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Radiosity Supplement

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Controlling RenderWorks 12 Radiosity

Radiosity is a technique whereby a computer can simulate the indirect lighting that would exist in a scene. It works by converting the entire model into triangles and calculating the light that bounces from each lit triangle to its neighbors. As light energy bounces around it will be absorbed by surfaces depending on their diffuse RGB color, or will escape out into space. The radiosity ?solution? can be stopped after a certain amount of energy has been considered. It is then rendered using the usual RenderWorks modes and options.

The effects seen in a radiosity rendering are more accurate and localized ambient lighting and washes of color from one surface to another.

Final Quality RenderWorks versus Final Quality Radiosity:

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The radiosity render modes are Fast Radiosity, Final Quality Radiosity and Custom Radiosity. These modes are available in the same locations as other RenderWorks modes, including the Object Info palette for viewports, the Render Bitmap tool, QTVR movie export, and batch rendering.

RenderWorks Radiosity Interface

Here is a listing of all of the radiosity controls and their locations:

The Fast and Final Quality Radiosity render modes are optimized for speed or quality and do not provide additional controls. They do respond to radiosity overrides, described later in this document.

The Custom Radiosity render mode does provide additional controls in the Custom Radiosity Options dialog, available from the View->Rendering menu.

The Custom Radiosity Options dialog has two panes. The Rendering pane is identical to the Custom RenderWorks mode?s options dialog. The Radiosity pane is unique to this render mode and shows several sliders and some other controls to manipulate the solution process:

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The default settings shown in the Custom Radiosity Options dialog are identical to those used with Final Quality Radiosity mode.

Clicking the Optimizations button in the Custom Radiosity Options dialog brings up the Custom Radiosity Optimizations dialog, which lists several optimizations that can be applied depending on the situation:

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Radiosity overrides are settings that can be applied per-texture through the Edit Texture dialog or per-object through the Object Info palette?s Render pane. You can force objects or textures to participate in the radiosity solution or not by applying these overrides.

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The amount of light that bounces off of a surface is determined solely by that surface?s Diffuse reflectivity value. For untextured objects this value is the same as a Matte reflectivity shader set to a Diffuse value of 100%. If you want to manipulate the amount of light that comes off of a surface you can assign a Reflectivity shader to that surface?s assigned texture, then change the Reflectivity shader?s Diffuse parameter.

Note that the Ambient, Specular, and other shader values will not affect the amount of light energy that bounces off of a surface. Only the Diffuse value and the color of the surface are significant during radiosity processing.

Controlling Radiosity

Two key points can help you manage the speed and quality of RenderWorks radiosity renderings:

  1. All objects are converted into triangles of a certain size.
  2. Light energy is ?shot? from the brightest triangles first.

Since all objects are converted into triangles, managing the triangle load is the main way to reduce the time to achieve a decent radiosity rendering.

In addition to the options of deleting or re-modeling objects for more efficient radiosity processing, RenderWorks provides multiple ways of managing this load to achieve faster and better results. These include:

Object inclusion settings

Detail settings

View-dependent optimizations

3D bounds optimizations

Per-texture and per-object overrides

Object Inclusion

The Fast and Final Quality Radiosity modes exclude small (Fast) and very small (Final) objects from the radiosity solution, so that the processor can concentrate its work on the larger surfaces that will contribute more significant amounts of light energy to the scene. The Custom Radiosity render mode allows you to control which objects will be included in the radiosity solution, based on the objects? size.

To use this control, open the Custom Radiosity Options dialog, check the Show Color-Coded Preview checkbox, and move the Obj Inclusion slider left and right. In the preview, gray surfaces are fully participating in the solution, red surfaces will receive indirect light but not re-emit light to their neighbors, and black surfaces will neither emit nor receive indirect light ? they will be rendered with direct lighting only.

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This control helps greatly in files that have small 3D details like knobs, plant leaves, wires, moldings, etc. You should set the Obj Inclusion slider just high enough to include the important surfaces that should re-emit indirect light to their neighbors and avoid surfaces that are not large enough to consider. Eliminating small curvy objects can greatly reduce the triangle load.

Detail Settings

Below the Obj Inclusion slider are two sliders that control the size of the triangles created from the model. The first is the Init Detail slider, which is the main control for affecting the triangle load. This slider sets the initial detail size, which is the maximum triangle size for surfaces. If a surface in the model is larger than this value it will be subdivided into smaller triangles until they are below this size. The Init Detail slider should be set so that the boundaries of any directly lit areas can be accurately captured. If this value is too low (large size) then smaller lit areas might be completely missed. If it is set too high (small size) then the triangle load will be higher than it needs to be and time will be wasted because of the large number of triangles. Because of this speed versus quality tradeoff it is recommended that the Init Detail slider be set to a value just high enough to capture the lit areas. The Init Detail setting is shown as yellow triangles in the color-coded preview.

Init Detail yellow preview triangles and directly-lit patches of floor:

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Below the Init Detail setting is the Small Detail setting. To resolve the edges of indirect shadows, the initial detail-sized triangles can be refined further to the Small Detail setting. In practice this value should be set equal to the Init Detail setting for speed, up to 1/4 the Init Detail size if high quality edges are desired. The Small Detail setting is shown as blue triangles within the yellow triangles in the color-coded preview. The Accuracy slider affects how readily the Init Detail triangles are subdivided to the Small Detail size.

