EXT_point_parameters
GL_EXT_point_parameters
$Date: 1997/08/21 21:26:36 $ $Revision: 1.6 $
54
SGIS_multisample affects the definition of this extension.
This extension supports additional geometric characteristics of points. It
can be used to render particles or tiny light sources, commonly referred
as "Light points".
The raster brightness of a point is a function of the point area, point
color, point transparency, and the response of the display's electron gun
and phosphor. The point area and the point transparency are derived from the
point size, currently provided with the <size> parameter of glPointSize.
The primary motivation is to allow the size of a point to be affected by
distance attenuation. When distance attenuation has an effect, the final
point size decreases as the distance of the point from the eye increases.
The secondary motivation is a mean to control the mapping from the point
size to the raster point area and point transparency. This is done in order
to increase the dynamic range of the raster brightness of points. In other
words, the alpha component of a point may be decreased (and its transparency
increased) as its area shrinks below a defined threshold.
This extension defines a derived point size to be closely related to point
brightness. The brightness of a point is given by:
1
dist_atten(d) = -------------------
a + b * d + c * d^2
brightness(Pe) = Brightness * dist_atten(|Pe|)
where 'Pe' is the point in eye coordinates, and 'Brightness' is some initial
value proportional to the square of the size provided with glPointSize. Here
we simplify the raster brightness to be a function of the rasterized point
area and point transparency.
brightness(Pe) brightness(Pe) >= Threshold_Area
area(Pe) =
Threshold_Area Otherwise
factor(Pe) = brightness(Pe)/Threshold_Area
alpha(Pe) = Alpha * factor(Pe)
where 'Alpha' comes with the point color (possibly modified by lighting).
'Threshold_Area' above is in area units. Thus, it is proportional to the
square of the threshold provided by the programmer through this extension.
The new point size derivation method applies to all points, while the
threshold applies to multisample points only.
* Does point alpha modification affect the current color ?
No.
* Do we need a special function glGetPointParameterfvEXT, or get by with
glGetFloat ?
No.
* If alpha is 0, then we could toss the point before it reaches the
fragment stage.
No. This can be achieved with enabling the alpha test with reference of
0 and function of LEQUAL.
* Do we need a disable for applying the threshold ? The default threshold
value is 1.0. It is applied even if the point size is constant.
If the default threshold is not overriden, the area of multisample
points with provided constant size of less than 1.0, is mapped to 1.0,
while the alpha component is modulated accordingly, to compensate for
the larger area. For multisample points this is not a problem, as there
are no relevant applications yet. As mentioned above, the threshold does
not apply to alias or antialias points.
The alternative is to have a disable of threshold application, and state
that threshold (if not disabled) applies to non antialias points only
(that is, alias and multisample points).
The behavior without an enable/disable looks fine.
* Future extensions (to the extension)
1. GL_POINT_FADE_ALPHA_CLAMP_EXT
When the derived point size is larger than the threshold size defined by
the GL_POINT_FADE_THRESHOLD_SIZE_EXT parameter, it might be desired to
clamp the computed alpha to a minimum value, in order to keep the point
visible. In this case the formula below change:
factor = (derived_size/threshold)^2
factor clamp <= factor
clamped_value =
clamp factor < clamp
1.0 derived_size >= threshold
alpha *=
clamped_value Otherwise
where clamp is defined by the GL_POINT_FADE_ALPHA_CLAMP_EXT new parameter.
   New Procedures and Functions |
void glPointParameterfEXT ( GLenum pname, GLfloat param );
void glPointParameterfvEXT ( GLenum pname, GLfloat *params );
Accepted by the <pname> parameter of glPointParameterfEXT, and the <pname>
of glGet:
GL_POINT_SIZE_MIN_EXT
GL_POINT_SIZE_MAX_EXT
GL_POINT_FADE_THRESHOLD_SIZE_EXT
Accepted by the <pname> parameter of glPointParameterfvEXT, and the <pname>
of glGet:
GL_POINT_SIZE_MIN_EXT 0x8126
GL_POINT_SIZE_MAX_EXT 0x8127
GL_POINT_FADE_THRESHOLD_SIZE_EXT 0x8128
GL_DISTANCE_ATTENUATION_EXT 0x8129
| Additions to Chapter 2 of the 1.0 Specification (OpenGL Operation) |
None
| Additions to Chapter 3 of the 1.0 Specification (Rasterization) |
All parameters of the glPointParameterfEXT and glPointParameterfvEXT
functions set various values applied to point rendering. The derived point
size is defined to be the <size> provided with glPointSize modulated with a
distance attenuation factor.
