On 2019 Apr 05 17:16:24, you wrote to All:

 GD> Presumably the graphics controller fills up a frame buffer for display,
 GD> so, if one is not at a certain point using the controller to draw
 GD> something, could one write to the frame buffer directly?

  https://en.wikipedia.org/wiki/Framebuffer

 GD> Also, AIUI, the RPi communicates with the GPU via memory-based
 GD> message buffers, so the GPU must be running some form of background
 GD> kernel?

 https://en.wikipedia.org/wiki/Graphics_processing_unit

 GD> I assume that this must be the purpose of only one of the GPU
 GD> processors as running full motion video, say, in an MPEG stream,
 GD> must take quite a lot of computer time?

see above... today's GPUs are a lot more powerful than those of yesteryear...

 GD> One part of the GPU must be simply concerned with taking the frame
 GD> buffer and emitting it as a composite video stream, so I wonder how
 GD> the aspect ratio is set up?

  https://en.wikipedia.org/wiki/Composite_video

 GD> Sorry, a bit unstructured in the above, just musing; it's something
 GD> that I pick up after a few months before dropping it again; I'm
 GD> struggling  to get to grips with the whole GPU thing.

see above but also understand that today's GPUs are basically multi-processor
computers dedicated to doing video work... because they are multi-processor
computers, they can also be used for other tasks at times... some examples
would be doing calculations for protein modeling, weather analysis, digital
coin generation, encryption, and more...

back in the day, i had a 12mhz 286 with two hard drives in it... i replaced the
existing HD controller card with a caching controller card... the caching
controller card had a 40mhz 286 chip on it with its own banks of memory on the
card... this caching controller card was more powerful and had more memory than
the machine it was installed in... i know this is not the same as with the GPUs
but the moving from the old-school stuff to today's stuff is very similar...

in any case, the above links should help you understand more of what is going
on and why...

-- 

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Always Mount a Scratch Monkey
Do you manage your own servers? If you are not running an IDS/IPS yer doin' it
wrong...
... Uncle Dirty Dave = crudely deviant.
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