mental ray Architectural and Design Visualization Shader Library

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.