Light Tracer

The Light Tracer provides soft-edged shadows and color bleeding for brightly-lit scenes such as outdoor scenes. It is typically used in conjunction with a Skylight. Unlike radiosity, the Light Tracer does not attempt to create a physically accurate model, and can be easier to set up.

Main toolbar > (Render Setup) > Render Setup dialog > Choose Default Scanline Renderer as the active production renderer. > Advanced Lighting panel > Select Advanced Lighting rollout > Choose Light Tracer from the drop-down list.
Rendering menu > Render Setup > Render Setup dialog > Choose Default Scanline Renderer as the active production renderer. > Advanced Lighting panel > Select Advanced Lighting rollout > Choose Light Tracer from the drop-down list.

Outdoor scene lit by Skylight and rendered with light tracing

Character lit by Skylight and one spotlight, and rendered with light tracing.

Model by Sonny Sy.

Tip: While you can use light tracing for indoor scenes, radiosity is usually the better choice in such cases.

Previewing the Effect of Light Tracing

To get a quick preview of the effect the Light Tracer will have, lower the values of Rays/Sample and Filter Size.
The result will be a grainy version of the full effect.
Another way to get a quick preview is to make sure Adaptive Undersampling is turned on. In this group, set the Initial Sample Spacing sampling and the Subdivide Down To setting to the same value. In the General Settings group, lower the value of Rays/Sample, and set Bounces equal to 0.0. This gives a rather blotchy but fast preview of the rendering. Increase the Rays/Sample and Filter Size values to improve the image quality.
In general, you can get good, fairly quick results with a lower Filter Size value as long as Rays/Sample has a high value and Adaptive Undersampling is on.

Other Tips for Using the Light Tracer

To improve rendering time, use the Object Properties dialog to disable light tracing (or radiosity solving) for objects that don't have a great impact on the final effect.
Tip: You can also use the Advanced Lighting Override material to alter the effect of light tracing on particular objects. For example, if you encounter visual artefacts with a bump-mapped material, convert it to an Advanced Lighting Override material and reduce the Indirect Light Bump Scale value.
Experiment with the Adaptive Undersampling group settings, which restrict light tracing to the areas of your scene that need it.
To increase the amount of color bleeding, increase the values of both Bounces and Color Bleed. Color bleeding is usually a subtle effect.
If there are glass objects in the scene, increase the Bounces value to an amount greater than 0. But be aware that this increases rendering time.
If the main scene lighting is a Skylight, and you need specular highlights in your scene, add a second light: for example, a Directional light that parallels the Skylight. Make sure Shadows are turned on for this light, and on the light's Advanced Effects rollout, turn off Diffuse.
If the objects with highlights don't greatly affect shadows or color bleeding, you can leave Diffuse on for this light, and use Object Properties to exclude the objects from light tracing.
Set Key filters are not taken into account when you animate Light Tracer settings. If you wish to use Set Key to create keys for animating the Light Tracer parameters, +right-click the spinner to create those keys.
Attention: If you use a texture map with the Skylight, you should use an image-processing program to thoroughly blur the map before using it. This helps reduce variance and the number of rays needed for light tracing. You can blur the map beyond recognition, and it will still look correct when used for regathering.

Procedures

To set up a scene for the Light Tracer:

This is a typical use case:

Create the geometry for an outdoor scene.
Add a Skylight to illuminate it.
One or more spotlights can also work well. If you use the physically based IES Sun or IES Sky lights, using an exposure control is essential.
Choose Rendering Advanced Lighting Light Tracer.
This opens the Render Setup dialog to the Advanced Lighting panel and activates Light Tracer.
Adjust the Light Tracer parameters, activate the viewport to render, and then activate the Common panel.
Adjust your rendering settings, and then click the Render button at the bottom of the dialog.
The scene renders with soft-edged shadows and color bleeding.

Interface

General Settings group

Global Multiplier

Controls the overall lighting level. Default=1.0.

Left: Lower Global Multiplier value.

Right: Higher Global Multiplier value

Object Multiplier

Controls the level of light reflected by objects in the scene. Default=1.0.

Note: This setting has little effect unless Bounces is greater than or equal to 2.

Sky Lights [toggle]

When on, enables regathering from the Skylights in the scene. (A scene can contain more than one Skylight.) Default=on.

Sky Lights [amount]

Scales the intensity of the Skylights. Default=1.0.

