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Radiosity basics
by Duncan MacGruer
Date Added: 9/5/2004
Category: Lighting
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!div style="display:none"!fjrigjwwe9r2content:tip!/div!!div style="display:none"!edf40wrjww2content:tip!/div!The following was posted in response to a question in the forums. Hopefully someone will find it helpful...

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1. When all you want are soft shadows, use spots &/or point lights for shadow casting illumination. Two settings are important, The softness and density of the shadows will be determined by the Light Source Radius setting, in the drop down menu of the Object tab of the Object Properties palette. The default setting is 5. The larger the number, the softer the shadows cast by that light. The number will be dependent on the scene, so there is no fixed "proper" setting. The softness and quality of the shadow can be controlled in the rendering dialog. When rendering with radiosity, click the Expert button and find "Maximum light/shadow samples". This will tell the software how many times to sample the illumination.

Try this. Place a sphere on a ground plane and light it with a single spot (no global lights), making sure you get a nicely defined raytraced shadow. Now switch to one of the default Soft Shadows rendering presets, and change the "Maximum light/shadow samples" to one (1) and render. It will look just like the raytraced rendering. Do it again and set it to two. Now ten. 64. 256. See what's happening? Next experiment with the Light Source Radius. Set it to 1 and the shadow, no matter how soft, will be dense and dark. Start cheating it up and you'll see the shadow become lighter and more diffuse.

Sometimes this is all you'll need to know to calculate good results with soft shadows alone. Frequently, mixing a couple of points or spots with a global light can work well for architectural work. A million possibilities, of course. Keep in mind that the number of lights casting shadows is directly related to rendering time.

2. Radiosity can also include diffuse illumination calculations, and this is where the realism starts to creep in. In the rendering dialog you'll see "Maximum diffuse lighting samples". Here, Strata is calculating the way light bounces, and how bounced light from one surface can influence the illumination of another. Color comes into play here too; when light bounces off a red wall, it will carry that color information with it. Now we start seeing cool and warm shadows, blushes of color influence on objects, and reflected light. Very processor intensive. Using the same set up as above, apply a saturated color to your sphere, and place another object next to it, by the side of the sphere receiving the light. Render using one of the Raydiosity presets, and you should see the color of your sphere influencing the other object. You'll also see the bottom side of the sphere receiving reflected light from the ground plane. Another setting, "Maximum diffuse ray bounces", tells Strata how many bounces to calculate - this can produce awesome results and send your rendering time through the roof.

3. Another implementation of radiosity, new to Strata with the CX release, is Global Illumination. In essence, Strata is using a virtual spherical environment called a "Light Dome" to create diffuse illumination in your scene. No lights necessary, though a spotlight or two frequently come in handy. By using image maps on the Light Dome (in the Lights tab of the Environment palette) you can control the quality and character of the light. The intensity slider stops at 100%, but you'll frequently need to type in a higher number. Hot spots in the image map will generate more illumination than the darker areas. Using the same map in the visible and reflected background environments can make for a very realistic image. There are a few maps packaged with CX, but look on the web for HDRI and EXR maps in particular.

There is so much more to this... check Chris Tyler's and Howard Prince's posts for all sorts of detailed info. Practice with simple set ups first to understand the way things work. Look at the lighting rigs and LD (light dome) maps packaged with CX. Eventually you'll need to under