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R11 GI Tutorial by Michael Vance, August 8, 2008. Part 1

Advance Render Global Illumination: Out with the old, in with the new.

R11 GI Tutorial Part 1
R11 GI Tutorial Part 2
R11 GI Tutorial Part 3

Image 1: GI Mode: IR+QMC (Still Image) / High Settings / TRT: 13 Minutes.
Render times are on a Mac Dual G5 and are for comparison only. New computers will be faster.
Scene File: http://mvpny.com/R11GITutorial/CausticRingMV6Public1.zip

CINEMA 4D Version 11 AR3 GI is a major upgrade from AR2 with a completely revamped new state of the art Global Illumination native render engine for Cinema 4D. VRay users may notice some similarities, while AR2 users will need to become acclimated to new terminology, settings and controls. Cinema users that until now wanted better GI in Cinema's native Advanced Render should appreciate the improved render performance and control.

There are 47 independent GI settings. Of those 28 fall within 7 presets. Next to each preset is a bullet that can be opened to edit all settings that fall within the preset category. To begin the experimentation process to judge which preset combination a scene will do best with only requires the user to activate GI and hit render using the GI default settings.

The default settings render quickly. Getting good GI results from AR3 doesn't generally require one to know everything described hereafter, but if you want to tweak your results, render an animation, or just want to know more about the inner workings of Cinema's new GI, this tutorial is for you.

Image 2:

Diving right into the GI General Illumination render settings, users select a GI mode (from the dropdown menu shown above), a Diffuse Depth, Primary and Secondary Intensity, and Gamma correction if desired. I'll return to the GI modes after a quick discussion about the other options found here in the general settings page of the render settings, since Diffuse Depth and Gamma are particularly useful settings to start with.

Diffuse Depth: The number of surfaces rays will test.

In the real world, light rays emit from a heated surface and travel in all directions, diffusing in intensity following the inverse square rule. For every doubling of the distance traveled, a contact surface will receive a quarter of the illumination. Light rays travel through a vacuum at 299,792.458 km/s until they hit other surfaces and are either absorbed (stopped), refracted (slowed), or reflected (reversed), often, as with glass, in a combination of all three. The output of any of these three, absorption, refraction, and reflection, never return more or less energy than went into it. To save processing time and still arrive at the same result, Global Illumination simulates this behavior, paradoxically, by working in somewhat the opposite direction .

Samples called stochastic rays are sent outward from surfaces into the scene to find surfaces that emit either direct or GI reflected light. A surface considered to emit light by AR3 GI in Cinema is one that has a material with an active illumination channel, or one that is directly lit by a standard Cinema light source. Depending on settings, hundreds or thousands of these rays are sent outward from each point or record area to sample the surrounding scene, with their results averaged to obtain a final brightness value for that surface point or area.  In order to find light sources that aren't in a direct line of sight, these stochastic rays must bounce off the first surface hit or emit new stochastic rays from the bounce surface to see what lies around the corner. Thus a diffuse setting of two (two because two surfaces are tested, the first one it hit and the one hit following the first bounce), represents one bounce, a setting of three equals two bounces, etc.

Although technically the rays go in the opposite direction, diffuse depth can be thought of as the real world number of bounces (minus one) that illumination from a surface will travel before it stops. For light to bounce off of walls and around corners, a depth greater than one must be selected. The choice of ray depth depends on how many corners you need the light to look around, and how physically accurate you need the final GI result to be. Typically, a scene that is lit only with material illumination sources will need at least one higher ray depth or more. A scene that is lit primarily with standard light sources may require one less or fewer. As with every GI setting, the selection of a ray depth is dependent upon the quality desired weighed against what the user deems an acceptable render time. The renders below (Image_03) show the result of higher diffuse depths from 1 to 4.

Image_03:  GI Mode IR (Still Image), Medium Settings, GI Portal w/Material Oversampling.
Render times are on a Mac Dual G5 and are for comparison only. New computers will be faster.
Scene File: http://mvpny.com/R11GITutorial/EggboxMV10NoFusion9Public1.zip

Primary and Secondary Intensities: More or less.

