On the Ranch, we render still images using our own technology called MultiBand. A still image is split into multiple horizontal strips in order to distribute them across several servers to speed-up the render process.
Global Illumination biased algorithms (Irradiance Map, Light Cache) uses random sampling to avoid artifacts caused by regular sampling (samples aligned in a grid). It means that from a render to another, the GI solution will not be exactly the same.
For instance, here is an animation of 10 renders (same frame/settings), calculated in a row, on the same computer:
If no pre-pass file is provided with the scene, each node of the farm will calculate its own random GI solution and, therefore, the strips will not match when stitched together, as illustrated on the following render:
There are two solutions to avoid this problem:
Here are two renders from the Cinema 4D Physical render using Global Illumination, one using Irradiance Cache and the other using QMC.
The settings are very low on purpose, to generate artifacts. Irradiance Cache creates splotches whereas QMC creates noise. Unlike the splotches, as the noise is one-pixel sized, there is no visible junction between the strips after the stitching when using QMC.
We recommend the OpenEXR format (.exr) to render still images on the Ranch.