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82 3 Sun and SkyThe subtle interplay of skylight, courtesy of the portal lights
3.9 Photometric Lights 833.9 Photometric Lights3.9.1 The ProblemOne difficulty in using base mental ray shaders to render a physically correct scenes with GI(using photons), is in balancing the photon energy and the direct light correctly. The reasonfor this is that the intensity is generally specified as an intensity for some given direction (forexample the peak intensity on a spotlight), but the energy is the sum of (or technically, theintegral) of all such intensities in all directions.So for example for a spotlight, for the same peak intensity, you would need different photonenergies depending on the width of the spotlight cone, and depending on the shape of theintensity distribution within that cone. And while this can be calculated for a mathematicallydefined intensity distribution (such as a spotlight cone) what about an arbitrary distributionas defined by a light profile?Furthermore, to ensure optimal convergence of the photon solution for a light which variesits intensity based on the direction of light (e.g. a spotlight or a light using a light profile),instead of emitting photons of different energy in different directions, one should emit photonsof similar energy in different densities. So the amount of energy-per-area (the density ofphoton energy) at a given point must match the direct light arriving at that point.3.9.2 The SolutionTo solve all these issues, we introduce the mia photometric light. It takes all the guesswork outof balancing the photon energy with the light intensity by peforming a numerical integrationof the chosen intensity distribution to calculate the proper photon energy automatically, aswell as takes care of adapting the photon density to the distribution, all automatically.To use it, simply use the same instance of the shader as both light and photon shader on alight source. The light must have an origin (i.e. the shader does not work on an infinite lightsource). The light must be set to emit photons, and must be given a nonzero energy value(although the actual value will be overriden by the shader, a nonzero energy value is required,or mental ray will not emit any photons at all for the light). The exponent should alwaysbe 2. If the light is a spotlight, a spread value should be used. See the mental ray manualabout lights.shader "myLight" "mia_photometric_light" ("on" true,"color" 1 1 1,...)light "theLight" = "myLight"emitter = "myLight"
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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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Page 55 and 56: 2.6 Tips and Tricks 49• For extre
Page 57 and 58: 2.6 Tips and Tricks 51Diagram for g
Page 59 and 60: 2.6 Tips and Tricks 53There is one
Page 61 and 62: 2.6 Tips and Tricks 55• Glass-Liq
Page 63 and 64: 2.6 Tips and Tricks 57Enjoy the tro
Page 65 and 66: 2.6 Tips and Tricks 59Many small ad
Page 67 and 68: Chapter 3Sun and Sky3.1 Introductio
Page 69 and 70: 3.4 Common Parameters 633.3 Importa
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
Page 77 and 78: 3.8 Sky- and Environment Portals 71
Page 87: 3.8 Sky- and Environment Portals 81
Page 91: 3.9 Photometric Lights 85• When i
Page 94 and 95: 88 4 Camera- and Exposure EffectsAl
Page 96 and 97: 90 4 Camera- and Exposure Effects4.
Page 98 and 99: 92 4 Camera- and Exposure Effects1,
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Page 102 and 103: 96 4 Camera- and Exposure Effectscr
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Page 109 and 110: Chapter 5General Utility Shaders5.1
Page 111 and 112: 5.2 Environment Blur 105replaced by
Page 113 and 114: 5.2 Environment Blur 107But what is
Page 115 and 116: 5.2 Environment Blur 109Glossiness
Page 117 and 118: 5.3 Light Surface 111An example of
Page 119 and 120: 5.3 Light Surface 113the other two
Page 121 and 122: 5.3 Light Surface 115Complex geomet
Page 123: 5.3 Light Surface 117Another exampl
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mental ray Architectural and Design Visualization Shader Library

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