ARB_point_sprite
GL_ARB_point_sprite
Matt Craighead, NVIDIA Corporation (mcraighead 'at' nvidia.com)
Mark Kilgard, NVIDIA Corporation (mjk 'at' nvidia.com)
Pat Brown, NVIDIA Corporation (pbrown 'at' nvidia.com)
No known IP issues.
Approved by the ARB on July 24, 2003.
Last Modified Date: July 22, 2003
Revision: 7
ARB Extension #35
Written based on the wording of the OpenGL 1.4 specification.
NV_point_sprite affects the definition of this extension.
Applications such as particle systems have tended to use OpenGL quads
rather than points to render their geometry, since they would like
to use a custom-drawn texture for each particle, rather than the
traditional OpenGL round antialiased points, and each fragment in
a point has the same texture coordinates as every other fragment.
Unfortunately, specifying the geometry for these quads can be
expensive, since it quadruples the amount of geometry required, and
may also require the application to do extra processing to compute
the location of each vertex.
The purpose of this extension is to allow such applications to use
points rather than quads. When GL_POINT_SPRITE_ARB is enabled,
the state of point antialiasing is ignored. For each texture unit,
the app can then specify whether to replace the existing texture
coordinates with point sprite texture coordinates, which are
interpolated across the point.
* Should this spec say that point sprites get converted into quads?
RESOLVED: No, this would make the spec much uglier, because then
we'd have to say that polygon smooth and stipple get turned off,
etc. Better to provide a formula for computing the texture
coordinates and leave them as points.
* How are point sprite texture coordinates computed?
RESOLVED: They move smoothly as the point moves around on the
screen, even though the pixels touched by the point do not. The
exact formula is given in the spec below.
A point sprite can be thought of as a quad whose upper-left corner has
(s,t) texture coordinates of (0,0) and whose lower-right corner has
texture coordinates of (1,1), as illustrated in the following figure.
In the figure "P" is the center of the point sprite, and "O" is the
origin (0,0) of the window coordinate system. Note that the y window
coordinate increases from bottom-to-top but the t texture coordinate
of point sprites increases from top-to-bottom.
^
+y| (0,0)
| +-----+
| | |
| | P |
| | |
| +-----+
| (1,1)
| +x
O--------------->
Applications using a single texture for both point sprites and other
geometry need to account for the fixed coordinate mapping of point
sprites.
* Is the ARB specification different from the NV version?
RESOLVED: Yes. The point sprite R mode has been removed. The
wording has also been updated to reflect version 1.4 of the core
OpenGL specification however. The enumerant values are unchanged.
* How do point sizes for point sprites work?
RESOLVED: This specification treats point sprite sizes like
antialiased point sizes, but with more leniency. Implementations
may choose to not clamp the point size to the antialiased point
size range. The set of point sprite sizes available must be
a superset of the antialiased point sizes. However, whereas
antialiased point sizes are all evenly spaced by the point size
granularity, point sprites can have an arbitrary set of sizes.
This lets implementations use, e.g., floating-point sizes.
* Should there be a way to query the list of supported point sprite
sizes?
RESOLVED: No. If an implementation were to use, say, a single-
precision IEEE float to represent point sizes, the list would be
rather long.
* Do mipmaps apply to point sprites?
RESOLVED: Yes. They are similar to quads in this respect.
* What of this extension's state is per-texture unit and what
of this extension's state is state is global?
RESOLVED: The GL_POINT_SPRITE_ARB enable is global. The
COORD_REPLACE_ARB state is per-texture unit (state set by TexEnv is
per-texture unit).
* Should there be a global on/off switch for point sprites, or
should the per-unit enable imply that switch?
RESOLVED: There is a global switch to turn it on and off. This
is probably more convenient for both driver and app, and it
simplifies the spec.
* What should the TexEnv mode for point sprites be called?
RESOLVED: COORD_REPLACE_ARB.
* What is the interaction with multisample points, which are round?
RESOLVED: Point sprites are rasterized as squares, even in
multisample mode. Leaving them as round points would make the
feature useless.
* How does the point sprite extension interact with fragment
program extensions (ARB_fragment_program, NV_fragment_program,
etc)?
RESOLVED: The primary issue is how the interpolated texture
coordinate set appears when fragment attribute variables
(ARB terminology) or fragment program attribute registers (NV
terminology) are accessed.
