Frontier Developments : Planet Coaster - Crowd Audio (2/3)

Excerpt from audiokinetic.com

Planet Coaster - Crowd Audio : The Crowd Soundbox System (PART 2)

The croud soundbox system

During pre-production, it became clear that using an 'emitter per guest' solution is not effective when crowds are thousands of guests strong. Such a system would cause the amount of CPU work to scale exponentially as more and more guests wandered into your park.

Gameplay and simulation code managing crowds face similar problems. Our programmers turned to a fluid dynamics simulation rather than path-finding to increase the number of fully-simulated guests on screen at any given time. If you would like to know more, our Principle Programmer, Owen Mc Carthy. has written an article for Gamastura on the topic here.

During pre-production, Frontier's audio programmers worked closely with Owen to align our work with his. This allowed audio to decouple the 'amount of CPU work' from the 'amount of guests'. Rather than placing a single emitter on each member of the crowd and filtering through the list to see where they are, audio code performs this step using data Owen had made available through a park-wide crowd grid.

The crowd simulation grid is a fixed size and subdivided into cells. The time to scan through all the cells and find where guests are is also fixed. Since both values are fixed, the workload is distributed over multiple frames so the amount of work this system does is predictable, at the worthwhile cost of slightly increasing lag.

For audio content, we ran various experiments using different crowd recordings and densities. Our goal was to recreate a crowd made from grain-like elements rather than using fixed loops. Audio designer Michael Maidment ran a lot of tests with coders Dan Murray and Jon Ashby, and from their experiments we decided that a crowd (around the camera) can be expressed by four emitters placed in the four directions of the compass. A fifth emitter is added to describe the crowd in the entire park.

Furthermore, the best performance-to-quality ratio for us was in using three audio assets for crowd size (small, medium, and large) divided into different combinations of crowd diversities (adult male/female, teenage male/female, child). We would have liked to go even more small-grain (individuals, groups of two or four) but found that to be a path of diminishing returns.

The Soundbox brings all of the above together and ties audio content to the emitters. It is a system that uses data to make an informed decision regarding where to put emitters and what to play on them at any given moment.

A Soundbox's cycle starts by collecting data which itanalyzesfor relevant information such as crowd density, size, diversity, location, andbehavior. Once the Soundbox has an understanding of what the clusters of crowds spread around in a park 'look' like, it can create and place emitters in approximate 'correct' locations around the camera.

The Soundbox creates sounds from data, rather than the traditional way of deriving audio content from pre-placed emitters on game objects which means work can be distributed across frames, emitters can be pooled, and it circumvents the pitfalls of scaling.

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