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Exterior Radiosity

Ted Boardman tedb@tbmax.com http://www.tbmax.com

In this column you will learn some the concepts and practices of using the new Autodesk VIZ 4 Radiosity or 3ds max 5 lighting and rendering for an exterior scene with sky mapped to a dome. Use of the Exposure Controls will also be covered as well as mixing Standard lights with Photometric lights.

Exterior Radiosity with Sky dome

It is common for VIZ and max users to want to do an exterior scene with a skydome with the new Radiosity rendering features and the photometric Daylight lighting setup. There are, however, several factors that can prevent this from being an easy, intuitive process.

Here we will walk through the steps and investigate both the concepts involved and the “gotchya’s” to help make the basic setup a more logical and straight-forward.
The skydome for the scene is simply a hemisphere with a Normal modifier to flip the face normals to be visible from inside the dome. It has a Gradient Ramp map with a Spherical UVW Map modifier that has been adjusted to show the full Gradient in the camera view. This gives the illusion of a blue sky with increasing haze toward the horizon. Any appropriate cloud or sky map could be used instead of the Gradient Ramp map. See Figure 1.

Figure 1: Exterior scene with skydome

Applying Daylight

The first step will be to apply the new Daylight system for the scene from the Create panel, System panel, Daylight. Pick in the middle of the scene and drag to create a Compass Rose, then release and position the Daylight icon before clicking to set it in position. In the Control Parameters rollout, set the sun for 11:00:00 am and 11/28/2002 for the date and time and change the Time Zone to –7. This is 7 hours before Greenwich time and will position the sun for test purposes.

With the Daylight object selected, go to the Modify panel and notice it is made up of two components; Sunlight and Skylight. The Sunlight is the direct rays of the sun shining on objects and the Skylight calculates the light that is normally bounced in the atmosphere to fill in the shadows. The light calculations are based on real physics so it is critical that your scene is modeled using real units. Note also in the Sun Parameters rollout that the Intensity of the sun is automatically set at nearly 80,000 lux. That will be important later. The shaded camera viewport looks burned out and a rendered image is black.

The new Daylight system has Ray Traced Shadows with an option called 2 Sided Shadows checked in the Parameters rollout. This causes the skydome to cast a shadow regardless of the fact that the normals are flipped. You will fix this by changing the skydome properties.

The Skydome

The skydome is just an object in the scene with a sky map that can be viewed from several angles. It’s sole purpose is to represent the sky and it should have no effect on lighting whatsoever in a Radiosity solution. You do not want the skydome to cast or receive shadows, nor do you want it to be included in the bounced light calculations of Radiosity. The bounced light in the exterior scene is handled by the Skylight component of the Daylight system.

Select the Skydome and right-click on it. Choose Properties in the quad menu. In the Object Properties dialog, switch to By Object from By Layer in the Rendering Control section and clear the Receive Shadows and the Cast Shadows checkboxes. Click on the Radiosity tab of Object Properties dialog. Switch to By Object and check the Exclude from Radiosity Processing checkbox. See Figure 2.

Figure 2: Use Object Properties dialog to disable Receive and Cast Shadows and to Exclude the skydome from Radiosity Processing.

Select the Daylight object in the scene and, in Modify panel, Sun Parameters rollout, click Exclude. In the Exclude/Include dialog select Sky in the left column and click the >> button to move it to the right. It is no longer lit by the Sunlight. A rendering will show a completely burned scene.

Tip: For faster rendering set the Max Quadtree Depth, in Ray Traced Shadow Params rollout, from the default 7 to 10 for this scene. On a test the render time dropped from 4:04 to 1:24.

Exposure Control

An important element in the successful use of Radiosity rendering is the Exposure Control settings. Think of radiosity as a camera with film. Used in bright daylight the film is overexposed if the amount of light entering the lens is not adjusted to compensate for the brightness of the sun. The same thing is true in Radiosity calculations.

Click the Exposure Control button or open it from the Rendering pull-down menu, Environment option. Make sure the Exposure Control is set to use Logarithmic, the best choice for most situations. In the Parameters rollout, check the Exterior option, Daylight should be checked by default, but the intensity must be set to Exterior to compensate for the bright daylight flooding the scene. See Figure 3. A test render shows a much more balanced lighting effect.

Figure 3: Checking the Exterior option for Logarithmic Exposure Control adjusts for the extremely bright daylight.

Radiosity Processing

Click the Radiosity Panel button or use the Rendering pull-down menu, Radiosity option to open the Radiosity dialog. Make sure VIZ Radiosity is active and click the Start button to process the current radiosity calculations. The shaded viewport will look reasonably well, but if you render the scene the sky will be pure black. You will correct that later.

In the Material Editor, choose an unused Sample Window and make the Diffuse color and light green (RGB = 35, 175, 25). Drag and drop the material to the object called Ground01 in the scene. In the Radiosity dialog, Radiosity Processing Parameters rollout, click the Reset All button, then click the Start button. In the shaded viewport and in the rendered image you will see green bleeding onto the trees and the house, especially at the eaves. You might also see some red bleeding from the window frames.