When point sprite is enabled and the GL_COORD_REPLACE_ARB state for
a given texture unit is GL_TRUE, the texture coordinate set for
that texture unit is (s,t,0,1) where the point sprite-overridden
s and t are described in the amended Section 3.3 below. The
important point is that r and q are forced to 0 and 1, respectively.
For fragment program extensions, r and q correspond to the z and w
components of the respective fragment attribute.
* How does this extension interact with PolygonMode?
RESOLVED: If a polygon is rendered in point mode and POINT_SPRITE_ARB
is enabled, its vertices will be rendered as point sprites.
* How does this extension interact with the point size attenuation
functionality in ARB_point_parameters and OpenGL 1.4?
RESOLVED: Point sprites sizes are attenuated just like the sizes of
non-sprite points.
* What push/pop attribute bits control the state of this extension?
RESOLVED: POINT_BIT for all the state. Also ENABLE_BIT for
the POINT_SPRITE_ARB enable.
* How are point sprites clipped?
RESOLVED: Point sprites are transformed as points, and standard point
clipping operations are performed. This can cause point sprites that
move off the edge of the screen to disappear abruptly, in the same way
that regular points do. As with any other primitive, standard
per-fragment clipping operations (scissoring, window ownership test)
still apply.
   New Procedures and Functions |
None
Accepted by the <cap> parameter of Enable, Disable, and IsEnabled, by
the <pname> parameter of GetBooleanv, GetIntegerv, GetFloatv, and
GetDoublev, and by the <target> parameter of TexEnvi, TexEnviv,
TexEnvf, TexEnvfv, GetTexEnviv, and GetTexEnvfv:
POINT_SPRITE_ARB 0x8861
When the <target> parameter of TexEnvf, TexEnvfv, TexEnvi, TexEnviv,
GetTexEnvfv, or GetTexEnviv is POINT_SPRITE_ARB, then the value of
<pname> may be:
COORD_REPLACE_ARB 0x8862
When the <target> and <pname> parameters of TexEnvf, TexEnvfv,
TexEnvi, or TexEnviv are POINT_SPRITE_ARB and COORD_REPLACE_ARB
respectively, then the value of <param> or the value pointed to by
<params> may be:
FALSE
TRUE
| Additions to Chapter 2 of the OpenGL 1.4 Specification (OpenGL Operation) |
None.
| Additions to Chapter 3 of the OpenGL 1.4 Specification (Rasterization) |
Insert the following paragraphs after the second paragraph of section
3.3 (page 66):
"Point sprites are enabled or disabled by calling Enable or Disable
with the symbolic constant POINT_SPRITE_ARB. The default state is for
point sprites to be disabled. When point sprites are enabled, the
state of the point antialiasing enable is ignored.
The point sprite texture coordinate replacement mode is set with one
of the commands
void TexEnv{if}(enum target, enum pname, T param)
void TexEnv{if}v(enum target, enum pname, const T *params)
where target is POINT_SPRITE_ARB and pname is COORD_REPLACE_ARB. The
possible values for param are FALSE and TRUE. The default value for
each texture unit is for point sprite texture coordinate replacement
to be disabled."
Replace the first two sentences of the second paragraph of section
3.3.1 (page 67) with the following:
"The effect of a point width other than 1.0 depends on the state of
point antialiasing and point sprites. If antialiasing and point
sprites are disabled, ..."
Replace the first sentences of the fourth paragraph of section 3.3.1
(page 68) with the following:
"If antialiasing is enabled and point sprites are disabled, ..."
Insert the following paragraphs at the end of section 3.3.1 (page
70):
"When point sprites are enabled, then point rasterization produces a
fragment for each framebuffer pixel whose center lies inside a square
centered at the point's (x_w, y_w), with side length equal to the
current point size.
All fragments produced in rasterizing a point sprite are assigned the
same associated data, which are those of the vertex corresponding to
the point, with texture coordinates s, t, and r replaced with s/q,
t/q, and r/q, respectively. If q is less than or equal to zero, the
results are undefined. However, for each texture unit where
COORD_REPLACE_ARB is TRUE, these texture coordinates are replaced
with point sprite texture coordinates. The s coordinate varies
from 0 to 1 across the point horizontally left-to-right, while
the t coordinate varies from 0 to 1 vertically top-to-bottom.
