Select the spotlight and press the "T" key to show the spotlight manipulators (or click on the "Show Manipulators" icon next to the shelf). Click sequentially on the manipulator sequencer button next to the spotlight until it cycles around to display the decay regions as shown. The indicator should point to about 7:00. Click on the lowest ring on the decay regions and pull it down far enough so that the cone completely covers the region you want to envelop in fog. Make a note of the resulting value, as you'll use it later.
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For the start of the fog, set one of the decay rings at the edge of the lamp, and observe the number. In this case, we want our spotlight to illuminate a volume beginning around 4.5 units, and extending to 22.08 units. We can achieve this using an intensity curve. In the Light Effects section of the spotlight Attribute Editor, click the "Create" button next to Intensity Curve. With the spotlight selected, open up the Graph Editor window.
The graph represents the intensity of the spotlight over distance. By default, the intensity curve starts at 1 and fluctuates between 0 and 1 over a distance of 100 units.
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Select keys 3, 4, 5, 6 and 7 and delete them. Select the last key, and enter a value of 0 in the first Stats box, and a value equal to the extent of the cone (as set above - in this case 22.083) in the second box.
Set the first key to (0,0), and set its Tangents type to "stepped". Set the second key to whatever value you want the initial intensity of the light to be. You may have to play with the end value a little, but the resultant curve will represent the intensity of the spotlight at any point over its distance. Setting the first key as we did enables us to eliminate the cone of light that exists behind the lamp shape.
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In this example, the light intensity is 0 until it reaches the edge of the lamp geometry. The intensity then jumps to 8 using step tangency to eliminate any light artifacts, then decays to an intensity of 4 at the end of the cone extent as measured before. The decay in the attribute editor is set to linear.
Finishing Touches
Finally, I want to add some finishing touches to the final image. I assigned a white blinn shader to the light bulb, added transparency, incandescence and glow to the shader. I also added a small amount of glow to the primitive objects being lit by the spotlight.
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If you are using multiple glows, remember that their effect is additive - the amount of glow given off by two shaders is greater than the sum. In order to avoid flickering glows in an animation, you need to turn off the Auto Exposure and set the glow exposure manually.
Create test renders until you achieve the glow result you want. Look in the output window, and read the output of the render. You'll see two lines referring to "normalization factors". The first is the glow intensity normalization factor, the second is the halo intensity normalization factor.
In the hypershade window, MMB drag and drop the Shader Glow node from the visor to the hypershade work area. The Shader Glow node is in the "Post Process" section of the visor. Open the attribute editor for the Shader Glow node, and in the Glow Attributes section, plug the numbers from the output window into the appropriate attribute settings. In the Common Shader Glow Attributes section, turn Auto Exposure to off. This will ensure that your glows remain even throughout your animation, instead of popping or flickering when new glows are introduced to your scene.
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