Scene Settings
Scene scale and epsilons
In Predict Engine, the scene itself has some characteristics :
The scale of the scene sets the scale of the geometries it contains (in meters, centimeters,...).
The direct and indirect lighting epsilons are parameters required the path tracing algorithm : they define the precision in the intersection computations in the scene.
Be aware that a wrong value of the epsilons may generate artifacts in the scene such as dark triangles or very dark shading zones. See next section for more details on the epsilons visual artifacts.
The scene settings can be defined in a UVR Scene Settings component placed anywhere in the scene. If there is no Scene Settings component in the scene, default settings will be used.
The default scene settings can be defined in the PredictSuite Preferences, section Engine Settings.
A default asset with the UVR Scene Settings component can be instantiated in the scene via the “GameObject/PredictSuite/Settings” menu.
Visual artefacts due to epsilons
Ray tracing techniques are plagued by self intersection problems since the beginning. A widespread workaround is to use a very small value, the so-called epsilon, to carefully offset the ray origin along the surface normal.
At the moment, Predictive Engine uses this solution. We have made a difference between the indirect and direct light ray epsilon to allow more flexibility.
Predictive Engine uses the default values of 1e-4 for the direct light epsilon and 1e-5 for the indirect light epsilon. If you observe artifacts in your rendering, here is a heuristic to follow in order to fix your epsilon problems :
Make sure you use properly tessellated geometries in your scene since it can lead to shadow terminator problem on curved surfaces (see scene (1)).
Progressively increase the direct light epsilon. Be careful not to put a too big epsilon since it will lead the intersector to ignore some surfaces which are too close to each other (see scene (3) : the plane below the glass cube is ignored by the intersector, see scene (4) : the area light plane is ignored). A too small epsilon can lead to numerical instability, so we do not advise to go below 1e-5 (see scene (2)).
Progressively increase the indirect light epsilon. The recommendations of the step above are valid here too.
Scene (1) : poorly tessellated geometry and too small direct light epsilon : 1e-6 on the left, 1e-1 on the right
Scene (2) : too small direct light epsilon : 1e-8 on the left, 1e-5 on the right
Scene (3) : too big indirect light epsilon : 1e-1 on the left, 1e-6 on the right
Scene (4) : too big direct light epsilon : 1e-5 on the left, 1e-6 on the right
If you can't find a good epsilon to get rid of all the artefacts in your scene, or if you see in the scene artefacts for an epsilon that is too small and too big at the same time, it can be caused by a bad placement of the geometries in their local framework.
Geometries should be centered on their local framework origin : when triangles are defined far from the origin (in the scene unit), it can introduce intersection artefacts. If the geometry is already centered on the local origin but is just very big, consider separating the geometry into several smaller meshes, all centered on their local origin.
If possible, it is also preferable to use a bigger scene scale (meter instead of cm or mm) in order to reduce the intersection artefacts. If you do so, make sure your materials (specular material especially) are defined in the right unit as well.
If the procedure does not work for you, do not hesitate to contact the UVR team to help you. In the future, one of our high priorities will be to develop and make available to our customers a robust and tweak free (without epsilons) method.