Final Gather process

What is Final Gather?

Final Gather is a process which exists for certain types of 3d rendering and particularly for the built in renderer Mental Ray within 3dsMax. It exists to help achieve Global Illumination within any given scene at rendertime. Global Illumination is a method of calculating the indirect light within a scene (light which is bounced off other objects in the scene) which contributes to the end lighting solution, producing more believable results than direct lighting alone (light coming directly from the light source).

FG_example

Figure 1: an indoor scene lwith direct lighting only (left) and with gobal illumination (right)

As can be seen above the additional contribution of light bounced from other surfaces can significantly contribute to the realism of the render.

How to use Final Gather?

Final Gather needs to use the mental ray renderer within 3dsMax, so make sure that is set as the current renderer. It can work with any of the sampling modes but the default Unified /Raytraced is best.

Within the Global Illumination Tab either Skylight Illumination from Final Gather or Skylight Illumination from IBL may be used. However, IBL based lighting provides better results with the least render times.

Final Gather has a number of settings, I will indicate a recommended process to use to produce acceptable quality and good render times.

A two-step process.

While Final Gather can be rendered with each image as part of the render process it is advisable to create a separate final gather map pass on a single machine and then use this to create the final renders from in a two step process. There are a number of reasons for doing so; speed on your final renders, flexibility (you can make render changes easily and still use the same FG map), and quality – one of the most difficult image artefacts to eliminate within a global illumination map rendered sequence is flickering between frames as the maps used to generate the lighting vary from frame to frame. This can be solved by increasing the quality (in some cases dramatically) but this has a time penalty of often unrealistic duration. Creating a separate final gather map creates a quality stable image that can be relatively quickly created and used to make much faster renders.

The first step: Creating the Final Gather Map (FGM).

Use these render settings: Indeed, the render settings can be set to a lower quality – .25 just make sure the minimum is no less than 1.

Step 1: The Renderer TabRenderer_1

Renderer_2

 GI_tab

Step 1: The GI Tab

Use these Global Illumination settings:

Note that the settings are high, although your particular settings may vary for your own scene.

The Project points are from the camera position (best for stills) and look particularly at the Reuse and Final Gather map section.

Use single file only, and have the Calculate FG/GI ticked.

Choose incrementally add FG points to FG map files and select …..

A networked file location and a file name. This will create a map of this name for you to use later in the next phase for rendering.

Step 1: The Common Tab

Finally for this first step. Look in the common parameters rollout, there you can choose how many frames to render.

Where there is little movement of objects in the scene you may skip every “n” number of frames. I have found every 20th frame works fine for modest camera speeds. In the next step of rendering the frames that are rendered will be blended together smoothly to cover those that are missing.

Note also that the size of the final gather map can be smaller than the final output – in this case it is half resolution (2048×2048 for an intended 4k resolution) it can be less – even 1k.

The Save file for the image itself can be anything you like as it will not be used, although it must still be present.

Another important note is that you can only render the final gather map on a single machine – so a single renderer that will not be interrupted must be available to generate the map, do not assign the job to multiple machines!

Use single file only, and have the Calculate FG/GI ticked.

Choose incrementally add FG points to FG map files and select …..

A networked file location and a file name. This will create a map of this name for you to use later in the next phase for rendering.

 Step 2: The Common TabCommonTAB2

STEP 2.

These are the settings used for rendering the final image sequence using the final gather map produced in the first step.

You may set the parameters for both steps 1 and 2 in scene states within the same scene, or if you prefer you can have both as separate scenes.

Note that this is your final render so full settings should be used – every frame, final render size, render elements (if any) etc.

Step 2: The Renderer Tab

These are the settings used for rendering quality. The quality setting can have a large effect on the final render times, and I found that a value of .35 still produces acceptable results. Do not reduce the minimum value below 1.

The other settings should work for a wide variety of scenes.

Step 2: Global Illumination Tab

These are the settings used to set the final gather map you created in step 1 ready to be used with the mental ray renderer.

A value of .5 is usually sufficient for the IBL shadows.

In Final gathering you will notice that the final gather points are set quite low which produces fast results. This piggybacks off the already rendered final gather map set in the Reuse section and creates additional points. In this case it was necessary to smooth out areas in the map that were not completely covered, so a small number of interpolation points and a few extra rays fixed this very well.

Note that maintaining a high number of rays and interpolation points at this stage will make your renders even slower than you would expect a normal render to take because it is in addition to an already highly detailed map.

Also note that diffuse bounces are not used, adding extra bounces dramatically increases render times for only a small improvement in realism (in most cases)

It is important to uncheck “Calculate FG/GI and skip final rendering” otherwise you will be overwriting your final gather solution and not rendering any images to disk!

Because the file you generated in step 1 was a single image file you must use this setting.

Make sure that the file you generated is the one selected in the Final gather map section of files to choose, and make sure you have “ Read FG points only from existing FG Map Files.”

You are now ready to render your file sequence out.

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