Render Item: Global Illumination

Allows you to create and save custom render settings as presets to reuse in other scenes. You can set any of the property settings within the sub-tabs and click the Save Preset button. To remove a render preset, click the Remove Preset button.

Adds an even illumination globally to all diffuse surfaces in a scene. This is most useful for simulating additional bounces of light. Some 3D artists recommend driving this value very low or off altogether and favor adding more complex lighting scenarios or using actual global illumination bounces to achieve adequate light fill. However, using a little ambient light can be a very useful optimization to any scene; if setup properly, it can look very natural and at very little cost in render time. Especially when compared to additional lights and indirect illumination bounces.

The ambient intensity control is a real world radiance value for lighting simulation, and simply acts as a multiplier for the ambient color. You can change it to show luminance units (candela per square meter) in the Input Preferences. Start with the default value of .05 W/srm and then work the value up or down until the desired lighting result is achieved.

Activating this toggle enables Modo to calculate global illumination.

This value represents the number of samples taken for each pixel in the image when using the Monte Carlo indirect illumination model (irradiance caching disabled). Increasing this value results in higher quality at the expense of longer render times.

Note:  When Indirect Illumination is enabled, the Indirect Samples option is also available in the Settings tab.

The default value of 1 means that a single bounce is used to calculate how a surface's environment affects it. This is less accurate than a multi-bounce solution since in the real world, photons bounce all over when illuminating an environment. However, the sacrifice in quality using fewer bounces is rewarded by increased performance. By increasing the number of indirect bounces, the indirect rays are fired from the initial surface and bounce off the first surface they hit, then continue traveling and hitting surfaces until the maximum number of bounces is met.

Note:  As you add bounces, the number of calculations and render time increases and while the result is technically more accurate, there are naturally occurring diminishing returns with each additional bounce. We recommend that you adjust your scene with a single bounce and then add additional bounces to see how great an impact they have on the end result. If a scene is mostly illuminated by a single small area of light, adding bounces can dramatically improve the overall brightness and look of the render.

Tip:  To reduce the number of bounces required, you can increase the Ambient Intensity value. Since the effect of multiple bounced illumination rays is to boost the overall lighting by some averaged color, you can reduce the number of bounces and set the Ambient Color and Intensity to mimic those final bounces. This saves time and yields similarly realistic results.

This value determines how far an indirect ray can travel before it is terminated. In the case of a ray being terminated due to the indirect range value, it is assumed that the ray eventually hits the environment background, so this value is returned to the shading engine. This is a very useful way to optimize render times. Reducing the indirect range generally improves render speed.

Note:  Setting this value too low creates unnatural lighting effects, as many rays that otherwise would have ultimately hit a geometric surface resulting in a shadow may get cut off early, illuminating the surface with the backdrop color, rather than shading it with the surrounding geometry. A setting of 0 disables indirect range, utilizing the full scale of your scene.

The following images show Indirect Range (IC enabled inside a 1m cube w/ default Environment), Ray Threshold 0%:

When rendering surfaces that employ subsurface scattering (SSS), you can choose how the indirect lighting in the scene affects those surfaces. When selecting Direct Only, Modo doesn't use any indirect (bounced) lighting, but only calculate SSS based on the direct lighting within the scene, that is, distant lights, spotlights, point lights, and so on. This mode is useful in many cases as global illumination's effect on SSS is usually fairly subtle. However, in instances where there are no direct lights in your scene, such as those lit entirely by image-based lighting (HDRI), you need to choose either Indirect Affects SSS or Both to see the effects of SSS for any given surface.

• Indirect Affects SSS - When selecting this option, Modo uses any global illumination in your scene, as well as the direct lighting to calculate the subsurface scattering effects on your object's surface.

• SSS Affects Indirect - When selecting this option, Modo calculates bounced indirect light, taking into consideration the subsurface scattering effects of any given surface.

• Both - This setting allows global illumination to affect subsurface scattering, as well as for the subsurface scattering to affect the indirect illumination calculations. This setting is the most render-intensive.

Importance sampling is a feature that takes into account the brightness of HDR images used for Image Based Lighting. Brighter areas of the image have a greater effect on the lighting result, so when enabled, samples concentrate around these brighter areas, producing a more accurate, smoother result with fewer IC rays (which means faster rendering). This feature essentially makes the process of applying a blurred low-resolution copy of the Environment HDRI unnecessary.

Note:  Importance sampling slightly changes the look of the final rendered image, most obviously producing more accurate shadows, which may or may not be a desirable result.

When rendering indirect illumination in Modo, irradiance caching is the default method. Disabling this toggle directs Modo to use the more intensive Monte Carlo method to calculate global illumination.

Modo offers two ways to calculate global illumination. By default, there is the very fast irradiance caching option that saves values and refers to them when needed. The second way is to disable IC, Modo then uses Monte Carlo path tracing. There is no caching of Monte Carlo values, so results can be slow and noisy, however, with enough rays and time, it can produce very clean and accurate results. The way ray evaluation works in Modo can be thought of as layers. When lighting with multiple bounces (having more than a single bounce defined in the Indirect Bounces control), each bounce is its own layer. The Irradiance Usage option allows you to control what layer of rays use irradiance caching during a render and which use Monte Carlo. The reasons for these options are to allow the strongest influencing rays (the first bounce) to use the more accurate Monte Carlo Method and blend that with the faster and smoother results of the subsequent IC evaluation. To mimic the results of Modo 701 and earlier, use the First and Second Bounce option.

• First Bounce Only - Only the first outward bounce from the camera uses IC, any subsequent bounces use Monte Carlo.

