Owlet features four types of material’s medium:
Here you can see them all in the Main parameters section of the material editor panel:
Let’s discuss them one by one.
This type of medium means “no medium”, so no refraction is performed and Owlet doesn’t treat such material as “thick”. This medium type is perfect for really thin objects like paper, leaves, various labels, cards and so on. Using this material on thick objects like balls or cubes gives physically incorrect transparency effects, while using other (manual or measured) modes on thin objects may give the results that are too dark.
This mode has no extra options.
This is one of the two main modes that you will be using most of the time (the second one is Opaque). This mode means that the material medium is translucent and probably has color:
You need to use this type of medium for all the glasses, liquids, semi-transparent plastics, gems and so on. Everything which is transparent will most likely end up here.
The mode comes with some settings:
The index of refraction parameter is quite simple and particular values can be easily found in the Internet. Note that it is usually a good idea to have this parameter equal to the one, defined in the layer, as in real life the same index of refraction is used for both reflection and refraction, so keeping them equal guarantees a realistic image.
Absorption and Attenuation parameters work as a pair and let you define the medium color. Think of green or brown bottle glass, various liquids and so on. Owlet calculates the path a ray does in the medium, then it takes its length, compares to the Attenuation value and tints the material with the Absorption color depending on the path’s length. The more light travelled inside the medium, the darker will be the color.
You can configure the medium color by either changing the Absorption color brightness, or by increasing the Attenuation parameter value. Here are four owlets, each having green ( RGB 0, 0.5, 0) absorption, but different attenuations:
From the left to right, the owlets have attenuations: 0.1, 1, 10 and 100. All the numbers are in centimeters. As you may see, the smaller the attenuation, the thinner the shape needs to be to become opaque. The bottom part of the shape is more solid, so it looks darker than the thinner top part.
Here we rendered the same owlets with the same attenuation level of 1, but this time we used different absorption colors:
From the left to right, the absorption colors are: RGB (0, 0.1, 0), (0, 0.5, 0), (0, 0.75, 0), (0, 1, 0). So you can change the medium color this way, too.
However, note the very right owlet and its bottom part. As you may see, its color doesn’t really depend on the thickness of the shape anymore. That’s because we used 100% green level in the absorption color. When you use such 100% colors, the math under the hood stops working and the medium color on that channel does not depend on the medium thickness anymore. Sometimes this may be exactly what you need, but for most of the cases, you’d better keep the colors below 90% level of the maximum. By the way, that’s exactly how the default medium color is set up. By default, Owlet uses white absorption color, which means “no absorption”.
As for priority, it is for cases when one medium is merged into another. Think about the water mesh inside of the glass mesh. The ray first enters the glass mesh, then in enters the water without leaving the glass and here Owlet needs to decide what medium to use now. It takes the priorities of both mediums and selects the one which is higher (it must be water in our case). This way you may not worry about the perfect border between the mediums, as everything will be done automatically by providing right priorities to materials.
That’s pretty much the same as the manual mode, but instead of providing IOR, attenuation and absorption values yourself, you simply load a special file that contains all these data measured professionally. This approach gives more realistic rendering, yet takes longer time.
Exactly as with manual medium mode, it is recommended to keep reflective layer’s settings similar to the medium’s one. So if you use measured data for medium, it is usually a good idea to use them for the layer, too.
In most cases, you will use the manual mode for translucent materials, but for rendering gems or other complex glass materials, it might be a good idea to find a proper file with measured data and use it instead.
That’s the second most popular medium type. It means that the medium is not transparent at all and there is no reason for Owlet to try rendering inside of it. Owlet just stops tracing a ray if it hits such medium.
This medium type is good for everything non-transparent: metals, stones, woods and so on.