INSIDE 3DS MAX® 7 [Electronic resources] نسخه متنی

Radiosity

We've thrown together a quick solution with the Light Tracer that looks pretty good. But there are times when we need absolute accuracy, when we want real-world lights and surface reflectance. This is where radiosity rendering comes in. This entire business about light being interreflected around a scene had its start as a tool for thermal engineers. They were concerned mostly with heat reflectance from surfaces, but their equations proved valuable to graphics programmers as well. Since thermal engineers are quite concerned about the relative sizes of things (it takes a lot more energy to heat up an aircraft hangar than it does to heat up a closet), they needed a solution that was size-specific.

Enter the radiosity solutionnot only a precise way to visualize bounced light, but also a means of assigning specific characteristics to lights and surfaces. The two aspects go hand in hand. Whereas the Light Tracer is a generalized solution that can be applied to most out door scenes regardless of scale, radiosity requires exact sizing of scene components to a real-world scale. Most radiosity rendering failures happen because this seemingly simple dictum wasn't heeded, so we'll put in a note for those skimming this introduction:

Note

In order to use radiosity in 3ds max, you must work under a real-world Units Setup, such as Metric or US Standard. If you don't follow proper sizing, your interior wall could be 8 inches, 8 feet, or 8 miles tall, and radiosity errors could result.

Light Sources for Radiosity

Another important consideration is to work with photometric lights instead of the standard 3ds max lights that you're used to. These are true representations of actual lights, and they throw light in the way that their actual counterparts do. You can get away with trying to use standard lights, but as these are converted to photometric anyway, you lose the control gained from doing it the right way.

Many manufacturers will give the specs for their lights in a catalog or on their Web site. A sampling can be found at the Erco Web site (www.erco.com), though you will have to sift through lots of technical data. Look for models compatible with VIZ 4, 3ds max's cousin, written more for architectural pursuits. You will also need the companion IES files that specify how the light spreads out from the fixture. The good news here is that the lamp is already modeled, and the light is attached (it's what 3ds max calls a luminaire: a grouped object/light combination). There is even an inverse kinematic link so that the fixture points wherever the target for the light is placed.

3ds max also has a "Common Lamp Values for Photometric Lights" help item. Many different types of lights are listed, but for now, here's a list of standard bulb-type lights that you might find in your home. They are all based on a Free Point Photometric Light type:

• 60-watt bulb = 70 candelas

• 75-watt bulb = 95 candelas

• 100-watt bulb = 139 candelas

To quickly drop in standard light elements, you can access the Photometric Light presets from the Create menu > Create > Lights > Photometric Lights > Presets.

Figure 20.27 shows the setup for a 100-watt bulb.

Photometric Lights are set up the same way. In addition to the method described above, they can be added to the scene by going to Create panel > Lights > Standard > Photometric. Point lights are similar to standard omnis. There are two unique light types, Area and Linear. The first, Area, simulates the light that might be given off by a glowing panel, like a fluorescent ceiling fixture. Linear lights mimic an individual fluorescent or neon tube. IES Sun is similar to a direct light and IES Sky is the radiosity version of a skylight.

Exposure Controls

Radiosity scenes tend to be too dark, and inexperienced users tend to keep bumping up the lighting power, trying to get something that they can see. This throws the lights totally out of whack. Imagine wearing dark sunglasses into a normally lit room and attempting to adjust the lights. 3ds max has Exposure Controls to rectify this situation. These controls are accessed from the Environment tab in the Environment and Effects dialog.

There are four choices available: Automatic, Linear, Logarithmic, and Pseudo Color. For most circumstances, Automatic will work fine. Linear can help scenes with not much lighting differential, while Logarithmic is better for very dynamic scenes. However, Logarithmic can give an over-saturation to files (Figure 20.28).

Finally, Pseudo Color is more of an analysis tool to show luminance and illuminance in the scene. A false-color rendering is made with a scale at the bottom (Figure 20.29).

Materials Considerations

Standard 3ds max materials tend to throw off too much light and cause light blooms. Material reflectance must be monitored closely, but this is difficult because colors tend to look too dark in the interface. If you're going to be working with radiosity, it's best to turn on Reflectance & Transmittance Information. Choose Customize > Preferences, and in the Radiosity panel select Display Reflectance & Transmittance Information. Once you do that, each material will give you a readout that allows for exact control (Figure 20.30).