• Second Bounce Only - The first outward bounce from the camera uses Monte Carlo and then IC for the second bounce and back to Monte Carlo for any subsequent bounces.

• First and Second Bounce - Both the first and second bounces use IC and subsequent bounces use Monte Carlo.

Irradiance rays are technically the same as indirect rays in that they are the rays fired out from the surface in order to sample indirect illumination. For convenience, Modo has both irradiance and indirect rays settings since the two forms of indirect illumination typically require vastly different numbers of samples. Irradiance caching relies on much higher quality samples distributed sparsely across the project, whereas indirect illumination without irradiance caching uses lower quality samples at every pixel. Having two values allows you to easily switch between irradiance caching and traditional indirect illumination without constantly adjusting the number of rays for each sample.

The following images show Irradiance Rays, Ray Threshold 0%:

Note:  When Irradiance Caching is enabled, the same option is available in the Settings tab, called Irradiance.

Note:  The Supersampling Toggle option is hidden by default. Click the More button at the bottom of the panel to display all controls.

After all rays have been fired for a particular irradiance evaluation, this feature looks at the resulting ray color of each hemisphere cell and then sends additional rays through those cells that differ a lot from their neighbors, getting a more detailed look at high contrast areas of the environment and therefore a more accurate estimate. Currently about 25% more rays are fired, so for a setting of a 100 rays, you really get 125, but the results are better than if you had just used a setting of 125 without supersampling. This is because the rays are going in the more important directions. It is recommended to leave this option on all the time. It generally improves your render quality with minimal performance impact.

Note:  The Outlier Rejection option is hidden by default. Click the More button at the bottom of the panel to display all controls.

This option specifies whether to disregard rays that are much brighter or darker than the average values in a particular direction when irradiance cache values are being computed and Supersampling is enabled. It can be useful for smoothing indirect illumination in some cases and help to eliminate noise produced by very small, very bright illumination in a given scene.

As irradiance caching smoothly blends between neighboring IC values, a little breathing room is necessary between samples to provide space for that blend to occur. Irradiance Rate provides that space as it sets a minimum distance between IC samples, calculated by pixels. So a setting of 2.5 means the minimum distance between two samples is 2.5 pixels away. Increasing this value causes IC values further away from each other to blend together, providing a smoother result, however increasing the value too high may result in reduced accuracy in areas of high detail.

Note:  The Ratio option is hidden by default. Click the More button at the bottom of the panel to display all controls.

While Irradiance Rate sets the minimum distance between IC values, Irradiance Ratio sets the maximum distance. The ratio is a multiplier of the rate, so a setting of 6 multiplied by a rate of 2.5 returns a value of 15, meaning that IC values are no closer than 2.5 pixels and no further apart than 15 pixels. The ratio ultimately reduces the number of samples necessary across smooth surfaces where shading changes are minimal.

Note:  The Interpolation Values option is hidden by default. Click the More button at the bottom of the panel to display all controls.

Specifies a minimum for how many nearby values to interpolate. Let's say it's set to 3. If we're shading a point and can only find two nearby previously computed values in the cache, then it forces the computation of a new value at the current point, and the final irradiance at that point is a blend of all three values. So it tends to smooth out the shading. Increasing this value generally improves render quality at the expense of render time.

When rendering an animation where only the camera is moving in the scene, such as for architectural walk-throughs, enabling the Walkthrough Mode option tells Modo to never delete any irradiance cache values. Each frame continues to calculate pre-passes, generating new values when necessary and reusing existing values when available. Since Modo never needs to generate a new (possibly different) value, this can help to eliminate jittery shading sometimes evident on pristine architectural surfaces. However, for longer sequences it can create a large number of IC values, which may slow down rendering of later frames.

Note:  The Store Direct Light checkbox is hidden by default. Click the More button at the bottom of the panel to display all controls.

This option includes illumination from direct light sources in addition to the usual indirect illumination in the secondary irradiance cache values, saving time whenever a first bounce indirect ray hits a surface that already has cached secondary values. This is useful for applications like architectural interior rendering.

Note:  This option is only applicable if there are at least two GI bounces and Irradiance Usage is set to Second Bounce Only or First and Second Bounce.

Note:  The Retrace Threshold option is hidden by default. Click the More button at the bottom of the panel to display all controls.

If an indirect ray travels less than this distance, it ignores the second level IC and reverts to path tracing. This helps prevent light leaks (especially when using Store Direct Light), and can speed up rendering highly-detailed geometry, since irradiance caching doesn't work so well in those cases.

Note:  This option is only applicable if there are at least two GI bounces and Irradiance Usage is set to Second Bounce Only or First and Second Bounce.

This checkbox tells Modo to calculate caustics for all reflective and transparent surfaces.

The total photons control sets a ceiling on the total number of photons fired in the scene. The photon count is divided amongst all active 3D lights in a scene with a bias based on the power of the light, ensuring energy conservation.

Local photons indicate the number of photons required for each pixel sampled. When the pixel is rendered a search along the surface locates the nearest photons up to the local photon count. The default setting of 32 indicates that 32 photons are used for each pixel rendered. Increasing this value creates a smoother overall caustic effect at the expense of detail, while decreasing the value results in a sharper caustic effect with increasing graininess.

When rendering animations containing caustics, where the camera is the only item moving in the scene, it is helpful to enable the Walthrough Mode option. This tells Modo to cache the caustic calculations, frame to frame, reusing the initial calculations, thus ensuring each subsequent frame matches precisely, producing a smoother rendered result.