The r and q coordinates are replaced with the constants 0 and 1,
respectively.
The following formula is used to evaluate the s and t coordinates:
s = 1/2 + (x_f + 1/2 - x_w) / size
t = 1/2 - (y_f + 1/2 - y_w) / size
where size is the point's size, x_f and y_f are the (integral) window
coordinates of the fragment, and x_w and y_w are the exact, unrounded
window coordinates of the vertex for the point.
The widths supported for point sprites must be a superset of those
supported for antialiased points. There is no requirement that these
widths must be equally spaced. If an unsupported width is requested,
the nearest supported width is used instead."
Replace the text of section 3.3.2 (page 70) with the following:
"The state required to control point rasterization consists of the
floating-point point width, three floating-point values specifying
the minimum and maximum point size and the point fade threshold size,
three floating-point values specifying the distance attenuation
coefficients, a bit indicating whether or not antialiasing is
enabled, a bit indicating whether or not point sprites are enabled,
and a bit for the point sprite texture coordinate replacement mode
for each texture unit."
Replace the text of section 3.3.3 (page 70) with the following:
"If MULTISAMPLE is enabled, and the value of SAMPLE_BUFFERS 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 a region centered at
the point's (x_w, y_w). This region is a circle having diameter
equal to the current point width if POINT_SPRITE_ARB is disabled, or
a square with side equal to the current point width if
POINT_SPRITE_ARB is enabled. Coverage bits that correspond to sample
points that intersect the region are 1, other coverage bits are 0.
All data associated with each sample for the fragment are the data
associated with the point being rasterized, with the exception of
texture coordinates when POINT_SPRITE_ARB is enabled; these texture
coordinates are computed as described in section 3.3.
Point size range and number of gradations are equivalent to those
supported for antialiased points when POINT_SPRITE_ARB is disabled.
The set of point sizes supported is equivalent to those for point
sprites without multisample when POINT_SPRITE_ARB is enabled."
| Additions to Chapter 4 of the OpenGL 1.4 Specification (Per-Fragment Operations and the Frame Buffer) |
None.
| 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.
| Interactions with NV_point_sprite |
ARB_point_sprite is a functional subset of NV_point_sprite.
The only functional difference between the extensions is that
NV_point_sprite provides the POINT_SPRITE_R_MODE_NV control. This mode
allows applications to specify how the r texture coordinates for point
sprites are replaced. The r coordinate can be replaced with the
corresponding s texture coordinate ("S" mode), left unchanged ("R" mode),
or replaced with the constant zero ("ZERO" mode). ARB_point_sprite always
replaces r texture coordiantes of point sprites with zero.
Since ARB_point_sprite is functionally compatible with the default r mode
from NV_point_sprite, the two extensions can coexist nicely. Enumerant
values from NV_point_sprite are reused.
If NV_point_sprite is supported, the language describing the replacement
of r coordinates for point sprites (forced to zero) is replaced with the
corresponding language from NV_point_sprite (controlled by
POINT_SPRITE_R_MODE_NV).
None.
(table 6.12, p. 220)
Get Value Type Get Command Initial Value Description Sec Attribute
--------- ---- ----------- ------------- ----------- ------ ---------
POINT_SPRITE_ARB B IsEnabled False point sprite enable 3.3 point/enable
(table 6.17, p. 225)
Get Value Type Get Command Initial Value Description Sec Attribute
--------- ---- ----------- ------------- ----------- ------ ---------
COORD_REPLACE_ARB 2* x B GetTexEnviv False coordinate replacement 3.3 point
enable
Initially based on the NV_point_sprite specification but updated for
OpenGL 1.4.
Rev. Date Author Changes
---- -------- -------- --------------------------------------------
7 07/22/03 pbrown Marked point parameter issue resolved.
6 07/18/03 pbrown Removed POINT_SPRITE_R_MODE_CONTROL.
Expanded on spec issue documenting the inversion
of the "t" texture coordinate relative to the
"y" window coordinate. Added issues on
interaction with PolygonMode, clipping, and
point parameters. Documented interaction with
NV_point_sprite. Removed now unneeded point
parameter interaction section and GLX protocol.
parameter interaction section and GLX protocol.
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