mental ray Architectural and Design Visualization Shader Library

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114 5 General Utility ShadersThere is a light hidden behind each of the patches in the image above. The leftmost isemitting both specular- and diffuse light. The middle one is only emitting specular light,and the rightmost is only emitting diffuse light. Furthermore, the rightmost patch has it’sfg contrib set to zero.As we can see, the leftmost patch creates too much illumination, since we get illuminationboth from the light and from the patch. The center patch generates the strange effect of asmall “highlight” (our light source is just a point light) inside the reflection of what we aretrying to sell as “the visible light”.In this case this effect is distracting, so we want it disabled - like we have done on the lighton the right. That is the way to go; let the surface handle the reflection, not the illumination,and let the light handle the illumination, not the reflection (i.e. no traditional “specularhighlight”).An alternative, when using area lights, is to allow the area light to create the specularhighlights:Area light specular highlights can also “work”This illustrates the alternative. The leftmost patch again uses a point light with specular on.This is what we do not want, since it looks strange. The other two patches use an area light.The middle patch has the area lights specularity turned off and the patch has refl contribset to 1.0, the right has the lights specular enabled, and a refl contrib of zero.As we can see, both variants on the right side “work”, so it is a matter of choice which methodis used for area lights.However, the middle method tends to be preferred, because often one is using a geometricallysimple area light (sphere, rectangle, etc.) in place of an object that is actually geometricallycomplex (a neon sign, for example). In those cases one definitely want to use the method inwhich the object is reflected, and the light does not emit specular light:
5.3 Light Surface 115Complex geometry require real reflectionsBoth neon signs use a rectangular area light for their illumination, and both cast nice areashadows (as witnessed by the small spheres). But the sign on the left has a blatantlyrectangular “reflection”, which does not make any sense. For this use, definitely use the methodon the right side, with the lights specularity off, and the surface’s refl contrib nonzero.5.3.2.3 Complex Light DistributionIn a real world luminaire, complex interactions between the bulb, the reflector, and variousoccluders determine the final distribution of light flying off into the scene.Naturally, one do not want to consider all those minute details just to make a rendering of adesk lamp, and indeed, the problem comes pre-solved in the form of measured data (i.e. IESor EULUMDAT files).The mia light surface shader can get its intensity from a light. This saves the user the timeof keeping the intensity and color of the “visible surface” in sync with the intensity and colorof the light source. If the use lights is on and some light is passed in the lights parameter,this happens automatically.This works fine for isotropic point lights and similar, but when using measured light data, onecan run into an issue:Picking up intensity/color from the lightsThe above scene contains three spheres. Lets assume they are the models of the light bulbsof some desk lamp. They each contain a light that has an IES profile fitting that luminaireapplied.
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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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2.4 Quick Guide to the Material Par
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2.5 Detailed Description of Materia
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2.6 Tips and Tricks 49• For extre
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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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2.6 Tips and Tricks 59Many small ad
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Chapter 3Sun and Sky3.1 Introductio
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Page 71 and 72: 3.5 Sun Parameters 65• saturation
Page 73 and 74: 3.6 Sky Parameters 67light"sun",# T
Page 75 and 76: 3.6 Sky Parameters 69• sun direct
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Page 111 and 112: 5.2 Environment Blur 105replaced by
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Page 115 and 116: 5.2 Environment Blur 109Glossiness
Page 117 and 118: 5.3 Light Surface 111An example of
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