Another tip is to set a very low amount of reflection for every object that might need it. If this object is not reflecting in real, we set the value to 0.01%. The convenience of that is that in the post we can blur and maybe color the reflection of the object and use it to create additional specular effects. For example, for materials like silk it is much easier than to try to find the right values and maps in the Material Editor. It does increase the render time, but it is still much less time consuming then searching and experimenting for complex materials that work the way you want them and also create the desired look in the final image.
Rendering - Render Pass Manager
Blessedly we could use Grant Adam's RPManager for the project. With this plugin it is possible to save different settings for the scene in passes (Fig.05). The first and second pass for this scene were the calculations of light cache and irradiance map, without actually rendering the beauty picture.
The third pass was the beauty pass, which loaded the light cache and irradiance map data and rendered out the final picture together with some render elements like lighting, global illumination and raw reflection. These render elements are very important when you want to experiment with the final look during post-production. For example, we were able to increase the sunny summer look of the picture by simply screening in the lighting element some more. The beauty pass also had a render element called Multimatte, which created a mask of objects that could be manually selected (in this case the glass, because it was hidden in the AO Falloff pass, which we usually render the masks in).
The fourth pass was a rendering of the scene completely without lights and camera exposure and an override with a falloff material. This is good to be able to make some color corrections that affect the sides of objects. By doing so, you can create effects of roughness or effect lacquer. This pass also renders an element called "VRayExtraTex", which allows us to load a map (in this case an Ambient Occlusion map). Additionally we also rendered an element for the ZDepth and elements for the masks. Because the glass is hidden in that pass, it is possible to render masks for objects which are usually occluded by it. All four passes can be seen in Fig.06.
For the animation of this project we decided to calculate the light cache and the irradiance map in separate passes to load them into the beauty pass. The advantage over calculating them directly with the beauty pass is that only one frame has to be calculated. By using the option Use Camera Path, it is possible to generate the map in the first frame and V-Ray will calculate the rays of the whole movement of the camera in the active timeline. The results can then be used together with the light cache results to render out the beauty images with multiple different machines that only have to look up the illumination data. By doing it that way, you can make sure to have a convenient result without major splotches or flicker.
Another option for the irradiance map would be to calculate the map by Add to Current Map every 10 frames, but the single-frame-method is faster and offers better results if there is only little animation in the scene like noisy movement of the drapes and plants. It is really important to set Distance Threshold to a higher value like 0.3 when using this method in conjunction with small object animations.
At the beginning we had the problem that the samples of the GI on the ceiling were too big. At first we tried to increase the samples and subdivisions of the irradiance map, but after some tests and the realization that this needed too much render time, we decided to go with another method.
So we went on and increased the subdivisions of the light cache, because we thought that the floor was mostly illuminated by the second rays and thus increasing the resolution of the light cache would minimize the splotches. Actually this didn't change a lot in that area, but we had better results in small corners or under the couch (that's why the subdivisions of the light cache were left at 2000 - the calculation of it still finished in only half an hour per scene).
At the end we found the solution by merging the irradiance maps of every scene into one map. Usually objects, which have big planar areas, receive not a lot of samples. So we calculated an irradiance map for a direct look at the ceiling with high settings and merged it into our existing irradiance map. At the end our irradiance map had a size of 1 GB, but it was worth it, because now we had almost no visible sampling problems on the ceiling. Fig.07 shows the resulting irradiance map. You can watch your irradiance maps with the imapviewer, which ships with V-Ray.