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Why you should build a render farm

December 6, 2018

Image courtesy of Antonio Arroyo and Oliver Villar 



In this article we'll explain exactly why a render farm is important using real data.


We'll get to the bottom of why they are the only way to render some projects and how you can create one using free software.



An Explanation - What is a render farm, how does distributed rendering work?


Render farms are basically groups of computers that do distributed rendering, which is where you render either a single frame or multiple frames using several computers instead of just one. We're going to go into more detail about why this is faster but for now we'll spell out the basics.


There are two main methods for distributing rendering. The first and slightly easier method frame splitting. This is where an animation is rendered by distributing its frames over several computers (those several computers make up the render farm).


Each computer renders some percentage of all the frames in the animation. We've made a simple example to follow along to where an animation of nine frames is to be rendered on three computers (three computers is not a huge render farm, but much faster than just one computer!).

With frame splitting, a "master" node usually coordinates the render, sometimes this node (a technical name for a computer) will render some of the frames, some times it acts as a manager and controls the render, but does not render any frames itself. In our example, all the computers render.


The "slave" or "render" nodes are given some of the frames of the animation to render, in this example; three frames each.


If all three computers are the same in terms of processing power, then they will all complete the render in a third of the time just one of them would take. This is why distributed rendering is so important to artists, you can scale down your render time by adding more computers. 


The second method for distributing rendering is "bucket rendering" or "tile splitting". This method takes a single frame and splits it into parts to be rendered on multiple computers.

The tile splitting technique is what Crowdrender uses. As you can see from the diagram above, tile splitting works differently from frame splitting. Tile splitting breaks each frame up into parts and each computer renders one of these parts of the frame. 

Tile splitting is more flexible than frame splitting for the obvious reason that tile splitting works for single frame renders. Frame splitting cannot make a single frame render faster! Tile splitting is therefore very useful for fast previewing of frames to ensure there are no errors and that the overall impression of the image is what the artist wants.


Tile splitting is not without its own problems, however. Where there is a lot of compositing required, only frame splitting can accelerate the compositing. This is because the entire frame is available on each node and in compositing, you generally need to have the entire frame.


In tile splitting, compositing usually has to be done on the master node when the tiles from each slave node are received and the entire image is finally available. This means that when using tile splitting the compositing pass must be done on the master and is therefore slower.


Knowing just these details about each technique will help immensely in having a good final render experience! For example, if you have a heavy compositing pass, frame splitting may be a better choice.


An Example, why its so important to have a render farm anyway? 


We conducted a benchmark test using a project from Antonio Arroyo (see the image above!). The test was simple, the baseline was a single computer rendering the scene. For this baseline test, we used a macbook pro with a fifth generation i5 processor operating four threads.