Saturday, January 28, 2017

Hardware Decoding in Chromium through QtWebEngine on Raspberry Pi

A few months back I decided to try to implement hardware decoding in WebKit. Unfortunately this task is always pretty long and complex for many reasons. I found the time to draft an implementation for WebKit1, which is pretty useless as WebKit1 in Qt is only used outside QML and JS is executed in the main thread. Unfortunately I never found the time to implement this in WebKit2, which runs in QML and is suitable for more fluid UIs. This was the result:
This is how YouTube was running with this implementation:




Now Qt has deprecated QtWebKit and is working heavily on QtWebEngine, which is built on Chromium, so I wanted to try this road. Unfortunately these kind of things always claim a lot of time, and I don't typically have that much, but I was able to start and get something done already.

Writing a complete solution in Chromium to decode and render video takes much time, so I thought of a shortcut: creating a custom VDA (Video Decode Accelerator) that loads the POT library and reusing its entire codebase to implement decode and rendering with little modifications. This proved to be possible and now I get something on the screen.

So, to summarise: a little patch to Chromium is needed to create a VDA that dynamically loads POT library into memory and uses it with a common interface. Data and calls are translated to POT structures and are sent to POT, which then processes the buffers properly. The result of the decode operation is then sent back to the VDA through the same interface and textures are then sent to Chromium for rendering.

Still many problems remain open, there is much to be done yet as you can see from the video, but something is drawn. Have a look at the demo:


Have fun! ;-)

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