Indirect shadows:

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View-Dependent Optimizations

At the top of the Custom Radiosity Optimizations dialog there is an option called Optimize for Static View. This item is on by default and allows the radiosity processor to do less work for surfaces that are not visible in the current view. This should be turned off if you are generating movies, or if you want to view multiple positions in a design layer.

A more aggressive setting is the checkbox below the Optimize for Static View checkbox, labeled Include Visible Surfaces Only. This checkbox does just what it says ? only surfaces that are visible when the radiosity process is started will emit and receive indirect light. Other surfaces not in view will not participate in the radiosity solution. Note that non-visible surfaces may actually be important to the scene ? you must determine if this is the case before turning on this option (or assign radiosity overrides to force inclusion of the non-visible surfaces). It may be that speed is more important than accuracy and it is more useful to turn this optimization on than wait for all surfaces. Glass and other transparent surfaces are considered to be opaque; objects seen outside windows will not be included in the radiosity solution.

One case where the Include Visible Surfaces Only checkbox is very useful is for exteriors of buildings. It may be that the rendered view is a 3/4 view of a building?s fa?ade, and it is not at all necessary to figure out the indirect lighting for the interior of the building or the surfaces on the roof or on the back side of the building. If this is the case then it is very useful to avoid calculating the indirect lighting for the non-visible parts of the building and this option will help you generate the solution much sooner.

3D Bounds Optimizations

Below the view-dependent checkboxes in the Custom Radiosity Optimizations dialog are two options to limit the radiosity solution within a bounding box. The first option, Derive from Static View, lets the radiosity processor determine the bounds from the objects that are visible when the solution is started. The radiosity bounds will be set to the bounding box that includes all of the visible objects in the current view.

This option can automatically limit the radiosity solution to a single room of a larger building if the objects in the current view do not extend too far from the room itself. For example, if the floor and ceiling continue over the entire building then this optimization will not be able to limit the bounds to just the single room. If however the floor and ceiling end at the room?s boundaries then the radiosity solution will only be calculated for the surfaces within the room and will complete many times faster than without the bounds optimization.

You can also set the radiosity bounds by entering XYZ coordinates manually, if the Derive from Static View option will include more surfaces than you want. The manual bounds is shown in the preview as a box with white dotted lines if the Show Color-Coded Preview checkbox is on. Surfaces entirely outside the bounds are shown in black, and surfaces that intersect the bounds are shown in gray or red.

Manual bounds preview:

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Radiosity Overrides

After all the other options and overrides are set, it is still possible to force a surface to participate or not despite the other radiosity settings by applying per-texture or per-object radiosity overrides. These controls allow you to set whether an object or texture will emit and/or receive indirect light in the radiosity solution. Turning off both emit and receive effectively removes objects from radiosity processing, saving a lot of triangles and processing time. Such objects and textures will be rendered with direct lighting only.

Some examples of using overrides are:

1. Since bright triangles are dealt with first in the radiosity processor, it may be necessary to set overrides on bright surfaces so that the radiosity processor gets to other more important surfaces sooner. For example, if you want to render an interior using a bright directional light for the sun, the radiosity processor may spend a lot of time shooting light energy from the surfaces on the exterior since they are brightly lit. It will take some time before the triangles within the room are processed; if they are a darker color than the exterior then all of the exterior triangles will have to be shot before any interior triangles are processed. Applying a no emit no receive texture or object override to the exterior surfaces of the building will mean that they are ignored and the interior triangles will be processed much sooner.

2. Ground plane polygons and large image props have a very large surface area, will generate very many triangles because of their size, and usually do not contribute significant indirect lighting to the scene. Therefore it is often necessary to set radiosity overrides for these objects and set them to not emit and not receive. If necessary, ground planes can be split into two objects - a non-participating majority and a participating minority close to the building.

3. Light fixture symbols may have geometry in them that do not need to be included in the radiosity solution. These objects should be set to not emit and not receive. The triangle load can be greatly reduced in this way, if the lighting fixture is repeated many times in the model. Translucent bulb and diffuser textures should be set to not emit and not receive.

4. Conversely, if a lighting fixture?s appearance relies on light energy bouncing off of one of its surfaces, that geometry should be overridden and set to always emit and receive. Otherwise it might accidentally be excluded by the view-dependent, bounds, or size optimizations.

5. Clear glass textures should be overridden to not emit and not receive indirect light, since they will never produce significant indirect lighting and generating triangles for them is wasteful.

6. It is possible to limit radiosity processing to just a few surfaces, by setting the Other Items radio buttons to the May Only Receive or May Neither Emit Nor Receive choices. For example, if you wanted to see just the light that bounces off a tablecloth onto the ceiling and walls, you can choose the May Only Receive radio button, and then set the tablecloth to override radiosity to emit and receive. Light will be emitted from the tablecloth alone, quicker than if other surfaces were allowed to emit before the tablecloth.

7. If you use the Include Visible Surfaces Only optimization for a building exterior, it might happen that several important surfaces are left out because they are not visible. You can force them to be included by setting them to emit and receive. An example would be a modernistic building with a flat roof above the first floor that should emit light up against the second floor?s walls.

8. Sloped roof surfaces can be set to not emit and not receive if they will not emit significant light onto any other surfaces. Since roofs usually have a large surface area this could save a significant number of triangles.

Conclusion

You now know the controls available in RenderWorks radiosity and what situations are appropriate for each of them. These controls allow you to tune the radiosity solution for more speed and higher quality by managing the triangle load and helping the radiosity processor concentrate effort on the most important parts of the model.

[ 11-22-2005, 11:50 AM: Message edited by: Andy Stratton ]

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