mental ray Architectural and Design Visualization Shader Library

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112 5 General Utility Shaderscolor is the overall color, and applies both to built in light or light derived from light sources.intensity is the intensity of the “built in” light, i.e. the surface will appear to the camera tohave an intensity of color multiplied by intensity (assuming use lights is off - see below).fg contrib is how much of the light that is visible to FG rays, and refl contrib how muchthat is visible to reflection rays.When use lights is on, the lights listed in the lights array are polled and their intensity(multiplied by the lights multiplier) is added to the output dictated by the intensityparameter; i.e. if L is the output of all lights in the lights list, the final output color ofthe shader is:color ∗ (L ∗ lights multiplier + intensity)When lights eval point is 0,0,0 the intensity of the light is evaluated (with shadows disabled)at the point in 3D space that is being shaded. Since this may vary in an undesirable way fora light that has an IES profile, one can specify an explicit point at which the light color isevaluated. This point is in the coordinate space of the light.5.3.2 Use Cases5.3.2.1 Illumination in GeneralIn the real world every light emitting object is visible, has an area, and emits light from thatarea. When using FG, mental ray will treat any surface that adds light energy into the sceneas if it was a light source. However, to get the best possible quality out of this, one need veryhigh FG settings, with long render times.Light sources in computer graphics can be either point sources or area lights, and the arealights themselves may, or may not, be visible in the rendering. In most cases It is simply moreefficient to use actual light sources:Emitting FG, Not emitting FG, And using a point lightThe image above shows 3 patches that are all using the mia light surface above a checkeredplane. The leftmost patch has it’s fg contrib set to 1.0 (and is hence illuminating the floor),
5.3 Light Surface 113the other two has it set to 0.0. But the rightmost patch has a point light hidden behind it.At close distances (or with very large light sources, like the entire outdoor sky) FG canilluminate objects just fine with very good quality. At long distances or with small sources,using an explicit light source is much more efficient.mia light surface gives separate control if the object should be “seen” by reflection rays orFG rays (and how much) via the fg contrib and refl contrib parameters. This image usesa slightly reflective checkered plane to illustrate this. No light sources are used behind thesepatches:Visibility to FG and reflectionsThe leftmost patch is visible both to FG (illuminates the plane) and reflection (reflects in thefloor). The center patch does not illuminate (fg contrib is zero) and the rightmost does notreflect (refl contrib is zero).5.3.2.2 Highlights vs. ReflectionsIn the real world, the visible light emitting objects are both visible to any cameraphotographing the scene, as well as reflect (glossily) in other objects. In computer graphics,light sources tend to be invisible to the camera, and their reflections are “cheated” with thehelp of “highlights” by material shaders.The mia material supports a protocol for light sources to tell it if they should generatehighlights or not. This is implemented by most OEM applications light sources, look forflags like “Affect Specular” or “Emit Specular” or similar on the lights.Light hidden behind patch with various flags.
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mental ray Architectural and Design
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Table of Contents1 Introduction 11.
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Table of Contentsix3.8.3 Examples .
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Chapter 2Architectural Material2.1
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2.2 Fundamentals 52.2.1.1 A Note on
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2.3 Features 7Hence, when using GI:
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2.3 Features 9The rules are as foll
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2.3 Features 11In the real world
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2.3 Features 13Used alone, an AO sh
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2.3 Features 152.3.7 Performance Fe
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2.6 Tips and Tricks 51Diagram for g
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2.6 Tips and Tricks 53There is one
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2.6 Tips and Tricks 55• Glass-Liq
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2.6 Tips and Tricks 57Enjoy the tro
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mental ray Architectural and Design Visualization Shader Library

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