Name

    ARB_point_parameters

Name Strings

    GL_ARB_point_parameters

Contact

    Michael Gold, NVIDIA (gold 'at' nvidia.com)

Notice

    Copyright (c) 2000-2013 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Status

    Approved by the ARB, 21 June 2000.

Version

    Revision Date: September 23, 2010
    Version: 0.6

    Based on:  EXT_point_parameters
               $Date: 1997/08/21 21:26:36 $ $Revision: 1.6 $

Number

    ARB Extension #14

Dependencies

    OpenGL 1.0 is required.
    ARB_multisample affects the definition of this extension.
    The extension is written against the OpenGL 1.2.1 Specification.

Overview

    This extension supports additional geometric characteristics of
    points. It can be used to render particles or tiny light sources,
    commonly referred to 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
    PointSize. 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.

IP Status

    None.

Issues

    * Does point alpha modification affect the current color ?

      No.

    * Do we need a special function GetPointParameterfvARB, or get by
      with GetFloat ?

      GetFloat is sufficient.

    * 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 overridden, 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. POINT_FADE_ALPHA_CLAMP_ARB

      When the derived point size is larger than the threshold size
      defined by the POINT_FADE_THRESHOLD_SIZE_ARB 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 POINT_FADE_ALPHA_CLAMP_ARB new
      parameter.
 
New Procedures and Functions

    void PointParameterfARB(enum pname,
                            float param);
    void PointParameterfvARB(enum pname,
                             const float *params);

New Tokens

    Accepted by the <pname> parameter of PointParameterfARB, and the
    <pname> of Get:

        POINT_SIZE_MIN_ARB
        POINT_SIZE_MAX_ARB
        POINT_FADE_THRESHOLD_SIZE_ARB

    Accepted by the <pname> parameter of PointParameterfvARB, and the
    <pname> of Get:

        POINT_SIZE_MIN_ARB              0x8126
        POINT_SIZE_MAX_ARB              0x8127
        POINT_FADE_THRESHOLD_SIZE_ARB   0x8128
        POINT_DISTANCE_ATTENUATION_ARB  0x8129

Additions to Chapter 2 of the OpenGL 1.2.1 Specification (OpenGL Operation)

    None

Additions to Chapter 3 of the OpenGL 1.2.1 Specification (Rasterization)

    In section 3.3, the following is inserted after the description of
    PointSize:

    The point size is multiplied with a distance attenuation factor
    and clamped as follows:

        derived_size = Clamp(size * sqrt(dist_atten(dist)))

    where dist_atten is specified as

                                 1
        dist_atten(d) = -------------------
                        a + b * d + c * d^2

    and 'd' is the eye-coordinate distance from the eye, (0, 0, 0, 1) in
    eye coordinates, to the vertex.

    The derived point size is clamped to a given range, and then
    clamped to the implementation-dependent point size range.

    If multisampling is enabled, an implementation may optionally fade
    the point alpha (section 3.12) instead of allowing the size to go
    below a given threshold.  In this case, the diameter of the
    rasterized point is

                   derived_size                 derived_size >= threshold
        diameter = 
                   threshold                    Otherwise

    and the fade factor is computed as follows:

                 1                              derived_size >= threshold
        fade =
                 (derived_size/threshold)^2     Otherwise

    The distance attenuation function coefficients, 'a', 'b', and 'c',
    the bounds of the clamp, and the point fade 'threshold', are
    specified with

        void PointParameterfARB( enum pname, float param );
        void PointParameterfvARB( enum pname, const float *params );

    If <pname> is POINT_SIZE_MIN_ARB or POINT_SIZE_MAX_ARB, then
    <param> specifies, or <params> points to the lower or upper bound
    respectively on the derived point size.  If the lower bound is
    greater than the upper bound, the resulting point size is
    undefined.  If <pname> is POINT_DISTANCE_ATTENUATION_ARB, then
    <params> points to the coefficients 'a', 'b', and 'c'.  If <pname>
    is POINT_FADE_THRESHOLD_SIZE_ARB, <param> specifies, or <params>
    points to the point fade threshold.

    This extension doesn't change the feedback or selection behavior of
    points.

    In section 3.11, the word "Finally" is removed from the first
    sentence.

    Add the following after section 3.11.

    Section 3.12  Multisample Point Fade

    If multisampling is enabled and the rasterized fragment results
    from a point primitive, then the computed fade factor is applied
    to the fragment.  In RGBA mode, the fade factor is multiplied by
    the fragment's alpha (A) value to yield a final alpha value.  In
    color index mode, the fade factor has no effect.

