases, the API for these extensions might change or might not
be supported at all.
All supported extensions have an associated macro definition in gl.h and
a corresponding token in the extensions string returned by glGetString.
For example, if the EXT_abgr extension is supported then the token
GL_EXT_abgr will be defined in gl.h and GL_EXT_abgr will appear in the
extensions string returned by glGetString.
The definitions in gl.h can be used at compile time to determine if an
extension's tokens and procedures exist in the OpenGL library. However,
the tokens returned by glGetString must be consulted at runtime to
determine whether the extension is supported on the particular display in
use at that moment.
As an alternative to parsing the extensions string, you can determine
which extensions are supported by examining the renderer string (to
determine which graphics subsystem is being used) and the version string
(to determine the release number of the software being used). For more
information see glGetString.
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glIntro(3G) OpenGL Reference glIntro(3G)
On some machines an extension may be incompletely implemented, and
therefore its name will not appear in the extensions string. In such
cases the only way to determine if the extension can be used is to query
the renderer and version strings.
GLX also has been extended. Refer to glXIntro for more information.
NOTES
When an OpenGL application uses indirect rendering, additional instances
of Xsgi, the SGI X server, process show up under ps. The additional
processes are multiple threads of the X server, used to implement
indirect rendering.
Do not mix OpenGL and IrisGL calls from within a single process.
BUGS
Different OpenGL processes which render to the same window using direct
rendering will not share the software ancillary buffers on that window.
If an OpenGL program does a server grab using its X connection, then for
the duration of the grab it should not render OpenGL into any window that
the client doing the grab did not create. Otherwise a deadlock occurs.
The client is still able to do X rendering. This holds for both local
and remote rendering.
If the OpenGL DSO (libGL.so) is unloaded, by calling dlclose(), before
the X connection is closed (or before the application is shutdown), a
segv will occur. To get around this, call XCloseDisplay before unloading
libGL.so.
glXCopyContext does not work correctly if the source context is not the
current context or if the destination context has never been made
current.
On High Impact and Maximum Impact systems, and on RealityEngine,
RealityEngine2, and VTX systems, GLX_BUFFER_SIZE is not the sum of
GLX_RED_SIZE, GLX_GREEN_SIZE, GLX_BLUE_SIZE, and GLX_ALPHA_SIZE for
visuals with 12-bit RGBA components.
glXSwapBuffers does not work for double-buffered GLX pixel buffers.
On XS, XZ, Elan, and Extreme systems, and Indy and XL systems,
glConvolutionFilter1DEXT, glConvolutionFilter2DEXT, and
glSeparableFilter2DEXT, will skip pixels and/or rows when the pixel zoom
is set so that it minifies (i.e., -1 < zoom < 1).
When the commands glHistogramEXT and glColorTableSGI are called between a
glNewlist and a corresponding glEndList, they are always stored in the
display list even when target is set to GL_PROXY_*. Proxy calls should
not be stored in a display list; instead they should be executed
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glIntro(3G) OpenGL Reference glIntro(3G)
immediately.
Even though the SGIX_texture_scale_bias extension is implemented on XS,
XZ, Elan, and Extreme systems, and on Indy and XL systems, it is not
possible to query GL_POST_TEXTURE_FILTER_BIAS_RANGE_SGIX and
GL_POST_TEXTURE_FILTER_SCALE_RANGE_SGIX. Doing so will result in a gl
error.
On XS, XZ, Elan, and Extreme systems, and on Indy and XL systems, calling
glHistogramEXT, glResetHistogramEXT, glMinmaxEXT, or glResetMinmaxEXT
between a glBegin and glEnd will not generate a GL_INVALID_OPERATION
error as it should.
No locking of display list structures is done on behalf of indirect
OpenGL contexts that share display list spaces. Applications that use
such contexts should use their own mechanisms to ensure mutual exclusion
when defining or destroying display lists.
You may notice some discrepancies between the OpenGL Reference Manual
which is available through InSight and the man pages (i.e., the ones you
get using the "man gl..." command in a shell window). If so, the man
pages contain the correct, up to date, information.
MACHINE DEPENDENCIES
Most machine dependencies are documented in the reference pages for the
commands with which they're most closely associated. Some more general
observations follow:
VGX and VGXT
On VGX and VGXT systems, OpenGL is implemented atop IrisGL. This
implementation passes the ``mustpass'' OpenGL conformance tests but,
nonetheless, the following inconsistencies exist: pixel centers lie on
integer coordinates (not half-integer coordinates), and diffuse and
specular light colors are ignored.
If any of the following are enabled, then all geometric primitives will
be rendered through software (i.e., no hardware acceleration will be
used):
o alpha test (unless test is GL_ALWAYS or GL_NOTEQUAL and reference
value is zero)
o texture
o lighting enabled but no lights enabled
o clock-wise front face
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glIntro(3G) OpenGL Reference glIntro(3G)
o fog enabled and color index visual
o drawing into both front and back buffer AND blend is enabled, or
logic op is enabled
Points will be rendered through software if evaluators are used (e.g.,
GL_MAP1_VERTEX3 is enabled).
Polygons and strips will be rendered through software if any of the
following are true:
o front mode is not the same as back mode and the polygon mode is not
GL_FILL
o polygon offset is enabled and the scale and bias offset factors are
not 1 and 0, respectively.
o points have to go through software and the polygon mode is GL_POINT
On VGXT fog is done per-vertex instead of per-pixel.
SEE ALSO
glXIntro
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