UNDERSTANDING FINAL GATHER

by CGAfrica in Tutorials,

on February 16th, 2017

Final Gather can be found in just about all of the most common render engines and is an essential step in creating realistic renders that closely mimic how light interacts with objects in the real-world. With that in mind, let’s go over Final Gather, how it works and where it can be implemented into your next render.

What is Final Gather?
In the real world a lot of the light that you see is the direct result of indirect illumination (e.g., light that isn’t coming directly from a source like a light bulb). This is the light that bounces off other surfaces to illuminate your surroundings. When creating realistic renders whether it be for an interior or visualization, indirect illumination is something you will want to consider using within your scene. Final Gather is a prominent feature in most rendering engines, including the render engine mental ray which is the main rendering engine for Maya, 3ds Max and Softimage.

Final gather

For example, if you look closely at the image above you’ll notice that the wall has a slight tint of the red couch that is really the light bouncing off of the red couch and hitting the wall. This type of effect can be simulated using Final Gather.

final_gather_points

To calculate its results, Final Gather points are cast out onto any surface in the camera view. These points send out rays that sample the brightness and color values of any object they come into contact with, creating the indirect illumination in the scene. Final Gather is an easy way to achieve indirect illumination results for scenes that need accurate lighting, producing nice soft shadows efficiently within the scene and blending the colors together smoothly.

object

Because Final Gather points are sent out from the camera, another feature of Final Gather is that you don’t need a direct light source to add lighting effects. You can actually illuminate your scene from an object or surface, for example by increasing the incandescence of the object as seen in the render above. Of course, Final Gather may not be needed for every render that you do. Typically the reason for simulating indirect lighting is to create real-world effects into your render. If you are going for a more stylized render for your scene you may not need Final Gather. There is no need to increase render time if it can be avoided.

How is Final Gather different from global illumination?
Final Gather is often rendered along with something like global illumination, although it typically takes a much shorter amount of time to render than global illumination. Even though both Final Gather and global illuminations goal is to achieve indirect illumination, the way that they accomplish it is quite different. For example, global illumination uses photons to calculate and bounce around light, whereas Final Gather uses points that are set out randomly in the scene. These points then send out individual rays that sample their surroundings and bring that information back to that point. Final gather is not as physically accurate as global illumination, but it makes up for it because your render results will often times come back much faster.

It’s important to remember that while Final Gather and global illumination differ in mental ray, in most other 3D applications and renderers global illumination is more closely related to Final Gather and doesn’t use photons to get the information.

Tips for setting up Final Gather in mental ray
As mentioned above, Final Gather is not as accurate as global illumination, but Final Gather and global illumination can both work together. Using them in conjunction can create the most accurate lighting effect for your scene, and Final Gather can help eliminate some of that splotchiness that often occurs with global illumination. This means you won’t need to increase the amount of photons for global illumination and, by extension, the render won’t take quite as long since Final Gather renders faster than global illumination. It would be as if you only increased the global illumination quality.

Accuracy

When needing to increase the smoothness and quality of your Final Gather render you can increase the accuracy. By adjusting the accuracy, it either increases or decreases the amount of rays that are being cast out from a single Final Gather point. The higher the amount, the more rays that will be sent out to get information and bring it back to that point. Of course the higher this number is the longer your render time will be. So the key to successfully rendering with Final Gather is often done with a lot of tweaking to try finding that fine line of quality and speed.

You can enable the use of Final Gather on only certain objects in your scene. This option is great for fine tuning where your Final Gather occurs, as well as speed up your render time significantly. Since every project is going to have different requirements, it’s always a good idea to take a few moments before setting up your renders to determine what your project calls for and if you are able to get away with only having Final Gather on certain objects.

A common issue many artists need to overcome when working with Final Gather is, when used with rendering an animation, it can often give the render a strobe like feel to it. This occurs because on each frame any object that moves will mean that a new set of points will need to be cast out and there is no way that each point will be placed in the exact same spot as the frame before. To help avoid this, mental ray has a feature in the Final Gather options called optimize for animations, and this will help to smooth out that problem. 

source: Pluralsight

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