The area under eaves and the porch roof is also showing the effects of bounced light. You would not get this effect with the standard sunlight in previous versions of VIZ or when not using radiosity. However, the distribution of the effect is blotchy and random.

Radiosity calculations are stored in a special mesh that is generated by the radiosity processing and the relative position of vertices of adjacent objects is critical in passing the light and shadow information across the surfaces.

Select all the objects belonging to the house in the scene. Right-click on any of the selected objects and choose Properties from the quad menu. Click the Radiosity tab and click the By Layer button to switch to By Object. Clear the Use Global Subdivision Settings checkbox and make sure Subdivide is checked and Meshing Size is set to 3’3 2/8” (one meter).

In the Radiosity panel, click Reset All and then click Start. Right-click on Camera01 viewport label and choose Edged Faces to show the new wireframe in the shaded viewport and a new distribution of the radiosity solution. See Figure 4.

Figure 4: Viewport showing radiosity solution and meshing at one meter. The tighter meshing results in a better distribution of lighting effects.

Right-click on the selected house objects again and change the Meshing Size to one foot. In the Radiosity dialog, click Reset All and Start to reprocess the solution. It will take longer and the viewport and rendered image look more blotchy and worse than ever. However, the distribution of color is better and meshing is an important first step. Tip: Meshing will also cause the file size to balloon rapidly. Use it only as necessary. Do not use Global Meshing parameters for that reason. You can clear the Display Radiosity in Viewport setting in the Radiosity dialog, Radiosity Processing Parameters rollout, to clear the viewport for easier editing.

The next challenge is to reduce the blotchy effect of the lighting. This can usually be accomplished by increasing the Refine Iterations in the Radiosity Processing Parameters rollout. Enter 5 in the Refine Iterations (All Objects) field, Reset All, and Start the processing. Refining looks at the information stored in the vertices and recalculates for better distribution.

To further smooth the results enter 3 in the Interactive Tools, Filtering field of the Radiosity Processing Parameters. This blurs the refined lighting for yet a better bounced light quality. Caution: Do not use high numbers in the Filter field, 5 is about as high as you will want to go. Filtering blurs the edges of areas of contrasting lighting and will effective remove your shadows if set too high.

The scene does not look good yet, but you see the process. The problem with this model is that it was not created specifically with Radiosity in mind and, for example, the vertices of the roof do not match up with the vertices in the walls nor do the foundation vertices line up with landscape vertices. This makes it difficult for max or VIZ to calculate the interaction of the light bouncing between objects. Smaller meshing sizes will help with a dramatic increase in processing times.

Lighting the Skydome

There is still no light on the skydome. Because it has been excluded from the Radiosity processing and from the Daylight you will need a new light specifically to make it light enough to see. You will use a Standard Omni light for this.
Choose Reset All in the Radiosity dialog and close all windows and dialogs. In the Left viewport place a Standard Omni light just below and centered on the Sky hemisphere. Center it on the Sky in both the x and y axis of the Top viewport. In the Modify panel, clear the Shadows On checkbox and click the Exclude button. In the Exclude/Include dialog, choose Sky in the left column, move it to the right column, and check the Include option at the top left. This lights only the sky and will not affect any new objects you might add to the scene.

Open the Radiosity dialog and Start the processing. When processing is complete, render the Camera01 viewport and the sky will still be very dark When Standard lights are mixed with the radiosity process the brightness levels are completely out of sync with each other. In order to correct this you must open the Exposure Control dialog and enter the strength of the brightest photometric light in the scene in the Physical Scale field of the Logarithmic Exposure Control rollout. This acts as a multiplier on the Omni light to allow the light to shine the distance to the hemisphere.
You can now fine-tune the scene with the Brightness, Contrast, and Mid-Tone settings in the Exposure Control dialog.

Summary

You have learned several important steps in setting up a exterior scene that uses a skydome with a mapped sky image with Radiosity lighting.

• A Daylight system made up of Sunlight and Skylight components.
• The skydome must be excluded from the Radiosity Processing and from the Daylight itself.
• The skydome must have it’s own Standard light that excludes all other objects.
• The Radiosity must be processed.
• Exposure Control must be used, preferably with Logarithmic Exposure Control.
• Physical Scale must be set in Exposure Control to the level of the brightest photometric light to multiply the effect of the Standard light.
• Meshing Parameters must be adjusted for the objects in the scene for enough vertices to store and transfer the bounced light effects.
• Refine Iterations and Filtering must be increased in the Radiosity Processing Parameters to smooth the lighting effects.
• When you have the basic scene rendered you will need to tweak the settings in both Radiosity and Exposure Control for an acceptable rendering. Exterior scenes are difficult to render with radiosity, mainly because of the density and size of the objects and the meshing required.

Use Exterior radiosity for scenes with large overhangs that will be lit from sunlight bouncing from walks and streets and the effects will be worth the production times involved. For everyday scenes or urban cityscapes you may be better off sticking to standard lighting techniques.

Good luck and have fun

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