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Name
3DFX_multisample
Name Strings
GL_3DFX_multisample
GLX_3DFX_multisample
WGL_3DFX_multisample
Contact
Paula Womack, 3dfx Interactive (paulaw 'at' 3dfx.com)
Status
Complete.
Version
Date: April 18, 2000; Version 1.1
Number
207
Dependencies
OpenGL 1.1 is required.
GLX 1.2 is required.
WGL_ARB_extensions_string is required.
WGL_ARB_pixel_format is required.
GLX_EXT_fbconfig and GLX 1.3 affect the definition of this extension.
This spec is written against OpenGL 1.2 and GLX 1.3
Overview
This extension provides a mechanism to antialias all GL primitives:
points, lines, polygons, bitmaps, and images. The technique is to
sample all primitives multiple times at each pixel. 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. Because each sample includes depth and stencil
information, the depth and stencil functions perform equivalently to
the single-sample mode.
An additional buffer, called the multisample buffer, is added to the
framebuffer. Pixel sample values, including color, depth, and
stencil values, are stored in this buffer. When the framebuffer
includes a multisample buffer, it does not also include separate
depth or stencil buffers, even if the multisample buffer does not
store depth or stencil values. Color buffers (left/right,
front/back, and aux) do coexist with the multisample buffer,
however.
Multisample antialiasing is most valuable for rendering polygons,
because it requires no sorting for hidden surface elimination, and
it correctly handles adjacent polygons, object silhouettes, and even
intersecting polygons. If only points or lines are being rendered,
the "smooth" antialiasing mechanism provided by the base GL may
result in a higher quality image.
This extension is a subset of SGIS_multisample. It differs in these
key ways:
* Fragment alpha values are not affected by the fragment sample mask
* The sample locations may or may not be fixed. Thus, there is no
support for rendering the scene multiple times with different
sample points.
* Fragment masks are not computed for images or for bitmasks.
Because of these differences a new extension was created. However,
it is not expected that this extension will co-exist with
SGIS_multisample. Because of this and the fact that there are only
32 push/pop bits the MULTISAMPLE_BIT_SGIS state value is the same as
MUTLISAMPLE_BIT_3DFX.
IP Status
No issues.
Issues
Should tbuffer be kept as a separate extension?
Yes. The ability to define a write mask for the fragment mask should
be kept separate. This feature is orthogonal to SGIS_multisample,
while 3DFX_multisample is not. 3DFX_multisample is a strict subset
of SGIS_multisample.
Should the multisample buffer be defined as a separate buffer?
Yes. It does not need to be implemented this way though. Since GL
rendering is done off screen and then blitted (to handle window
clipping) the multisample buffer can actually be the same as the
offscreen front and back buffers. The blit engine handles reads and
writes to/from AA buffers so ReadPixels will work correctly.
Should we allow the depth values for the different samples to differ
when AA is off?
No. This will be a bug on hardware that operates this way. In
practice it should not be a big issue.
New Procedures and Functions
None
New Tokens
Accepted by the <attrib_list> parameter of glXChooseFBConfig and
glXChooseVisual and by the <attribute> parameter of
glXGetFBConfigAttrib and glXGetConfig:
GLX_SAMPLE_BUFFERS_3DFX 0x8050
GLX_SAMPLES_3DFX 0x8051
Accepted in the <piAttributes> parameter array of
wglGetPixelFormatAttribivARB, and wglGetPixelFormatAttribfvARB, and
in the <piAttribIList> and <pfAttribFList> parameter arrays of
wglChoosePixelFormatARB:
WGL_SAMPLE_BUFFERS_3DFX 0x2060
WGL_SAMPLES_3DFX 0x2061
Accepted by the <cap> parameter of Enable, Disable, and IsEnabled,
and by the <pname> parameter of GetBooleanv, GetIntegerv, GetFloatv,
and GetDoublev:
MULTISAMPLE_3DFX 0x86B2
Accepted by the <pname> parameter of GetBooleanv, GetDoublev,
GetIntegerv, and GetFloatv:
SAMPLE_BUFFERS_3DFX 0x86B3
SAMPLES_3DFX 0x86B4
Accepted by the <mask> parameter of PushAttrib:
MULTISAMPLE_BIT_3DFX 0x20000000
Additions to Chapter 2 of the 1.2 Specification (OpenGL Operation)
None
Additions to Chapter 3 of the 1.2 Specification (Rasterization)
If SAMPLE_BUFFERS_3DFX is one, the rasterization of all GL
primitives is changed, and is referred to as multisample
rasterization. Otherwise primitive rasterization operates as it is
described in the GL specification, and is referred to as
single-sample rasterization. The value of SAMPLE_BUFFERS_3DFX is an
implementation dependent constant, and is queried by calling
GetIntegerv with pname set to SAMPLE_BUFFERS_3DFX.