Primary intensity sets the first ray depth GI strength, which in turn will affect all subsequent depths. Secondary Intensity multiplies its value with the Primary Intensity value to determine the relative strength of secondary depths only. Thus, the secondary intensity will normally be left at 100% and need not be changed to match that of the primary intensity. The default 100% settings are suitable for most situations, but the option to change them gives the user the option to boost or decrease the relative strength of the GI.

Gamma: The malleable falloff curve.

GI light bounce intensity automatically falls off using the physically correct inverse square rule. Gamma correction effectively changes the falloff curve and can thus be used to lighten up the GI result to, for example, compensate for a lack of diffuse depth, though it won't add light where there is none. Because it works on the unclamped bit depth, it produces a smother result than if gamma correcting a 24 bit image.

GI Modes Compared: Choosing the best modes and settings for the scene at hand.

Two principle GI engines are employed in the new AR GI. The first is QMC, which is closest in principle to the old Advance Render GI stochastic solution, and the second is Irradiance Cache, which is closest in principle to the Radiosity method of previous AR GI versions. (There is one more special mode called Sky Sampler which I will discuss later) While there is just one QMC preset, Irradiance has six options suited to different purposes. The selection of a GI mode is the most important scene dependent choice you will make and the presets are aptly enough named to make the choice easy. Let's look at QMC first and then examine the IR modes.

GI Mode: QMC: The carpet bombing approach.

Image_04

If the GI Mode QMC is selected in General Settings (Image_04), each scene shading point is sampled. That is, an array of stochastic samples (like an array flower) will blossom from every single point in the scene visible to the camera. Sample Count determines the number of stochastic rays sent outward to sample the scene for each shading point. The term QMC is also used in four of the IR modes, as well as in two special material sampling options, so it may be helpful to first examine what QMC is and how it is used in Cinema.

QMC stands for Quasi Monte Carlo, and is an improvement on the more primitive Monte Carlo method, a term first coined in the 1940's by physicists working on nuclear weapon projects in the Los Alamos National Laboratory. (http://en.wikipedia.org/wiki/Monte_Carlo_method) With a Monte Carlo sample, a random array of hemispheric stochastic rays is sent outward to sample scene lighting. The term Monte Carlo, named after Monaco's famous casino district, derives from the fact that the exact directional distribution of rays is driven entirely by chance, with the probability of a representative sample increasing proportionately to the number of samples used. Because the distribution of stochastic rays is completely random with Monte Carlo, an undesirably uneven distribution of rays can occur, especially when fewer rays are used, and this can lead to noise. QMC insures a more spatially even distribution of rays, especially at lower sampling rates, thus insuring a more representative, better overall array sample.

Image_05

While QMC ray distribution is more even, it is still important that a sufficient number of rays are used to obtain an accurate scene sample. The QMC specific controls shown below (Image_05) determine the number of stochastic rays used for each shading point.

Image_06

If a scene material light source is small, the chance of a small stochastic sample count finding that source and accurately determining the degree of weight it should be given in relation to other scene surfaces is greatly lessoned. Though the default stochastic sample count is 64 (Image_06), stochastic counts into the hundreds or thousands might actually be necessary, depending upon the relative size of the illuminating surfaces

Image_07   GI Mode: QMC / Sample Count: 750 / Diffuse Depth: 1
Render times are on a Mac Dual G5 and are for comparison only. New computers will be faster
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750 stochastic samples per shading point were used to render the image above (Image_07). The render took 2 hours and 8 minutes, but the graininess tells us that even this many samples were not sufficient to render every pixel evenly given the relatively small light source. Because so many stochastic samples for each shading point sample can be so prohibitively render intensive and time costly, especially at higher diffuse depths, R11 offers some methods new to Cinema to either target rays directly toward selected materials using special Material Sampling Modes, or by altering record and stochastic ray count on a per object basis using compositing tags.

