FEATURE IN DEVELOPMENT STAGE
Preview image by Xpert Mihai of www.maxwellzone.com
Traditionally, having a dielectric object enclosed inside another dielectric object (like a liquid inside a glass) is a complex geometric situation, mainly because of defining the surface that delimits those mediums. As the liquid fills all the volume leaving no gap, that limit surface becomes geometrically difficult to define with polygons. The typical approaches to this situation, and that don't produce a correct result, are:
- model the liquid body to have coinciding faces with the glass body(with their polygons in exactly the same coordinates). In this case the render process falls in precision errors as, at each point, half of the rays will evaluate a glass surface and ignore the liquid and the other half with evaluate only a liquid face, which is not what happens in the real situation (see picture below).
- model the liquid body slightly smaller than the glass to avoid those coinciding faces. In this case the small "air" gap in between will not produce the typical TIR (Total Internal Reflection) effect that is visible in these situations, making the result visually different than what is expected.
- model the liquid body bigger than the glass, invading its volume. With this approach the invading volume will be assumed to be composed by a mix of glass and liquid materials, also causing a wrong result.
Modeling the glass and liquid bodies with coinciding faces produces precision errors in those coinciding walls
The "infinitesimal gap" solution
In fact, no approach involving two coexisting surfaces (the glass and the liquid walls) will produce a correct result. The real situation is more complex, it doesn't have two surfaces but one single limit surface that behaves like either glass or liquid depending on the direction of the incoming ray. And as this behaviour depends on the direction of the incident ray, it can not be recreated with a back-face material either.
So far, the only way to overcome this situation and produce a correct result was the "infinitesimal gap" that is explained in this page: Tips and Tricks: Liquid in a glass.
This method involves the liquid body to be completely enclosed inside the glass body, which doesn't have inner walls where the liquid exists. This way there is only one single wall, instead of two, to define the limit area between the glass and the liquid medium. The method also involves a third material -glass exit material- over the liquid upper surface to provide the correct order for the rays exiting the glass medium in certain directions.
This method has the disadvantage that makes the models setup very complex, less flexible and not suitable for animations.
The "infinitesimal gap" method was the only way to ensure every ray enters and exits the mediums in the correct order
Nested dielectrics NEW IN MAXWELL 3.2
The best way to overcome this solution is by using the nested dielectics approach. Using this approach, two dielectric bodies can be overlapped and the render engine can automatically "boolean" them in render time and remove the surplus volume leaving just one single polygon wall, and assign the proper material for that single wall depending on the direction of the incident ray.
This method works by sorting the two dielectric materials in contact regarding a priority order you can custom define, indicating that when those dielectric bodies overlap, the renderer will automatically fill the overlapping volume only with the material of your choice.
The Nested Priority value is defined in the material basis, and is set in the Global Properties level. The lower this value, the higher its priority, meaning that where a body with priority 0 and a body with priority 1 overlaps, the overlapping volume will be filled only with the priority 0 material.
Setting the Nested Priority parameter of a dielectric material on the Global Properties level
In our liquid inside a glass example, the liquid can now me modeled slightly bigger, invading the glass body, and set the glass priority to 0 and the liquid priority to 1 (or any value higher than the used by the glass). This way the overlapping volume with be filled with only glass, the liquid wall will be removed, and the remaining walls will later properly get either the glass or the liquid material depending on the ray direction.
By nesting the dielectric objects and assigning the correct priorities, the objects setup becomes easier allowing overlapping (left).
In render time, the liquid overlapping volume gets automatically removed leaving one single face and the materials are properly assigned regarding each ray direction (right)
What some ice cubes?
If we want to through some ice cubes into the liquid, we will end up with the a similar situation: inside the ice cubes volume there exists at first two different materials coexisting (liquid and ice), which is not what would happen in the real world, where only ice fills that space. So in this case we will need to assign a higher priority (lower value) to the ice than to the liquid.
The priorities assignment in this example should be:
- glass material: priority 0 (0>2: where glass and liquid overlaps, glass remains)
- liquid material: priority 2
- ice material: priority 1 (1>2: where ice and liquid overlaps, ice remains)
This way, the liquid gets "booleanized and removed" by both of the rest.
Values are just set to establish the proper order when they overlap (the material with lower priority value remains) but the numerical value itself is irrelevant (setting 0 and 1 for two dielectric materials produces the same result as setting them to 7 and 11).