During multisample rasterization the contents of a pixel fragment
are changed in two ways. First, each fragment includes a coverage
mask with SAMPLES_3DFX bits. The value of SAMPLES_3DFX is an
implementation dependent constant, and is queried by calling
GetIntegerv with pname set to SAMPLES_3DFX. Second, each fragment
includes SAMPLES_3DFX depth values, instead of the single depth
value that is maintained in single-sample rasterization mode. Each
pixel fragment thus consists of integer x and y grid coordinates, a
color, SAMPLES_3DFX depth values, texture coordinates, a coverage
value, and the SAMPLES_3DFX-bit mask.
The behavior of multisample rasterization is a function of
MULTISAMPLE_3DFX, which is enabled and disabled by calling Enable or
Disable, with cap set to MULTISAMPLE_3DFX. Its value is queried
using IsEnabled, with cap set to MULTISAMPLE_3DFX.
If MULTISAMPLE_3DFX is disabled, multisample rasterization of all
primitives is equivalent to single-sample rasterization, except that
the fragment coverage mask is set to all ones. The depth values may
all be set to the single value that would have been assigned by
single-sample rasterization, or they may be assigned as described
below for MULTISAMPLE_3DFX-enabled multisample rasterization.
If MULTISAMPLE_3DFX is enabled, multisample rasterization of all
primitives differs substantially from single-sample rasterization.
It is understood that each pixel in the framebuffer has SAMPLES_3DFX
locations associated with it. These locations are exact positions,
rather than regions or areas, and each is referred to as a sample
point. The sample points associated with a pixel may be located
inside or outside of the unit square that is considered to bound the
pixel. Furthermore, the pattern (relative location) of sample points
may be identical for each pixel in the framebuffer, or it may
differ. The locations of the sample points are fixed by the
implementation and it is not possible to query the actual sample
locations of a pixel.
If the sample patterns differ per pixel, they should be aligned to
window, not screen, boundaries. Otherwise rendering results will be
window-position specific. The invariance requirement described in
section 3.1 is relaxed for all enabled multisample rasterization,
because the sample patterns may be a function of pixel location.
3.3.2 Point Multisample Rasterization
If MULTISAMPLE_3DFX is enabled, and SAMPLE_BUFFERS_3DFX is one, then
points are rasterized using the following algorithm, regardless of
whether point antialiasing (POINT_SMOOTH) is enabled or disabled.
Point rasterization produces a fragment for each framebuffer pixel
with one or more sample points that intersect the region lying
within the circle having diameter equal to the current point width
and centered at the point's (Xw,Yw). The coverage value for each
fragment is 1. Mask bits that correspond to sample points that
intersect the circular region are 1, other mask bits are 0. All
depth values of the fragment are assigned the depth value of the
point being rasterized. The data associated with each fragment are
otherwise the data associated with the point being rasterized.
Point size range and number of gradations are equivalent to those
supported for antialiased points.
3.4.4 Line Multisample Rasterization
If MULTISAMPLE_3DFX is enabled, and SAMPLE_BUFFERS_3DFX is one, then
lines are rasterized using the following algorithm, regardless of
whether line antialiasing (LINE_SMOOTH) is enabled or disabled. Line
rasterization produces a fragment for each framebuffer pixel with
one or more sample points that intersect the rectangular region that
is described in the Antialiasing section of 3.4.2 (Other Line
Segment Features). If line stippling is enabled, the rectangular
region is subdivided into adjacent unit-length rectangles, with some
rectangles eliminated according to the procedure given under Line
Stipple, where "fragment" is replaced by "rectangle".
The coverage value for each fragment is 1. Mask bits that correspond
to sample points that intersect a retained rectangle are 1, other
mask bits are 0. Each depth value is produced by substituting the
corresponding sample location into equation 3.1, then using the
result to evaluate equation 3.3. The data associated with each
fragment are otherwise computed by evaluating equation 3.1 at the
fragment center, then substituting into equation 3.2.
Line width range and number of gradations are equivalent to those
supported for antialiased lines.
