How should specular coefficient be applied?

[anotheruserofgithub]

Questions:

1. https://github.com/modelica/ModelicaStandardLibrary/blob/2308ab206ffa874188d24b64962fa42dfeb3c81f/Modelica/Mechanics/MultiBody/Types/SpecularCoefficient.mo#L2 -> Why no max=1?
2. Not all components default to world.defaultSpecularCoefficient -> Is that intentional or should it be fixed?
3. From the references below, I understand that instead of a single specular coefficient, there should be a specular color and a specular exponent (shininess), meaning, there are usually two degrees of freedom (color and shininess) to define the specular component. But with a single coefficient, I assume that it is meant to scale a white specular color, and that the shininess is fixed arbitrarily. That's how I implemented it for OpenModelica (see this commit of https://github.com/OpenModelica/OpenModelica/pull/10362), trying to get as close as possible to the example images below. Did I misunderstand something? @HansOlsson Maybe you would be able to give me a deeper insight and share how this is done in Dymola for instance?

References:

• https://learnopengl.com/Lighting/Materials §4: "The specular material vector sets the color of the specular highlight on the surface (or possibly even reflect a surface-specific color). Lastly, the shininess impacts the scattering/radius of the specular highlight."
• https://people.inf.ethz.ch/fcellier/Lect/MMPS/Refs/Dymola5Manual.pdf p.167: "Specular is a coefficient defining white specular reflection."
• https://www.maplesoft.com/documentation_center/online_manuals/modelica/Modelica_UsersGuide_ReleaseNotes.html#Modelica.UsersGuide.ReleaseNotes.Version_2_2_1 Modelica.Mechanics.MultiBody.all models: "All components with animation information have a new variable specularCoefficient to define the reflection of ambient light. The default value is world.defaultSpecularCoefficient which has a default of 0.7. By changing world.defaultSpecularCoefficient, the specularCoefficient of all components is changed that are not explicitly set differently."
• https://github.com/modelica/ModelicaStandardLibrary/blob/2308ab206ffa874188d24b64962fa42dfeb3c81f/Modelica/Mechanics/MultiBody/Types/SpecularCoefficient.mo l.3: "Reflection of ambient light (= 0: light is completely absorbed)"
• https://github.com/openscenegraph/OpenSceneGraph/blob/b4152d9634984d75d0c591957feb9db534b890a6/src/osg/Material.cpp
• http://glprogramming.com/red/chapter05.html#name7
• https://www.claytex.com/blog/radar-modelling-reflection-of-an-em-wave/
• https://www.tomdalling.com/blog/modern-opengl/07-more-lighting-ambient-specular-attenuation-gamma/
• https://en.wikipedia.org/wiki/Specular_reflection
• https://en.wikipedia.org/wiki/Specularity

Examples:

• https://github.com/modelica/ModelicaStandardLibrary/blob/2308ab206ffa874188d24b64962fa42dfeb3c81f/Modelica/Resources/Images/Mechanics/MultiBody/Types/SpecularCoefficient.png
• https://github.com/modelica/ModelicaStandardLibrary/blob/2308ab206ffa874188d24b64962fa42dfeb3c81f/Modelica/Resources/Images/Mechanics/MultiBody/Examples/Loops/EngineV6_CAD.png
• https://github.com/modelica/ModelicaStandardLibrary/blob/2308ab206ffa874188d24b64962fa42dfeb3c81f/Modelica/Resources/Images/Mechanics/MultiBody/Examples/Systems/RobotR3/r3_fullRobot.png
• https://github.com/modelica/ModelicaStandardLibrary/blob/2308ab206ffa874188d24b64962fa42dfeb3c81f/Modelica/Resources/Images/Mechanics/MultiBody/Examples/Elementary/DoublePendulum.png

[tobolar]

The usage of the specular coefficient seems not to be done consequently.

• default value by a number (not by world.defaultSpecularCoefficient)
  • Modelica.Mechanics.MultiBody.Visualizers.Rectangle
  • Modelica.Mechanics.MultiBody.Visualizers.PipeWithScalarField
  • Modelica.Mechanics.MultiBody.Visualizers.Torus
  • Modelica.Mechanics.MultiBody.Visualizers.VoluminousWheel
  • Modelica.Mechanics.MultiBody.Visualizers.Advanced.PipeWithScalarField
  • Modelica.Mechanics.MultiBody.Visualizers.Internal.Lines

(but in other visualizers, the default world.defaultSpecularCoefficient is indeed used, e.g. Modelica.Mechanics.MultiBody.Visualizers.SignalArrow

• defined but not used

  • Modelica.Mechanics.MultiBody.World (is this on purpose?)
  • Modelica.Mechanics.MultiBody.Joints.Constraints.Universal (no visualization components here)

• others (how does the matching of actual arguments for arrow visualizer works here?)

  • Modelica.Mechanics.MultiBody.Sensors.AbsoluteSensor
  • Modelica.Mechanics.MultiBody.Sensors.RelativeSensor

[anotheruserofgithub]

Any answer to question 3? Maybe @HansOlsson?

[HansOlsson]

Any answer to question 3? Maybe @HansOlsson?

As far as I can see it is a multiplicative factor for the color; so a red ball can only have various levels of red specular color. The shininess is not influenced.

[anotheruserofgithub]

Thank you! I will try that.

[anotheruserofgithub]

Well, after more experiments, it really seems to me that the specular coefficient is meant to multiply a white specular color with a fixed shininess around 8 (OpenGL). Explicit support for a shininess input in MSL would be great, otherwise vendor-specific annotations can be put in place if there is any use case for that.

[tobolar]

@anotheruserofgithub Feel free to suggest a necessary documentation in e.g. Modelica.Mechanics.MultiBody.UsersGuide.Tutorial.