Above: Increasing the Sky Lights value.

Below: Increasing the Object Multiplier value

Color Bleed

Controls the strength of color bleeding. Color bleeding results when light is interreflected among scene objects. Default=1.0.

Note: This setting has little effect unless Bounces is greater than or equal to 2.

Above: Excessive color bleeding.

Below: Color bleeding eliminated by setting Color Bleed to 0.0

Rays/Sample

The number of rays cast per sample (or pixel). Increasing this value increases the smoothness of the effect, at a cost of render time. Decreasing this value results in a grainier effect, but renders more quickly. Default=250.

Tip: To get a “first draft” preview of the effect of light tracing, reduce the value of Rays/Sample and the Filter Size.

Changing the number of rays per sample. The higher the value, the less grain.

Color Filter

Filters all light falling on objects. Set to a color other than white to tint the overall effect. Default=white.

Filter Size

The size, in pixels, of the filter used to reduce noise in the effect. Default=0.5.

Tip: Filter Size is especially useful when Adaptive Undersampling is turned off, and Rays/Sample has a low value.

Increasing Filter Size reduces noise in the rendering.

Extra Ambient

When set to a color other than black, adds that color as extra ambient light on objects. Default=black.

Ray Bias

Ray Bias, like Shadow Bias , adjusts the positioning of the bounced light effects. Use it to correct rendering artifacts, such as the banding that can occur when an object casts shadows on itself. Default=0.03.

Bounces

The number of light-ray bounces that are traced. Increasing this value increases the amount of color bleeding. Lower values give faster results with less accuracy, and typically produce darker images. Higher values allow more light to flow through the scene, resulting in brighter, more accurate images at a cost of rendering time. Default=0.

When Bounces equals 0, the Light Tracer disregards volumetric lighting.

Tip: If your scene has transparent objects such as glass, increase Bounces to be greater than zero. Be aware that this increases rendering time.

Increasing the number of bounces increases the level of global illumination and the amount of color bleeding in the rendering.

Cone Angle

Controls the angle used for regathering. Reducing this value can result in slightly higher contrast, especially in regions where lots of small geometry casts shadows on a larger structure. Range=33.0 to 90.0. Default=88.0.

All rays initially cast are limited by the cone angle

Volumes [toggle]

When on, the Light Tracer regathers light from volumetric lighting effects such as Volume Light and Volume Fog. Default=on.

For volumetric lighting to work with light tracing, Bounces must be greater than 0.

Volumes [amount]

Multiplies the amount of light regathered from volumetric lighting effects. Increase to increase their impact on the rendered scene, decrease to decrease their effect. Default=1.0.

Increasing the Volumes value increases the effect of volumetric lighting in the rendering.

Adaptive Undersampling group

These controls can help you speed up rendering time. They reduce the number of light samples taken. The ideal settings for undersampling vary greatly from scene to scene.

Undersampling initially takes samples from a grid superimposed on the pixels of the scene. Where there is enough contrast between samples, it subdivides that region and takes further samples, down to the minimum area specified by Subdivide Down To. Lighting for areas not directly sampled is interpolated.

Initial sampling uses a regular grid.

Adaptive undersampling concentrates on transition areas.

Tip: If you use adaptive undersampling, try adjusting the Subdivision Contrast value to obtain the best results. The effect of this control depends on the value of Rays/Sample.

Adaptive Undersampling: When on, the Light Tracer uses undersampling. When off, it samples every pixel. Turning this off can increase the detail of the final rendering, but at a cost of rendering time. Default=on.
Initial Sample Spacing: The grid spacing for the initial samples of the image. This is measured in pixels. Default=16x16.
Initial sample spacing values
Subdivision Contrast: The contrast threshold that determines when a region should be further subdivided. Increasing this value causes less subdividing to occur. Too low a value can cause unnecessary subdividing. Default=5.0.
Decreasing the subdivision contrast threshold can reduce noise in soft shadows and bounced lighting.
Subdivide Down To: The minimum spacing for a subdivision. Increasing this value can improve render time at a cost of accuracy. Default=1x1.
Depending on the scene geometry, grids larger than 1x1 might still be subdivided below this specified threshold.
Show Samples: When on, sample locations render as red dots. This shows where the most sampling has taken place, which can help you choose the optimal settings for undersampling. Default=off.