Oversampling, QMC Sampling, and Per Pixel QMC Sampling:

Image_08

Whereas QMC evens out stochastic sample rays, Oversampling creates extra rays that target only selected illuminating materials. In the illumination tab of each material in Cinema is found a special Sampling Mode menu (Image_08). When one of these oversampling options is selected, extra stochastic rays sent from each GI record in a scene target the material light sources directly, discovering their full characteristic, size, shape and color, with fewer rays, ensuring both better visual result and faster render performance than if the entire scene were so sampled.

Oversampling: An all around good player.

This setting is suitable for most situations where you have a medium to large illuminating surface. It greatly improves render quality by stochastically oversampling the illuminating material, and is fast. An illuminating material is again one that has a luminance channel active or a surface directly lit by a standard light source.

QMC Sampling: When you're not in a hurry.

Similar to Oversampling, still more stochastic rays are focused on the illuminating material. A bit slower than Oversampling but better suited to small illuminating surfaces.

Image_09

GI Mode QMC results for material sampling modes Normal, Oversampling, and QMC Sampling are illustrated above (Image_09). The yellow rays represent the extra stochastic samples used to target and thus oversample the illuminating material.

Per-Pixel QMC Sampling: When every pixel counts.

When using IR render modes, this material sampling mode forces a sample of the illuminating material from every shading point in the scene, much like if you did a pure QMC render targeted at that surface only. When rendering in an IR mode, this setting produces dramatically better shading and shadow definition, but is correspondingly slow. Like the QMC Sampling mode, this mode is most suited for use with small illuminating surfaces. The mode is redundant when rendering in QMC GI Mode.

Image_10

GI Mode IR results for material sampling modes Normal, Oversampling, QMC Sampling, and Per-Pixel QMC Sampling are shown above (Image_10). The yellow rays represent the extra stochastic samples used to target and thus oversample the illuminating material.

Image_11   GI Mode: QMC / Sample Count: 75 / Diffuse Depth: 1.
Render times are on a Mac Dual G5 and are for comparison only. New computers will be faster.

GI Mode QMC results for material sampling modes Normal, Oversampling, QMC Sampling and Per-Pixel QMC Sampling are shown above. (Image_11). Notice how much faster the QMC Sampling and Per-Pixel QMC Sampling modes have rendered the image with less noise. You might think that these are good options to use always, and for many situations this is true, but notice also how the shadow and the caustic from the foreground light source is missing behind the marble. This is because the rays targeting the foreground light directly can't correctly calculate the transparent shadow and caustic accurately. (The caustics in the front are from the HDRI image used in this scene which is not oversampled.) Even the Oversampling mode (Top Right), though having a better shadow, has lost some of the shadow with caustic and thus some of the luster of the scene, especially noticeable in the refraction of the blue cats eye portion of the marble.

Below (Image_12) the marble is rendered from another angle which shows this better.

Image_12   GI Mode: QMC / Sample Count: 75 / Diffuse Depth: 1.
Render times are on a Mac Dual G5 and are for comparison only. New computers will be faster.

Most of the time the material sampling modes can render better quality much faster, but they do not handle refractive caustics well, and they don't work with reflective caustics at all.

GI Portal: The window to better results.

Image_13

Another material specific setting, GI portal enables the material oversampling modes just discussed to be used with a transparent material such as a window. This mode is perfect for indoor renders where the main illuminating source, a sky for example, comes through a small portal or window. A material must have its transparency channel activated before the GI Portal option can be selected. When enabling the portal option, be sure to also enable one of the oversampling modes, as it has no effect when used with normal sampling. They must be used together.

The image below (Image_14) compares a cornell box having a window at the top rendered first without the portal option checked (top left), and then with the portal option checked in combination with the different oversampling modes.

Image_14:     GI Mode: AR (Still Image) / Diffuse Depth: 3 / Low Settings
Render times are on a Mac Dual G5 and are for comparison only. New computers will be faster.
Scene File: http://mvpny.com/R11GITutorial/CornellBoxMV2.zip

R11 GI Tutorial Part 1
R11 GI Tutorial Part 2
R11 GI Tutorial Part 3

Proceed to Part 2