Additions to Chapter 4 of the OpenGL 1.2.1 Specification (Per-Fragment
Operations and the Frame Buffer)

    None

Additions to Chapter 5 of the OpenGL 1.2.1 Specification (Special Functions)

    None

Additions to Chapter 6 of the OpenGL 1.2.1 Specification (State and
State Requests)

    None

Additions to Appendix A of the OpenGL 1.2.1 Specification (Invariance)

    None

Additions to the GLX / WGL / AGL Specifications

    None

GLX Protocol

    Two new GL rendering commands are added. The following commands are
    sent to the server as part of a glXRender request:

        PointParameterfARB
            2           8+4*n           rendering command length
            2           2065            rendering command opcode
            4           ENUM            pname
                        0x8126    n=1   POINT_SIZE_MIN_ARB
                        0x8127    n=1   POINT_SIZE_MAX_ARB
                        0x8128    n=1   POINT_FADE_THRESHOLD_SIZE_ARB
            4           FLOAT32         param

        PointParameterfvARB
            2           8+4*n           rendering command length
            2           2066            rendering command opcode
            4           ENUM            pname
                        0x8126    n=1   POINT_SIZE_MIN_ARB
                        0x8127    n=1   POINT_SIZE_MAX_ARB
                        0x8128    n=1   POINT_FADE_THRESHOLD_SIZE_ARB
                        0x8129    n=3   POINT_DISTANCE_ATTENUATION_ARB
            4*n         LISTofFLOAT32   params

Dependencies on ARB_multisample

    If ARB_multisample is not implemented, then the references to
    multisample points are invalid, and should be ignored.

Errors

    INVALID_ENUM is generated if PointParameterfARB parameter <pname> is
    not POINT_SIZE_MIN_ARB, POINT_SIZE_MAX_ARB, or
    POINT_FADE_THRESHOLD_SIZE_ARB.

    INVALID_ENUM is generated if PointParameterfvARB parameter <pname>
    is not POINT_SIZE_MIN_ARB, POINT_SIZE_MAX_ARB,
    POINT_FADE_THRESHOLD_SIZE_ARB, or POINT_DISTANCE_ATTENUATION_ARB

    INVALID_VALUE is generated when values are out of range according
    to:

    <pname>                                 valid range
    --------                                -----------
    POINT_SIZE_MIN_ARB                      >= 0
    POINT_SIZE_MAX_ARB                      >= 0
    POINT_FADE_THRESHOLD_SIZE_ARB           >= 0


New State

(table 6.11, p. 201)
                                                                Initial
    Get Value                           Type    Get Command     Value   Description Sec.    Attribute
    ---------                           ----    -----------     ------- ----------- ----    ---------
    POINT_SIZE_MIN_ARB                  R+      GetFloatv       0.0     Attenuated  3.3     point
                                                                        Min point
                                                                        size

    POINT_SIZE_MAX_ARB                  R+      GetFloatv       M       Attenuated  3.3     point
                                                                        Max point
                                                                        size

    POINT_FADE_THRESHOLD_SIZE_ARB       R+      GetFloatv       1.0     Threshold   3.3     point
                                                                        for alpha
                                                                        attenuation

    POINT_DISTANCE_ATTENUATION_ARB      3xR     GetFloatv       (1.0,0.0,0.0)       3.3     point
                                                                        Attenuation
                                                                        coefficients

    M is the larger of the max antialiased and non antialiased point
    sizes.

New Implementation Dependent State

    None

Revision History

    11/09/1999  0.1
        - First ARB draft based on the original SGI and EXT drafts.

    12/07/1999  0.2
        - clarified behavior when POINT_SIZE_MIN exceeds POINT_SIZE_MAX
        - clarified when the point size is clamped to the supported range
        - removed issues from "Errors" section
        - fixed various typos
        - Updated to new extension template
        - added GLX protocol

    04/20/2000  0.3
        - rewritten to fit within the context of the 1.2 specification
        - added language describing where the fade alpha is applied.
        - added language which indicates that some implementations may not
          implement POINT_FADE_THRESHOLD_SIZE_ARB

    06/20/2000  0.4
        - removed alternate behavior for fade alpha, since it is optional
        - added new section describing fade alpha application

    03/12/2002  0.5
        - added GLX protocol for PointParameterfARB and assigned ropcodes

    09/23/2010  0.6 (pbrown)
        - add "const" qualifier to <params> argument to PointParameterfvARB