Rendering Mental Ray: Final Gather

From TOI-Pedia


Many render engines don't calculate reflections of the light. To be more precise: they don't calculate the diffuse reflections. Diffuse reflections occur when light bounces of in all directions from non-reflective surfaces. In most cases this effect should somehow be taken into account to get a nice, realistically looking rendering.

Mental Ray offers a method that allows to calculate these diffuse light reflections: Final Gather. Final Gather is a method to approximate the diffuse reflections that occur in the real-world, while trying to keep render times as low as possible by using a smart calculation model.

Final Gather works best for scenes that have (relative) uniformly distributed light, typically: daylight exteriors and interior scenes with a fairly large area for incoming daylight or other lightsources with a large area.

Example of a Mental Ray Render

With Final Gather the calculations of light are divided in two components:

  1. Direct Illumination
  2. Indirect Illumination
    • Indirect Illumination from other surfaces (light bounces of direct light)
    • Indirect Illumination from the environment (eg: skydome)

Direct Illumination Indirect Illumination from direct light Indirect Illumination from environment

Direct Illumination
the light from (active) light sources, such as a pointlight, directional light, etc.
Indirect Illumination
light from other surfaces (reflected light) or the environment

Final Gathering diagram

Final Gather first calculates the direct illumination in the same way a normal renderer would. In this stage the basic light distribution across a scene is calculated.

In the second stage, Final Gather really kicks in. It determines the light intensity at any given point by examining the color values found within the hemisphere around that point. This is added to the light intensity calculated from the direct illumination.

The environment color is special within Final Gather: it will always have an effect within Final Gather. This means that Final Gather doesn't require active lights to get properly lit scenes. It can use the color value (e.g. white) from the environment or even a surface as light 'source' as well.


In most cases it's recommended to setup your indirect illumination from the environment first. Tweak the light intensity until it's about right (e.g. for a somewhat cloudy day). Then, add direct illumination with shadows ('sun'). You can adjust the light intensity for your direct light source to get the right results.

Enable Final Gather in the Render Settings with basic (low) settings for fast preview rendering.

Indirect illumination

Global Illumination using Image Based Lighting (IBL)

Indirect Illumination takes into account the effect of light 'bouncing' of other surfaces. The environment plays an important role for indirect light calculations, as it provides a large 'surface' that emits diffuse light (you can best compare it to a cloudy sky dome in the real world).

There are several ways to create diffuse light from your environment:

Render settings - Mental ray tab

Anti-Aliasing Quality

See: Rendering Mental Ray: Anti Aliasing Settings

Rendering Features

Activate Final Gathering to use the final gather render option.

Final Gathering

Indicates how much rays are used during rendering. A higher value improves the result because the indirect lighting will be better defined. Higher values mean higher rendering time and also the memory requirement will increase. Start testing with low values.
Value: Test render: 20 Final render for 'simple' light setups: 20 - 60 (eg: exterior scenes) Final render for 'complex' light setups: 60 - 500 (eg: interior scenes or exterior night/evening scenes; really depends on your scene, other FG settings and the available light sources)
Point Density
Mental Ray will heuristically determine the points for which Final Gather calculations are necessary. To achieve higher overall quality, increase this number.
Value: Test render: 0.4 - 0.6 Final render: 0.8 - 1 (only increase above 1 - but always below 2 - if really necessary)
Point Interpolation
Mental Ray can do some interpolation to achieve nice and smooth results without too much (expensive) Final Gather calculations. Increase this number to smoothen your render. It's not as accurate as increasing your Accuracy, but much, much faster. This setting determines the number of Final Gather points that should be considered for interpolation.

Test render: same as accuracy value - three times the accuracy value (20 - 60) Final render: 1/5th of accuracy value for accurate renders (with a min of 10) Final render: same as accuracy for quick smooth renders (with a max of 150)

Final gather scale
Reduces (or increases) the effect of final gather lighting contribution. If your scene is lit to brightly, you can reduce the final gather scale. It might be needed on some (rare) occasions, but isn't physically accurate.

Final Gathering Options

Use Radius Quality Control
You can enable the old (deprecated) radius quality control. Leave disabled for normal use. Quality control with Point Density and Interpolation is the preferred method.
This setting will determine how much of the white spots will be filtered out which are caused by reflective materials. These spots are easy to recognize. If they occur use this setting. If you increase the setting less spots will be visible. Only use this setting if it is necessary. Values higher than 2 are rarely used.

Using the Filter option is known to cause problems in low-light scenes, i.e.: your scene renders almost completely black, because the few bright spots FG finds, are removed in the filtering process. The only solution in these cases is to increase the FG Point density and accuracy.
Fall Off start & stop
The fall off is the distance which the light travels trough your scene. The light bounces trough your scene and travels a certain distance. De distance is measured from the light source. If the maximum distance is reached the light will switch from the reflected value to emitting a value which is the same as the emitting value of the environment. This means if you decrease the stop value the light will switch over to the light strength of the dome or background. The effect will be that the room itself can become much brighter because the value of the background is higher then the original reflected value.

In most cases, you can leave the Fall Off at 0. If you change the value to something other than 0, light will always penetrate your room even if you have no windows. These setting are only used if you want to make a nice image. The effect can give a non realistic image.
The amount of reflections a sub ray makes. Only increase above 2 - 3 when really necessary. Make sure you enable Secondary Diffuse Bounces (see below) if you want this setting to have any effect on diffuse bounces.
The amount of refractions a sub ray makes. The default value is 1 and should only be increased when using more complex refractions within your scene.
Max Trace depth
The maximum number of reflections and refractions that a single ray can make, added together. For architectural application, a setting of 4-10 is recommended to get 'realistic' light behavior (bounces).
Secondary Diffuse Bounces
Use this setting to include diffuse bounces of the light. In scenes where indirect lighting (by light 'bouncing' of walls) is important, you should enable this feature, which is the case for most interior scenes. The number of secondary diffuse bounces is only limited by the Maximum Trace Depth (in Maya 2009 and later).

Note on Secondary Diffuse Bounces

In Maya 8.5 and 2008, there are some differences in how to use secondary diffuse bounces. Refer to Mental Ray Diffuse Bounces in older versions for more information.

Back to Maya

Back to Lighting and rendering

Back to Mental ray rendering

Personal tools