What is final gathering?Final Gathering - Final Gathering is computed from the point of view.
When the primary ray hits the point on an object, final gather is
activated and a series of rays are diverted at random angles into the
scene to calculate the light energy (colour information) to be
calculated around the point for which we are computing colour for. This information is stored in the photon map. The photon
map is then consulted and is used to add the effect of the bounced
light in the ray-tracing process, this, essentially turns every object
into a light source.
Maya / Mental Ray Final Gathering TutorialFirst off, and most importantly, switch off Maya's default light. To do this, select Maya's default Render Globals, Windows > Rendering Editors > Render Globals (Maya Software) ...., now scroll down to the Render Options, and open them up, now find Enable Default Light,
and deselect it. This will ensure now that you have no other lighting
in your scene. Ok, let's start with a simple scene, a polygon sphere on
a poly plane, with the same poly plan duplicated and moved up a little
to provide the 'lighting' in the scene : -
Material wise, I've used a Lambert for the poly plane, diffuse set at 8.00, All Translucency options set to 0, for the sphere, I have used the following values : -
And for our 'Light' object, I have another Lambert material, with Translucency options
all set to 0, diffuse set at 1, and Incandescence set to 1 (full
white). Ok, don't worry about the values too much so far, we'll return
to them later.
Now we're going to set up the final gathering, Press F5 to go into the Rendering menu subset, now select Render > Render Using Mental Ray. Open up the Render Globals, the tab where it says mentalrayGlobals, scroll down to the Quality section. Open it up and where it says Render Quality, select the box at the end. This will take you into mentalrayOptions1, now find the final gather section and open it, click on to set it up, now you will have some options available to you, Final GatherFinal Gather Rays, Min Radius, and Max Radius.
These are currently set at 1000, 0.000 and 0.000 respectively, we are
going to change these values, so they are more in line with what we
require in our scene. Now Final Gather Rays, set to 300 (we will change this again later). The Minimum and Maximum Radius can be set up using the following calculation which is based on scene size, the general rule is, the Maximum Radius should be set at 5-10% of the scenes size (in Maya Units), to do this let's quickly use the Measure Tool In Maya, go to the top view, then go to Create > Measure Tools > Distance Tool, and grid snap the following way : -
So, from that we can see our scene size in units is 12 units (always go
for the bigger measurement in non square scenes), so from the
calculation above, that means our Maximum Radius should be set at 1.2, Now to calculate the Min Radius, the Min Radius should be set at 10% of the Maximum Radius, so that means Minimum Radius should be set at 0.12. Ok, now let's try a quick render. 
ok, that's not too bad, we're getting somewhere, now open up the standard Render Globals for Mental Ray, and scroll down and beneath Final Gather is Sampling Quality,
open this up and make sure you have the following values entered, for
Minimum Samples, set this to -2, and for Max Samples, set this to 2.
Now re-render. 
Now, that's looking much sharper around the edges and cleaner than the previous version. Unfortunately, the shadows are looking a little blotchy, go back to the Render Globals, and increase your Final Gather Rays to 1000,
to increase the quality of the final gather, (the more rays, the longer
render time), also in the sampling quality options, switch on Jitter, this helps reduce banding problems, and whilst your at it, change the filter type from Box to Guass Filter,
defaults are fine, too much and your scene will just look blurred, set
Scanline (Scanline will render parts of the scene that don't need
raytracing) to off within the Rendering section, and re-render : - 
Now, you can see where we're going with this, the blotchiness has been
reduced and overall, the image is much more pleasing to the eye.In this
final test render, I have set the Final Gather Rays to 5000 Rays, probably a bit much, as I take a hit on rendering time, but the shadows are much more pleasing : -
Tips:
[A] Within each objects material, there is a mental ray tab, within
this are some extra options, the Irradiance, and Irradiance Colour. The
option we are most interested in, is the Irradiance Colour, this
basically controls how much 'lighting' an object will receive, it is
currently set at pure white, which means it get completely illuminated,
lowering the values helps you to work out how much final gather effect
an object receives in a scene, for instance, in the example above, if
we increase the size of the 'light' object, i.e. the object for which
we have mapped white into the incandescence, the scene in itself
receives more 'lighting', now to counteract the extra light in the
scene, we can do one of two things, open the materials attributes and
within the Mental Ray tab, change the Irradiance Colour to a lower
value, and so receive less 'light', essentially your 'balancing' out
the extra light in the scene, or alternatively, if you increase the
size of the 'light' object, you can also lower the incandescence value,
which will globabally lower the lighting effect.
Also, there is the Photon Attributes, these are normally fine and not
worth tampering with, so the 'Derive From Maya' is usually the best
option here. Similarly, Within each objects attributes section, there
is also a Mental Ray section, where you can change it from 'Derives
From Maya' to your own options, where you can set up useful things like
whether or not an object can cast or receive photons, or do both,
useful when using Global Illumination, so you can set up, say a ring
object, to be an object casting caustics, but not receiving them.
[B] Within the attribute editor for the Mental Ray globals, there is a section called Translation
open this section up, now, where it says Export Verbosity, you will
notice that as a default it is set to 'Warning Messages', change this
to some of the other options for very useful information available in
your Maya Output Window, for instance, changing this to 'Info Message'
will give you all sorts of information/statistics about your scene,
which are quite helpful, but one that most 3d artists would find useful
would be the frame render time, allowing you to see 'exactly' how long
it took a scene to render.
[C] BSP - Binary Space Partition, The BSP is the way Mental Ray
partitions a scene down into workable chunks in memory, process it and
move onto the next chunk. So, in the Mental Ray Render Globals, in the
Memory and Performace section you will find options to alter the way
the BSP tree is used, there are three options available : -
BSP Size, BSP Depth and BSP Max Memory. Don't worry about BSP Max
Memory, the BSP Size is the number of triangles in a scene voxel that
Mental Ray will process, and the BSP Depth is the maximum number of
times Mental Ray will subdivide to get to this depth. A Setting of 4, 5
or 6 works best for Size, and
for Depth, you can use 1 of 3 different values, 25 for scenes
containing light geometry, 40 for medium, and 55 for scenes with heavy
geometry. Going over a value of 60 will give you a performance hit on
your render times.
(D) Colour Bleeding - Now, remember, right at the beginning of this
tutorial, we switched off our default light to start with, this was
essentially to make
sure we were concentrating on only 'object' lights and to make sure
there was nothing else 'external' affecting the scene, if we had have
left the light on, our objects would also have shown colour bleeding,
i.e. what this means is, if I had a yellow box on a white plane, you
would probably notice that where the white plane is near the yellow box
in line with the light, there would be colour bleeding, basically, you
would be seeing yellow illumination on the white plane. Now this can be
a wanted or unwanted affect in your scene, for those that wish to make
use of colour bleeding, I'd recommend using a light or more (up to you
the user and your scene requirements). See the figures below for
examples : -
Here we have a quick test render with no lights, only 'object' illumination
And in this figure, we have a spotlight added (0.5% Intensity), notice
the yellow colour bleeding the plane object. This can lead to some very
realistic lighting within your scenes if used correctly.
Thanks for reading and I hope you find this tutorial useful.
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