Note that the gray material in Figure 20.30 is actually the white paint on the wall. The material is toned down by adjusting the Diffuse color's Value. This is accomplished by clicking the color swatch next to Diffuse and dialing down the Value in the Color Selector.

In the case of a bitmapped material, it's done somewhat differently. You have to open up the Bitmap slot and go down to the Output rollout at the very bottom. Adjust the RGB Level to get the desired Reflectance (Figure 20.31).

The maximum reflectance of even the whitest of whites should be no more than 80%. A value of 100% would mean that all of the light hitting the wall would be transmitted, a physical impossibility. If your materials start to look too glowing, try turning down the Reflectance until the problem goes away.

Transmittance has to do with clear or translucent materials; it's the amount of light that goes through. A completely opaque material has a Transmittance value of 0%.

Architectural materials have a Templates rollout that assists you in determining Reflectance and Transmittance values. You can set the material to behave like Semi-Gloss Paint, Paper, Masonry, and so on.

Note

Make sure that your geometry has lots of faces, at least one face every 2 feet. Incorrect radiosity solutions can result from not having enough vertices to hang the solution on. Also, try not to have sloppy intersections of wallsthey should have connected vertices at all edges.

Using the Radiosity Renderer

The radiosity settings are activated much the same way as the Light Tracer's: You open the Render Scene dialog and click the Advanced Lighting tab, then select Radiosity from the drop-down menu. You are greeted by the settings area (Figure 20.32).

The major difference between Radiosity and the other renderers in 3ds max is that an additional processing step must be accomplished before rendering. The processing puts a hidden web onto the scene's geometry that stores the radiosity solution. The good news is that the entire scene is processed at once, so that any subsequent renders can use the stored information. An architectural fly-through animation would need only one pass with the processing. The bad news is that any time you make changes, the whole scene must be reprocessed. The processing is controlled by the settings in the top part of the dialog shown in Figure 20.32.

The Initial Quality default value of 85% is enough to see the radiosity effect, and subsequent passes may be added to bring up the light detail in the scene or on specific objects. There is a point of diminishing returns, about 90% to 95%, as there can never be a total 100% solution. If your scene looks blotchy, increasing the Initial Quality will decrease the variation of shadows on a given surface.

Filtering can also improve the look of the scene without adding the overhead of refining passes. A setting of 3 or 4 can smooth out lumpy shadows.

Note

Sometimes the radiosity solver gets overloaded with too many changes in the parameters and gives back a "Solution is invalid at current time frame" error. Memorize your settings, and flush out the settings by choosing <no lighting plug-in> from the Select Advanced Lighting rollout up at the top. Then choose Radiosity again, and re-enter your settings. Reset All will not help. This can fix many of the problems you'll encounter.

Opening two of the lower rollouts, Radiosity Meshing Parameters and Rendering Parameters, we see the controls pictured in Figure 20.33.

The Radiosity renderer must break down the surfaces into smaller areas that it can map with the lighting information. The Global Subdivision Settings are used to specify the size. With good modeling, they shouldn't really need to be put into play, and can be left at a high value. If you are setting up to Regather, this also can be left at the default.

Selecting Re-Use Direct Illumination from Radiosity Solution gives the shadowing in the scene an inaccurate look. Selecting Render Direct Illumination gives a far better result (Figures 20.34 and 20.35).

In the Regather Indirect Illumination option, light is re-processed for a more exact solution. Of primary interest in this section is the Filter Radius (pixels) spinner. This needs to be changed depending on the size of the finished rendering. A small rendering needs a small value, like the default value of 2.5; a large rendering will need a greater value. Rays per Sample is an overall quality control, but turning it up too high can dramatically increase your rendering times. Adaptive Sampling works much the same way as its counterpart in the Light Tracer: More effort is given to contrasting areas where objects change than to large blank areas. Turning on Regather Indirect Illumination increases rendering time, so Adaptive Sampling can help to reduce it a bit.

More Information on Radiosity

The 3ds max file room.max has been included on the DVD so that you can experiment with radiosity settings. Try importing some of your own models into the scene to see how radiosity can work for you. Those who want to delve further into radiosity and peek at some of the underlying math can find an excellent introductory paper at