General

Weight

The transmission allows light to scatter through the surface, for materials such as glass or water.

Note that you must also disable Opaque for the mesh that has been assigned the Standard Surface shader.

If you can only see black where there should be transparency, you may not have a high enough refraction ray depth value (found in the Ray Depth section of the Render Settings). The default value is two.

Color

This filters the refraction according to the distance traveled by the refracted ray. The longer light travels inside a mesh, the more it is affected by the transmission_color. Therefore green glass gets a deeper green as rays travel through thicker parts. The effect is exponential and computed with Beer's Law. It is recommended to use light, subtle color values.

transmission_color: red (transmission_depth: 1 to 10)

If you use a fully saturated color like (1, 0, 0), the way that is interpreted is that all red light is allowed to pass through, and no green and blue light. transmission_color values near 0 make the interior of the mesh very dense to block all light, and then set the depth multiplier to a small value like 0.001 might not make much of a difference because the depth is large anyway. 

Fully saturated colors for transmission_color are not recommended


If this value has a color and shadows tinted with that color are required, then disable opaque for the mesh that has been assigned the standard_surface shader. In the example below, you can see that with opaque enabled the rays cannot pass through the sphere. Whereas with opaque disabled, the rays can pass through the sphere and absorb the color set by the transmission_color, thereby creating the effect of colored shadows. 

Note that transmission_color will not work for single-sided geometry unless thin_walled is enabled.

Depth

Controls the depth into the volume at which the transmission color is realized. Increasing this value makes the volume thinner, which means less absorption and scattering. It is a scale factor so that you can set a transmission_color and then tweak the depth to be appropriate for the size of your object.


The effect of increasing transmission_depth can be seen in the animation below. Note that a transmission_scatter color has also been used in this case.

Depth is scene scale-dependent and can have a dramatic effect on its appearance. The transmission_color and depth control transmittance/absorption, and that depends on the object scale. So for a small object to see anything you might need to set a quite low depth, or for a big object a high one. If you cannot see the effect of depth, then you may need to check the size of your scene.


When the scene scale is too small, the transmission_color (orange) appears incorrect (left image) with a transmission_depth of 1. Lowering the transmission_depth fixes it (right image). It is recommended to model to real-world scale to avoid these situations.

Thin Walled

Thin_walled can also provide the effect of a translucent object being lit from behind (the shading point is 'lit' by the specified fraction of the light hitting the reverse of the object at that point). It is recommended that this only be used with thin objects (single-sided geometry) as objects with thickness may render incorrectly.  


Thin_walled is ideal for thin (single-sided) objects, like bubbles for example.

Thin Walled Translucency

The appearance of this effect is like a thin sheet of paper letting some light through to the backside.

For diffuse surfaces such as paper, enable thin_walled and set subsurface_weight to, for example, 0.5 to have half the light reflected and half transmitted.


In certain situations, thin_walled may work fine with thickness. However, you should ensure that the diffuse_ray_depth level is above 1 when using thin_walled with objects that have thickness.

Exit to Background

This will cause the standard_surface shader to trace a ray against the background/environment when the maximum GI reflection/refraction depth is met and return the color that is visible in the background/environment in that direction. When the option is disabled, the path is terminated instead and returns black when the maximum depth is reached.

Advanced

Extra Roughness

Adds some additional blurriness of refraction computed with an isotropic microfacet BTDF. The range goes from -1 to 1, where 0 means no roughness. It is computed as 

transmission_roughness = specular_roughness + transmission_extra_roughness.


Negative values result in a lower roughness for transmission than for reflection.

specular_roughness: 0.3

Dispersion Abbe

Specifies the abbe number of the material, which describes how much the index of refraction varies across wavelengths. For glass and diamonds, this is typically in the range of 10 to 70, with lower numbers giving more dispersion. The default value is 0, which turns off dispersion. The chromatic noise can be reduced by either increasing the global Camera (AA) samples or the Refraction samples.

transmission_dispersion is ideal for gemstone materials like a diamond

Transmit AOVs

When enabled, transmission will pass through AOVs. If the background is transparent, then the transmissive surface will become transparent so that it can be composited over another background. Light path expression AOVs will be passed through so that for example a diffuse surface seen through a transmissive surface will end up in the diffuse AOV. Other AOVs can also be passed straight through (without any opacity blending), which can be used for creating masks for example. 

Alpha masks (rollover for beauty)


Ensure that the Background (Backplate) is set to None when using transmit_aovs.

An example scene file can be found here.

Dielectric Priority

Specifies how to resolve overlapping dielectrics into a well-defined medium, so that higher priority (higher number) dielectrics override lower priority ones which are effectively removed. This is used to correctly set up cases with adjacent dielectric media such as a glass of water with ice.

dielectric_priority is an integer (default 0) which can be positive or negative, where higher priority numbers override lower priorities. So for example, if glass with priority 2 overlaps water with priority 1, then in the overlap region, only the glass survives. Negative priorities are allowed, so, for example, a priority 0 object would override priority -1 (as it may be convenient to use negative priorities sometimes to specify a lower priority medium than the default 0).

dielectric_priority: glass: 3, ice & bubbles: 2, liquid: 1.

Rollover image to view the default dielectric_priority: 0 (not physically correct).

Nested dielectrics is enabled by default and can be found in the render settings -> Sampling -> Advanced (dielectric_priorities). You will notice changes to the look of old scenes. Disable this option if you wish to revert to previous non-physical results.

More information about nested dielectrics can be found here.

Scatter

Color

Transmission_scatter is suitable for any liquid that is fairly thick or where there is enough of it for scattering to be visible, such as a deep body of water or honey. If you have a glass of water, there is not that much scattering, however, for an ocean, it is required. Other examples include materials like ice, opalescent glass, or milky glass.

For scattering to work, ensure that opaque is enabled for the mesh that has been assigned the standard_surface shader. 

Marble look using a file texture → transmission_scatter 

Scatter Anisotropy

The directional bias, or anisotropy, of the scattering. The default value of zero gives isotropic scattering so that light is scattered evenly in all directions. Positive values bias the scattering effect forwards, in the direction of the light, while negative values bias the scattering backward, toward the light.

 

 

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