Category: mental ray > Light
Shader Family: Light
Output: Color
Portal Light (mia)
This shader is most often applied to a rectangular area light which is placed in the window of a room scene. The shader obtains
its proper intensity and color from the sky outside the window (for example, an environment shader such as the mia_physicalsky shader or similar) and how much of that sky can be seen.
When used with final gathering, the mia_portal_light acts as a final gathering concentrator, so that instead of having to
send thousands of final gather rays around the scene to "find" the window, the portal light actually blocks final gather rays,
and instead converts light from beyond the window to direct light, including high-quality area shadows.
Final gathering will now see a well lit room rather than a black room, and can use much lower final gather ray counts. Furthermore,
since the light from the window is now direct, you gain one extra light bounce for "free".
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The name of the shader node displayed in the render tree. Enter any name you like, or leave the default.
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Enables or disables the light.
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A multiplier for the intensity of the light.
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Modifies the light color. When tint_color is white, and the Intensity is 1.0, the light emitted is equal in intensity and color to the environment light
that final gathering would have seen if allowed to send many thousands of (unfiltered) final gather rays.
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The light normally shines in the positive Z direction of the light object's coordinate space. If is on, it shines in the negative Z direction.
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This is a performance optimization option. Any light below this level is ignored, and no shadow rays are traced (which is
what consumes the bulk of the render time of an area light). Of course, this makes the scene slightly darker since light is
ignored, but can save a lot of excess render time.
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Turns shadows on or off.
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When off, the mia_portal_light shader gets the color of the light from the global environment shader defined for the scene.
When on, the mia_portal_light shader calls the shader that is connected to its input port in order to get the color of the light.
While this shader is intended to be a portal to an environment, you can also use it as a "light card" by connecting a shader
that returns a solid background color into its custom_environment input port. If use_custom_environment is on, and no custom
environment is actually passed, the shader behaves like a white light card.
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When is on, the mia_portal_light shader calls the shader that is connected to this parameter port in order to get the color of
the light.
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Specifies whether or not the light emitting surface is visible in the render. When off, eye rays, reflection rays, etc. go straight through so that the portal light itself remains unseen and you can still
"see" out the window.
When on, the actual light emitting surface becomes visible to eye rays, reflection rays, etc. and you cannot "see" out the
window anymore, although you can still "see" the environment shader's result. This is useful when using the mia_portal_light
as a light card shader (see option).
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When off, a ray is cast into the environment and the environment shader is called to compute the color of the light. However, some shaders behave differently if they are called by a final gather ray or by another ray (for example, the mia_physicalsky
shader does not show the "image" of the visible sun to final gather rays). Since the main purpose of the mia_portal_light
shader is to act as a final gather concentrator, it should therefore follow that behavior. So when lookup_using_fg_rays is
on, it calls the environment with the final gather ray type, so that shaders that switch their behavior based on this can
return the color appropriate for a final gather ray.
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If shadow_ray_extension is 0 (zero), the mia_portal_light shader begins tracing shadow rays at the light. When positive, the
shadow rays actually start at that distance "outdoors". So if there is a large object just outside the window, its shadow
will be taken into account. Conversely, a negative value allows the shadow rays to begin at that distance inside the window,
which allows the shadow generation to skip over troublesome geometry near the window (such as flowers, curtains, etc.) that
would otherwise just introduce noise into the shadows.
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If emit_direct_photons is on, the light only shoots photons and does not actually emit any direct light at all.
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The transparency has two functions:
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When is on, is a multiplier for the "visible color" of the area light. When set to white, the directly "visible" color is the one dictated
by the laws of physics for a surface that emits that amount of light.
Modifying the parameter values away from white allows you to artificially change the balance between the visible result (which
is changed by changing this parameter) and the intensity of the emitted light (which is not affected by this parameter). This
can be useful to avoid noise
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When is off, defines the transparency of the area light. This allows the mia_portal_light shader to double as a "gel" on the window, to
subdue the intensity of what is seen outdoors, which otherwise tends to appear overexposed and blown out. The actual emitted
light intensity is not affected by this, nor does this affect the intensity of other light rays travelling through the window,
it only affects what is visible to the eye, in refractions or reflections.
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