Final Gather

FINAL GATHER

Speeding Up Renders With Final Gather
1. For test renders switch point density right down.

Point Density 1 is very high for test renders.

While testing lets go for .1 or even .01

Be sure to bump it back up for final renders.

2. Make sure your final gather points are smoothing together
Make sure your final gather points are blending together. When final gather points are not blending together render times go up sharply! This can be tweaked easily with the Point Interpolation setting by default it should be pretty good. But play with it. To tweak this manually see “Use Radius Quality Control” under “Smoothing Out Final Gather” in the sections below.

3. While rendering moving cameras with no moving objects/characters make sure you’re caching your final gather. See “Saving/Caching Final Gather point data” below.

4. For animating objects or characters, render the character and the background separately.

The background is rendered first by using the cache settings in the “Saving/Caching Final Gather point data”.

To render the character or moving objects (which cannot cache final gather) create a HDR Image of the scene in the form of a spherical map. And for the contact surfaces use a “use background” shader.

Create a HDRI map out of a Maya Scene

Its also common for reflections to have lo res proxy objects instead of the hires so reflects seem true but they don’t have to have all the details of the hires objects in them. For instance you could have a very low poly character with no displacement and basic shaders rather than dense textures to reflect in objects of the scene. Its a cheat but the audience won’t pick it up and it can severely decrease render times.

Main Final Gather Settings

To see some of the changes visually you can click on “Diagnose Final Gather”

Accuracy

Is how many rays are cast out by each final gather point or dot.

Take the accuracy down to one and you’ll see lots of blotches. 100 is the default and is a good starting point. Values up to 500 is common especially with rendering animating which is more prone to flickering.

Density

Is how many dots are dropped onto the objects. With an accuracy setting of 100 for our test renders we can bring the density values down to .1 or .01. Bump it back up for final renders. 1 is a good upper range value, you can go higher if you like.

Accuracy and density work together. Density is how many dots, accuracy is how many points per each main point.

Point Interpolation

Is how many points are blended together to smooth out the final gather. Raise this number up to smooth out. Lower the number for less washing out but higher render times.

10 is a good starting value. Go higher and it can be fine but we start to loose detail in our renders.

Saving/Caching Final Gather point data
The following video shows how to cache out final gather point data to speed up render times.
http://www.digitaltutors.com/lesson/3691-Saving-and-re-using-Final-Gather-point-data

This is a way of saving render times when the scene is still and the camera is moving but no objects are moving.

Find the Final Gather area under Render Globals then the setting…

rebuild

Rebuild on Off or Freeze will use a cached file that you need to specify, its a file on the hard disk

Rebuild on will calculate the FG on every render.
Rebuild Off will add to the final gather points as the camera moves.
Rebuild Freeze will use only the final gather cache data and not create anything new.

Enable Map Visualiser and Preview Final Gather Tiles make sure Rebuild is set to On and render this will create a cache file of the point data. You can physically find it in your project under the folder

renderData/mentalray/finalgMap

We should be able to see the points in our 3d scene, if not go to

window > Rendering Editors > Mental Ray > Map Visualiser

you can manually browse to the final gather map file.

If you move the camera you can see the areas that have been missed. Anything that’s not visible to them original camera position.

Switch the rebuild to freeze and render times will be much faster but there’ll be artefacts behind the objects etc. .

Switch to rebuild to off and rerender from a new camera postion. The empty spaces will be filled and its adding points for the areas missed. The final gather point data file will grow in size.

Once all the angles are mapped switch rebuild back to freeze.

Note: Once everything is cached. Changing the point accuracy and density etc will not affect our renders.

Smoothing out final gather
http://www.digitaltutors.com/lesson/3692-Using-Min-Max-radius-settings-to-smooth-Final-Gather-results

Optimise for animations.
Turns on the “multiframe final gather animation”.
Final gather techniques are tricky with flickering as a common problem. Switch on this setting to help with flickering.

Point Interpolation
As discussed above set this number higher for smoother renders.

Use Radius Quality Control
Use Radius Quality Control overrides the Point interpolation setting and allows us to manually control this. Point Interpolation is the way that the points are blended or averaged together to stop artefact noise in Final Gather.

Max radius and min radius are in maya units and tell the computer how far to look at surrounding points in order to blur them. Very low values will give spotty results when the points don’t blend together. Also when points aren’t blending together render times can go up quite quickly.

Good Max radius values are roughly 10% of the viewable scene size min values about 1%.

View Pixel Size
With view pixel size on, the Max Radius and Min Radius values become measurements in pixels not in maya units. World space is technically better because you can change the image size and you don’t have to readjust these values.