Slow scrubbing for videos

Hi,

We have spotted that video scrubbing can be quite slow using VideoContext. It can drop to 4/5 fps. There are obviously a lot of factors that will come into play for that:

• Canvas drawingbuffer size
• Video dimensions
• Video encoding (whether it is optimised for fast decoding or not)
• Number of nodes that play in parallel

To make sure that we would have a fair benchmark, @sacharified came up with a reduced test case that compares:

• Native video element scrubbing
• Scrubbing and painting on a 2dRenderingContext
• Scrubbing and painting on a WebGLRenderingContext with the most simplistic shaders possible
• Scrubbing using VideoContext with a single node attached to the graph

You can try it for yourself: https://codepen.io/Sacharified/pen/dgderG?editors=0010

We are currently trying to investigate what the bottleneck is but it is slightly unclear as there are a lot of boilerplate code. Is there some documentation somewhere that explain the WebGL program structure? That would really help to debug this issue further. If you have any lead please let me know

[gsouquet]

I forgot to update this issue.

I think this is more a video encoding problem rather than a VideoContext problem. I dug deep on this one and realised that the way that VC approached this was as optimised as it could be.

Changing the encoding parameters of our videos made a huge impact here.

[gsouquet]

Safari just released a canvas debugger. It exposes all of the valid shaders for a WebGL canvas. https://webkit.org/blog/8452/canvas-debugging/

It might help us to better understand the structure of VideoContext