The parameters GL_POINT_SIZE_MIN_EXT and GL_POINT_SIZE_MAX_EXT simply
define an upper and lower bounds respectively on the derived point size.
The above parameters affect non multisample points as well as multisample
points, while the GL_POINT_FADE_THRESHOLD_SIZE_EXT parameter, has no effect
on non multisample points. If the derived point size is larger than
the threshold size defined by the GL_POINT_FADE_THRESHOLD_SIZE_EXT
parameter, the derived point size is used as the diameter of the rasterized
point, and the alpha component is intact. Otherwise, the threshold size is
set to be the diameter of the rasterized point, while the alpha component is
modulated accordingly, to compensate for the larger area.
The distance attenuation function coefficients, namely a, b, and c in:
1
dist_atten(d) = -------------------
a + b * d + c * d^2
are defined by the <pname> parameter GL_DISTANCE_ATTENUATION_EXT of the
function glPointParameterfvEXT. By default a = 1, b = 0, and c = 0.
Let 'size' be the point size provided with glPointSize, let 'dist' be the
distance of the point from the eye, and let 'threshold' be the threshold size
defined by the GL_POINT_FADE_THRESHOLD_SIZE parameter of
glPointParameterfEXT. The derived point size is given by:
derived_size = size * sqrt(dist_atten(dist))
Note that when default values are used, the above formula reduces to:
derived_size = size
the diameter of the rasterized point is given by:
derived_size derived_size >= threshold
diameter =
threshold Otherwise
The alpha of a point is calculated to allow the fading of points instead of
shrinking them past a defined threshold size. The alpha component of the
rasterized point is given by:
1 derived_size >= threshold
alpha *=
(derived_size/threshold)^2 Otherwise
The threshold defined by GL_POINT_FADE_THRESHOLD_SIZE_EXT is not clamped
to the minimum and maximum point sizes.
Points do not affect the current color.
This extension doesn't change the feedback or selection behavior of points.
| Additions to Chapter 4 of the 1.0 Specification (Per-Fragment Operations and the Framebuffer) |
None
| Additions to Chapter 5 of the 1.0 Specification (Special Functions) |
None
| Additions to Chapter 6 of the 1.0 Specification (State and State Requests) |
None
| Additions to the GLX Specification |
None
| Dependencies on SGIS_multisample |
If SGIS_multisample is not implemented, then the references to
multisample points are invalid, and should be ignored.
INVALID_ENUM is generated if PointParameterfEXT parameter <pname> is not
GL_POINT_SIZE_MIN_EXT, GL_POINT_SIZE_MAX_EXT, or
GL_POINT_FADE_THRESHOLD_SIZE_EXT.
INVALID_ENUM is generated if PointParameterfvEXT parameter <pname> is
not GL_POINT_SIZE_MIN_EXT, GL_POINT_SIZE_MAX_EXT,
GL_POINT_FADE_THRESHOLD_SIZE_EXT, or GL_DISTANCE_ATTENUATION_EXT
INVALID_VALUE is generated when values are out of range according to:
<pname> valid range
-------- -----------
GL_POINT_SIZE_MIN_EXT >= 0
GL_POINT_SIZE_MAX_EXT >= 0
GL_POINT_FADE_THRESHOLD_SIZE_EXT >= 0
Issues
------
- should we generate INVALID_VALUE or just clamp?
Get Value Get Command Type Initial Value Attribute
--------- ----------- ---- --------- ---------
GL_POINT_SIZE_MIN_EXT GetFloatv R 0 point
GL_POINT_SIZE_MAX_EXT GetFloatv R M point
GL_POINT_FADE_THRESHOLD_SIZE_EXT GetFloatv R 1 point
GL_DISTANCE_ATTENUATION_EXT GetFloatv 3xR (1,0,0) point
M is the largest available point size.
| New Implementation Dependent State |
None
Backwards Compatibility
This extension replaces SGIS_point_parameters. The procedures, tokens,
and name strings now refer to EXT instead of SGIS. Enumerant values are
unchanged. SGI implementations which previously provided this
functionality should support both forms of the extension.
functionality should support both forms of the extension.
 |  | Last update: November 14, 2006.
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