Shadow Matte shader used to capture shadow on the floor plane (rollover image)
shadow_matte is a specific shader, used typically on floor planes to 'catch' shadows from lighting within the scene. It is useful for integrating a rendered object onto a photographic background. You can create a custom shadow pass with this shader. You can also render out shadows separately for use in a compositing package. For example, you may want to change the shadow density or color of the shadow to match some live-action footage.
An example scene file can be downloaded here.
This can be set to either scene_background (default) or background_color, which allows connecting a specific texture in the background_color parameter slot.
The color of the shadow. You can use it to 'tint' the color of the shadows to match a photographic backplate.
Determines how 'opaque' or dark the shadow appears. Higher values will produce a lighter shadow. The captured shadow is visible in the alpha channel. If you have a very hilly or bumpy ground, you may need to increase the Transparency Depth in the Render Settings.
Controls whether the alpha should be opaque or contain the shadow mask.
The color used to determine the overall indirect diffuse contribution in the scene.
Diffuse Color: Red
If enabled, the background color is used to determine the overall indirect diffuse contribution in the scene. Otherwise, the color defined in the diffuse color is used.
The amount of diffuse contribution.
When enabled, it takes into account the backlighting illumination. Backlighting provides 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.
Switch for enabling/disabling capture of the indirect diffuse light.
Switch for enabling/disabling capture of the indirect specular light.
The color the specular reflection will be modulated with.
The specular weight. Influences the brightness of the specular highlight.
Controls the glossiness of the specular reflections. The lower the value, the sharper the reflection.
The IOR parameter (Index of Refraction) defines the material's Fresnel reflectivity and is by default the angular function used. Effectively the IOR will define the balance between reflections on surfaces facing the viewer and on surface edges. You can see the reflection intensity remains unchanged, but the reflection intensity on the front side changes a lot.
Per light group shadow mattes. More information about light groups can be found here.
A list of available AOVs available for the Shadow Matte shader. Each option creates a separate AOV render pass for that component. Note that the AOV must also be enabled in the Render Settings window.
Direct light shadow AOV.
A difference AOV which can be used to eliminate the shadow from the direct component.
This AOV can be used in comp to localize and tweak the shadow.
There may be areas visible in the specular reflections which are outside of the background plate; so-called offscreen areas. This parameter defines the color used for these offscreen areas; you can link a texture for instance.
To define the color used for these offscreen areas, you must add an Arnold Tag to the Background object and set the Offscreen color parameter. To link a texture you must set the Offscreen color link parameter instead.
The Shadow Matte shader requires more than 1 sample to avoid noisy shadows. This is because it is computing a ratio of shadowed/unshadowed lighting, and to get a good estimate of that it requires multiple samples.