GL_EXT_point_parameters |

- Name
- Name Strings
- Version
- Number
- Dependencies
- Overview
- Issues
- New Procedures and Functions
- New Tokens
- Additions to Chapter 2 of the 1.0 Specification (OpenGL Operation)
- Additions to Chapter 3 of the 1.0 Specification (Rasterization)
- Additions to Chapter 4 of the 1.0 Specification (Per-Fragment Operations and the Framebuffer)
- Additions to Chapter 5 of the 1.0 Specification (Special Functions)
- Additions to Chapter 6 of the 1.0 Specification (State and State Requests)
- Additions to the GLX Specification
- Dependencies on SGIS_multisample
- Errors
- New State
- New Implementation Dependent State

Name |

EXT_point_parameters

Name Strings |

GL_EXT_point_parameters

Version |

$Date: 1997/08/21 21:26:36 $ $Revision: 1.6 $

Number |

54

Dependencies |

SGIS_multisample affects the definition of this extension.

Overview |

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.

Issues |

* 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 );

New Tokens |

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.

Errors |

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?

New State |

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.

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