3.5.7 Polygon Multisample Rasterization
If MULTISAMPLE_3DFX is enabled, and SAMPLE_BUFFERS_3DFX is one, then
polygons are rasterized using the following algorithm, regardless of
whether polygon antialiasing (POLYGON_SMOOTH) is enabled or
disabled. Polygon rasterization produces a fragment for each
framebuffer pixel with one or more sample points that satisfy the
point sampling criteria described in section 3.5.1, including the
special treatment for sample points that lie on a polygon boundary
edge. If a polygon is culled, based on its orientation and the
CullFace mode, then no fragments are produced during rasterization.
Fragments are culled by the polygon stipple just as they are for
aliased and antialiased polygons.
The coverage value for each fragment is 1. Mask bits that correspond
to sample points that satisfy the point sampling criteria are 1,
other mask bits are 0. Each depth value is produced by substituting
the corresponding sample location into the barycentric equations
described in section 3.5.1, using the approximation to equation 3.4
that omits w components. The data associated with each fragment are
otherwise computed by barycentric evaluation using the fragment's
center point.
If POLYGON_OFFSET_FILL is enabled then the offset value computed in
equation 3.7 is added to all sample depth values.
The rasterization described above applies only to the FILL state of
PolygonMode. For POINT and LINE, the rasterizations described in
3.3.2 (Point Multisample Rasterization) and 3.4.4 (Line Multisample
Rasterization) apply with the additional requirement that the sample
depth values must be offset by the value computed in equation 3.7
when POLYGON_OFFSET_POINT or POLYGON_OFFSET_LINE is enabled.
3.6.5 Multisample Rasterization of Pixel Rectangles
If MULTISAMPLE_3DFX is enabled, and SAMPLE_BUFFERS_3DFX is one, then
pixel rectangles are rasterized using the following algorithm. Let
(Xrp,Yrp) be the current raster position. (If the current raster
position is invalid, then DrawPixels is ignored.) If a particular
group (index or components) is the nth in a row and belongs to the
mth row, consider the region in window coordinates bounded by the
rectangle with corners
(Xrp + Zx*n, Yrp + Zy*m)
and
(Xrp + Zx*(n+1), Yrp + Zy*(m+1))
where Zx and Zy are the pixel zoom factors specified by PixelZoom,
and may each be either positive or negative. A fragment representing
group n,m is produced for each framebuffer pixel with one or more
sample points that lie inside, or on the bottom or left boundary, of
this rectangle. Each fragment so produced takes its associated data
from the group and from the current raster position, in a manner
consistent with SGIX_pixel_texture (if it is implemented) or in a
manner consistent with the discussion in the Conversion to Fragments
subsection of section 3.6.4 of the GL specification. All depth
sample values are assigned the same value, taken either from the
group (if it is a depth component group) or from the current raster
position (if it is not).
A single pixel rectangle will generate multiple, perhaps very many
fragments for the same framebuffer pixel, depending on the pixel
zoom factors.
3.7.1 Bitmap Multisample Rasterization
If MULTISAMPLE_3DFX is enabled, and SAMPLE_BUFFERS_3DFX is one, then
bitmaps are rasterized using the following algorithm. If the current
raster position is invalid, the bitmap is ignored. Otherwise, a
screen-aligned array of pixel-size rectangles is constructed, with
its lower-left corner at (Xrp,Yrp), and its upper right corner a
(Xrp+w,Yrp+h), where w and h are the width and height of the bitmap.
Rectangles in this array are eliminated if the corresponding bit in
the bitmap is zero, and are retained otherwise. Bitmap rasterization
produces a fragment for each framebuffer pixel with one or more
sample points either inside or on the bottom or left edge of a
retained rectangle.
Additions to Chapter 4 of the 1.2 Specification (Per-Fragment Operations
and the Frame Buffer)
4.1.9 Multisample Fragment Operations
If the DrawBuffers mode is NONE, no change is made to any
multisample or color buffer. Otherwise, fragment processing is as
described below.
If MULTISAMPLE_3DFX is enabled, and SAMPLE_BUFFERS_3DFX is one, the
stencil test, depth test, blending, and dithering operations
described in sections 4.1.4, 4.1.5, 4.1.6, and 4.1.7 are performed
for each pixel sample, rather than just once for each fragment.
Failure of the stencil or depth test results in termination of the
processing of that sample, rather than discarding of the fragment.
All operations are performed on the color, depth, and stencil values
stored in the multisample buffer (to be described in a following
section). The contents of the color buffers are not modified at this
point.
