3D World’s question of the month

Producing clean and splotch-free interior renders using GI (Global Illumination) has always been a major issue in Cinema 4D. This is simply due to the way GI works and just how time-consuming the calculation is. Since CPU power is limited, GI has a number of ways to compromise accuracy for speed to make it usable. However, for this tutorial we will be using a trick to help the GI calculation.

With Cinema 4D R9.5, a number of new lighting features were added, including better area lights. Rectangular area lights had already been available for quite a while but the new area lights – which now also come in many different shapes – are much faster to calculate. This allows for a very interesting technique when rendering interiors with GI. The idea is to pre-light the scene with area lights and use GI on top of that to make the light ‘bounce further’, thereby replacing the first GI pass. This initial pass is usually where most of the patchiness occurs when using GI only.

With only a window to provide light to a room, the GI settings have to be very high to avoid patchiness. The smaller the window, the more difficult this becomes. To calculate GI, Cinema 4D needs to send out sampling rays for all the areas in a scene. The rays are sent out randomly. Let’s say one area on the wall sends out 10 rays and three of them happen to hit the window. The neighbouring area on the wall also sends out 10 rays but due to the randomness of the rays only one hits the window. As a result, those two areas will be very different in brightness, which causes patchy illumination on the wall. To fix this, we could increase the number of these rays, but the gain in quality probably wouldn’t make up for the extremely long render times. Therefore, it’s very difficult and time-consuming to illuminate a room with GI alone.

This is where area lights come in – if the room is already pre-lit and large illuminated areas already exist before the GI calculation, the GI rays will pick up much more balanced lighting information and thus produce more accurate illumination. GI is no longer used as the sole method of illuminating the scene, but merely as the finishing touch. Another side effect is perfectly smooth area shadows since those are now cast by the area lights and not created as a result of the inaccurate GI calculation.

Click here to download the support files (2.55MB)