Weight

The following page provides advanced information on the Weight parameter.

Note: : It is highly recommended to set Weight to true (except in interactive simulation) for faster simulation convergence rate. Deactivating this option can be useful to understand certain phenomena as absorption.

Weight Description

The Weight represents the amount of energy a ray propagates. In real life, a ray looses some energy (power) when it interacts with an object.

When a ray is emitted by a source in a direct simulation or by a sensor in inverse simulation, it starts with a weight = 1.

Weight is unit less:
  • In Direct simulation, Weight is converted to actual energy at the end of the simulation when normalizing the results.
  • In Inverse simulation, Weight is not energy: Speos propagates an "importance" from the sensor to sources. Energy actually flows the other way around.
Weight behaves differently during the ray propagation:
  • Weight activated: when the ray hits a surface with 20% absorption, the ray is always reflected (or transmitted) with 80% of its weight. At the beginning the ray is 1, at the end the ray is 0.8.
  • Weight deactivated: the same ray hitting the same surface would have 20% chances to be stopped and fully absorbed. If the ray is not stopped, it would keep 100% of its initial weight.
In other words, statistically for 100 rays you will have:
  • Weight activated: 100% of the ray are reflected, and each ray have a weight of 0.8.
  • Weight deactivated: 20% of the rays stop their propagation on the spot. 80% carry on with their propagation with weight = 1.
Weight activated Weight deactivated




In case of a Ray/Volume interaction, let’s consider rays propagating inside an absorbing material.
  • Weight activated: rays' energy decreases exponentially according to the material absorption value and the path of rays through it.
  • Weight deactivated: rays have a probability to be absorbed or transmitted according to their path through the material.

Weight and Minimum energy percentage

Minimum energy percentage is useful only when Weight is activated.

The Minimum energy percentage parameter defines the minimum energy threshold to continue to propagate a ray with weight. This parameter helps the solver to better converge according the simulated lighting system.

In details: at some point ,Speos might propagate rays with very low energy: this is computationally expensive. Besides, these rays might have low significance for the result. Thus, using the parameter Minimum energy percentage, when the weight goes below 0.005 (the default 0.5% value) then the propagation behavior changes. A ray now now behaves as if Weight was deactivated: this stops the weight from decreasing any further and gives the ray a probability to be stopped or to continue propagating according to the probability laws.

Weight Deactivation

Deactivating the weight is useful in two specific cases.

  • When you analyze phenomena such as absorption. Considering a material with absorption, here is the observation of the absorbed rays using an interactive simulation.

    Interactive simulation result when weight is activated Interactive simulation result when weight is deactivated

  • If you want a simulation performance improvement in a closed system.

    Let us consider an integrating sphere with a light source and sensor inside of it.

    The surface inside the sphere has a high reflectivity value. The system is set so the sensor is protected from direct illumination from the light source.



    In this context, activating the Weight would highly extend simulation time.

    When weight is activated, simulation time corresponds to 1747.

    When weight is deactivated, simulation time corresponds to 440.

    This difference is due to the fact that low energy rays are still propagating after several bounds in the system for simulations using weight whereas the probability the rays still propagate decreases each bound they make for simulations not using weight.