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74 3 Sun and Skymultiplier sets the intensity and tint color modifies the light color. When it is white, andthe multiplier is 1.0, the light emitted is equal in intensity (and color) to the environmentlight that FG would have seen if allowed to send many thousands of FG rays 6 .The light normally shines in the positive Z direction of the light instance’s coordinate space.If reverse is on, it shines in the negative Z direction.cutoff threshold 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 arealight). Of course, this makes the scene slightly darker since light is ignored, but can save a lotof excess render time.shadows can turn shadows on and off.If use custom environment is off, the shader looks in the global camera environment forthe color of the light. If it is on, it calls the shader passed as custom environment to findthe color.Tip: While the shader is intended to be a portal to an environment, one can also treat it as a“light card” shader by putting a shader returning a solid color as the custom environment,for example using mib blackbody creates a light card with a given color temperature.If use custom environment is on but no custom environment is actually passed, theshader behaves as a white light card.visible defines if the light emitting surface is visible or not. When off, eye rays, reflectionrays etc. go straight through so the portal light itself remains unseen (and we still “see”out the window). When on, the actual light emitting surface becomes visible to eye rays,reflection rays etc (and one do not “see” out the window any more, although one still “see”the environment shaders result). The on mode is useful when using mia portal light as a lightcard shader.When lookup using fg rays is off, the environment shader is looked up with a normal callto mi trace environment(). However, some shaders behave differently if they are called byan FG ray or by another ray (the mia physicalsky shader, for example, does not show the“image” of the visible sun to FG rays). Since the idea of mia portal light is to act as an “FGconcentrator” it should therefore follow that behavior. So when lookup using fg rays is on,it calls the environment with the ray type set to miRAY FINALGATHER, so that shadersthat switch behavior based on this can return the color appropriate for a FG ray.If shadow ray extension is zero, the shader begins tracing shadow rays “at” the light. Whenpositive, the shadow rays actually start that distance “outdoors”. So if there is a large objectjust outside the window, it’s shadow will be taken into account. Conversely, a negative valueallows the shadow rays to begin that distance inside the window, which can allow them to“skip” over troublesome geometry near the window (flowers, curtains) that would otherwisejust introduce noise into the shadows.6 For FG filter 0, the unbiased mode.
3.8 Sky- and Environment Portals 75If emit direct photons is on, the light only shoots direct photons, and does not actuallyemit any direct light at all.The transparency parameters has two functions.• When visible is on, it is a multiplier to the “visible color” of the area light. When this iswhite, the directly “visible” color is the one dictated by the laws of physics for a surfacethat emits that amount of light.Changing the parameter away from white allows one to artificially change the balancebetween the visible result (which is changed by changing this parameter) and theintensity of the emitted light (which is not affected by this parameter). This can beuseful to avoid noise• If visible is off it defines the transparency of the area light.This allows the mia portal light shader to double as a “gel” on the window, to subduethe intensity of what is seen outdoors, which otherwise tends to appear overexposed andblown out. The actual emitted light intensity is not affected by this, nor does this affectthe intensity of other light rays travelling through the window, it only affects what isvisible to the eye, in refractions or reflections.3.8.3 ExamplesIn this section we will examine the benefits of using the portal lights compared to what hasbeen possible in previous versions of mental ray.We are using the following scene 7 :Our scene, using the portal lights, GI and FG.The scene is only lit by the sun and sky, there are no light sources inside the room of anykind.7 Large parts of the example scene geometry is provided courtesy of Giorgio Adolfo Krenkel.
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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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2.5 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
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Page 89 and 90: 3.9 Photometric Lights 833.9 Photom
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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
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mental ray Architectural and Design Visualization Shader Library

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