Stencil, depth, blending, and dithering operations are performed for
a pixel sample only if that sample's fragment mask bit is 1. If the
corresponding mask bit is 0, no operations are performed for that
sample. Depth operations use the fragment depth value that is
specific to each sample. The single fragment color value is used for
all sample operations, however, as is the current stencil value.
If MULTISAMPLE_3DFX is disabled, and SAMPLE_BUFFERS_3DFX is one, the
fragment may be treated exactly as described above, with
optimization possible because the fragment mask must be all 1's.
Further optimization is allowed, however. An implementation may
choose to identify a centermost sample, and to perform stencil and
depth tests on only that sample. Regardless of the outcome of the
stencil test, all multisample buffer stencil sample values are set
to the appropriate new stencil value. If the depth test passes, all
multisample buffer depth sample values are set to the depth of the
fragment's centermost sample's depth value, and all multisample
buffer color sample values are set to the color value of the
incoming fragment. Otherwise, no change is made to any multisample
buffer color or depth value.
After all operations have been completed on the multisample buffer,
the color sample values are combined to produce a single color
value, and that value is written into each color buffer that is
currently enabled, based on the DrawBuffers mode. The method of
combination is not specified, though a simple average computed
independently for each color component is recommended.
4.2.2.5 Fine Control of Multisample Buffer Updates
When SAMPLE_BUFFERS_3DFX is one, ColorMask, DepthMask, and
StencilMask control the modification of values in the multisample
buffer. The color mask has no effect on modifications to the color
buffers. If the color mask is entirely disabled, the color sample
values must still be combined (as described above) and the result
used to replace the color values of the buffers enabled by
DrawBuffers.
4.2.3.5 Clearing the Multisample Buffer
The color samples of the multisample buffer are cleared when one or
more color buffers are cleared, as specified by the Clear mask bit
COLOR_BUFFER_BIT and the DrawBuffers mode. If the DrawBuffers mode
is NONE, the color samples of the multisample buffer cannot be
cleared.
Clear mask bits DEPTH_BUFFER_BIT and STENCIL_BUFFER_BIT indicate
that the depth and stencil samples of the multisample buffer are to
be cleared. If Clear mask bit DEPTH_BUFFER_BIT is specified, and if
the DrawBuffers mode is not NONE, then the multisample depth buffer
samples are cleared. Likewise, if Clear mask bit STENCIL_BUFFER_BIT
is specified, and if the DrawBuffers mode is not NONE, then the
multisample stencil buffer is cleared.
4.3.2 Reading Pixels
[These changes are made to the text in section 4.3.2, following the
subheading Obtaining Pixels from the Framebuffer.]
Follow the sentence "If there is no depth buffer, the error
INVALID_OPERATION occurs." with: If there is a multisample buffer
(SAMPLE_BUFFERS_3DFX is 1) then values are obtained from the depth
samples in this buffer. It is recommended that the depth value of
the centermost sample be used, though implementations may choose any
function of the depth sample values at each pixel.
Follow the sentence "if there is no stencil buffer, the error
INVALID_OPERATION occurs." with: If there is a multisample buffer,
then values are obtained from the stencil samples in this buffer. It
is recommended that the stencil value of the centermost sample be
used, though implementations may choose any function of the stencil
sample values at each pixel.
This extension makes no change to the way that color values are
obtained from the framebuffer.
Additions to Chapter 5 of the 1.2 Specification (Special Functions)
None
Additions to Chapter 6 of the 1.2 Specification (State and State Requests)
An additional group of state variables, MULTISAMPLE_BIT_3DFX, is
defined by this extension. When PushAttrib is called with bit
MULTISAMPLE_BIT_3DFX set, the multisample group of state variables
is pushed onto the attribute stack. When PopAttrib is called, these
state variables are restored to their previous values if they were
pushed. Some multisample state is included in the ENABLE_BIT group
as well. In order to avoid incompatibility with GL implementations
that do not support 3DFX_multisample, ALL_ATTRIB_BITS does not
include MULTISAMPLE_BIT_3DFX.
