Matte Shadow (mip)

Category: Illumination

Shader Family: Surface Material

Output: Color

Often you want to include synthetic objects into an existing photographic background plate filled with real world objects. For example, adding a yet-to-be-constructed building into an empty lot, adding a virtual car onto a road, or having a virtual character walk through a scene and realistically interact with objects in the real world scene. In general, the term "synthetic" refers to additional objects to be inserted in the scene, and "real world" refers to objects that are already there.

Two main shaders exist to facilitate this work: the Camera Map (mip_Camera_Map) shader which "projects" an image from the camera onto geometry, and the Matte Shadow (mip_Matte_Shadow) shader which takes care of generating hold-out mattes, as well as allowing the real world objects in the photographic plate to both cast and receive shadows, as well as receive reflections and indirect light.

The Matte Shadow shader is used to create "matte objects" (objects that are used to represent existing real world objects in a photographic plate) for the following purposes:

Blocking another synthetic object from the camera's view (to allow the synthetic objects to go behind the real world object).
Allowing synthetic objects to cast shadows, occlude, and receive shadows from the real world objects.
Adding reflections of synthetic objects onto real world objects.
Allowing the interplay of indirect light between synthetic and real world objects.

In all the above mentioned cases, the Matte Shadow shader is applied to an object representing the real world object, and the synthetic object uses a traditional material.

This shader can also function as a "shadows only" shader which only shows how much in shadow a point is compared to the incoming light, but ignoring the actual amount of incoming light itself (only the occluded percentage of it).

Shadows

Catch	When enabled, allows other objects to cast shadows on this object.
No Self-Shadowing	When enabled, causes objects using the Matte Shadow shader not to receive shadows from any other such object.
Consider Light Angle	Specifies whether or not the angle to the light is considered when calculating the incoming amount of light.
Shadow Coloring Factor	If the Shadow Coloring Factor is 0.0, all shadows are cast in grayscale. If it is set to 1.0, the shadows have full color. For example, if the surface is lit by one red and one green light, the red light will have a green shadow, and the green light will have a red shadow.

Ambient Occlusion

Enable	When enabled, a built-in ambient occlusion is applied. The ambient occlusion respects the No Self-Shadowing option and will not cause ambient occlusion from objects with the same material as itself.
Samples	The number of ambient occlusion rays that are shot into the scene.
Maximum Distance	Sets an upper limit on the distance ambient occlusion rays can travel. If the value is set to zero, the rays reach infinitely far (no limit). Short rays increase performance dramatically, but localize the ambient occlusion effect.

Reflections

Catch	Enables reflections.
Samples	Sets the number of glossy reflection samples. If it is set to zero, mirror reflections are used.
Glossiness	Sets a Ward glossiness for reflections.
Maximum Distance	Limits the reach of glossy reflection rays. If the maximum distance is set to 0.0, the reach of the glossy reflection rays is infinite. For values greater than 0.0, the reach of the glossy reflection rays is limited to this distance, and the color of the reflection is faded toward the environment color as the length of the ray approaches this distance. Use Maximum Distance to improve performance and reduce excessive noise caused by high-contrast objects in the distance.
Falloff	The rate for fading into the environment. The default of 2.0 means that the falloff is by distance squared. A value of 3.0 means distance cubed, and so on. This parameter has no effect if Maximum Distance is 0.0.

Indirect

Catch

Enables the gathering of indirect light.

Additional Illumination

Catch

Enables the use of illuminators to light the scene.

Illuminators

Specifies the lights listed as illuminators which are tested and used to actually light the scene.

The difference between lights and illuminators is that the lights are only used to cause shadows on the background, whereas the illuminators are used to throw actual light on it. This means that the lights list should contain lights that are already present in the background plate, and the illuminators list contains any additional lights introduced by a CG element, such as the headlights of a CG car.

For more information, see Using Light Lists [Direct Illumination].

Render Tree Usage

The Matte Shadow shader has multiple outputs. All outputs are as "raw" as possible to be maximally useful as layers in post production. For example, to ensure that the reflections have not been multiplied with the reflection color, and so on.

result	The combined result.
shadows_raw	The raw full-color shadow pass on white background, suitable for compositing on top of a background in "multiply" mode.
ao_raw	The raw ambient occlusion.
refl_raw	The raw reflections.
indirect_raw	The incoming indirect light.
illumination_raw	Light gathered from any lights in the illuminators list.

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