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The rendering pipeline of Eve follows the producer/consumer design pattern. In order to create output, timelines must be able to create a producer to render content in a one-use, low-cost way. The render pipeline then acts as the consumer for this producer, feeding frames from the timeline into the output (whatever that might be — the output is abstracted from the rendering pipeline).

The first drawback with the producer/consumer pattern involves the idea of passing around buffers, the designed abstraction of low-level media data for Eve’s purposes. Because we can implement the Buffer interface any way we choose, we can pass around buffers that do not need to be copied, making the cost of permeating data through the pipeline negligible relative to the cost of the actual work being done. The input layer, rendering layer, and output layer no longer have to agree on Buffer implementations, just that these Buffers must have the same media type. The problem then lays in how we pass data between the layers of the pipeline.

The solution to this is two-fold: first, introduce marshaling and unmarshaling methods for Buffers passing into and out of the rendering stage, respectively. This lets the rendering pipeline — the one with most of the heavy lifting to do — mandate which buffer format is best for its job. However, it would be a mistake (and go against the idea of loose binding) to force the renderer to account for every type of buffer that the input layer could throw at it; indeed, years later, we want the exact same renderer to be able to work with any new input layers that have been added on. So the second step is to create a common, “native” Buffer format to use when all else fails. This nets us the performance benefit when the renderer knows how to efficiently encapsulate its input, the ease of programming when the input layer actually¬†uses this format, and the flexibility to use any input layer we want, as the renderer can request a conversion of the input buffer to the native format before encapsulation. Finally, this solves the unasked question: what kind of buffer should the output layer be passed?

I’d like to close with a discussion on the hierarchical nature of Producers. Like the timelines they are meant to express, Producers too are used in a recursive fashion — each Clip creates a delegate Producer to pass into the render queue. Because these could be track or timeline clips, they could create their own hierarchy of Producers within. The one remaining problem with this hierarchical producer/consumer pattern is: how do we make it fast? I won’t lie: a plan is in the works, but it hasn’t been finalised. The hope is that the performance part of the code will be separated from the functionality part of the code, allowing Eve first to be correct, and then to be usable in real-time.

Who’s writing this?

My name is Cameron Gorrie. I'm an undergraduate student at the University of Toronto, with a strong interest in Artificial Intelligence and Computer Graphics. You can read more about me, or read my CV. If you have work or research opportunities in my interest areas, please do not hesitate to contact me.
April 2021