Additions to the GLX 1.3 Specification
The parameter GLX_SAMPLE_BUFFERS_3DFX is added to
glXGetFBConfigAttrib. When queried, by calling glXGetFBConfigAttrib
with attribute set to GLX_SAMPLE_BUFFERS_3DFX, it returns the number
of multisample buffers included in the visual. For a normal visual,
the return value is zero. A return value of one indicates that a
single multisample buffer is available. The number of samples per
pixel is queried by calling glXGetFBConfigAttrib with attribute set
to GLX_SAMPLES_3DFX. It is understood that the number of color,
depth, and stencil bits per sample in the multisample buffer are as
specified by the GLX_*_SIZE parameters. It is also understood that
there are no single-sample depth or stencil buffers associated with
this visual -- the only depth and stencil buffers are those in the
multisample buffer. GLX_SAMPLES_3DFX is zero if
GLX_SAMPLE_BUFFERS_3DFX is zero.
glXChooseFBConfig accepts GLX_SAMPLE_BUFFERS_3DFX in attrib_list,
followed by the minimum number of multisample buffers that can be
accepted. Visuals with the smallest number of multisample buffers
that meets or exceeds the specified minimum number are preferred.
Currently operation with more than one multisample buffer is
undefined, so the returned value will be either zero or one.
glXChooseFBConfig accepts GLX_SAMPLES_3DFX in attrib_list, followed
by the minimum number of samples that can be accepted in the
multisample buffer. Visuals with the smallest number of samples that
meets or exceeds the specified minimum number are preferred.
The multisample FBConfig attributes are added to table 3.4 as follows:
Attribute Default Selection Criteria Sort Priority
--------- ------- ------------------ -------------
GLX_SAMPLE_BUFFERS_3DFX 0 Smaller
GLX_SAMPLES_3DFX 0 Smaller
If the GLX implementation is 1.2 or less, then
GLX_SAMPLE_BUFFERS_3DFX and GLX_SAMPLES_3DFX are accepted by
glXChooseVisual and glXGetConfig.
If the color samples in the multisample buffer store fewer bits than
are stored in the color buffers, this fact will not be reported
accurately. Presumably a compression scheme is being employed, and
is expected to maintain an aggregate resolution equal to that of the
color buffers.
Additions to WGL
The parameters WGL_SAMPLE_BUFFERS_3DFX and WGL_SAMPLES_3DFX are
added to wglGetPixelFormatAttribivARB and
wglGetPixelFormatAttribfvARB. WGL_SAMPLE_BUFFERS_3DFX indicates the
number of multisample buffers included in the pixel format. For a
normal pixel format, the return value is zero. A return value of one
indicates that a single multisample buffer is available.
WGL_SAMPLES_3DFX gives the number of samples per pixel. It is
understood that the number of color, depth, and stencil bits per
sample in the multisample buffer are as specified by the WGL_*_BITS
parameters. It is also understood that there are no single-sample
depth or stencil buffers associated with this pixel format -- the
only depth and stencil buffers are those in the multisample buffer.
WGL_SAMPLES_3DFX is zero if WGL_SAMPLE_BUFFERS_3DFX is zero.
wglChoosePixelFormatARB accepts WGL_SAMPLE_BUFFERS_3DFX in
piAttribIList and/or pfAttribFList, followed by the minimum number
of multisample buffers that can be accepted. Pixel formats with the
smallest number of multisample buffers that meet or exceed the
specified minimum number are preferred. Currently operation with
more than one multisample buffer is undefined, so the returned value
will be either zero or one.
wglChoosePixelFormatARB accepts WGL_SAMPLES_3DFX in piAttribIList
and/or pfAttribFList, followed by the minimum number of samples that
can be accepted in the multisample buffer. Pixel formats with the
smallest number of samples that meet or exceed the specified minimum
number are preferred.
Errors
None
New State
Get Value Get Command Type Initial Value Attribute
--------- ----------- ---- ------------- ---------
MULTISAMPLE_3DFX IsEnabled B TRUE multisample/enable
New Implementation Dependent State
Get Value Get Command Type Minimum Value
--------- ----------- ---- -------------
SAMPLE_BUFFERS_3DFX GetIntegerv Z+ 0
SAMPLES_3DFX GetIntegerv Z+ 0
Dependencies on GLX 1.3 and GLX_EXT_fbconfig
If GLX 1.3 is not supported and GLX_EXT_fbconfig is not supported
then all references to glXGetFBConfigAttrib and glXGetConfig are
removed.
Revision History
Version 1.1 - April 18, 2000 (Jon Leech, SGI)
Specified value of MULTISAMPLE_BIT_3DFX. Assigned values to
GLX_SAMPLE_BUFFERS_3DFX and GLX_SAMPLES_3DFX. Conversion from
HTML -> text and formatting cleanup.