[osg-users] Thoughts on Vulkan

[Robert Osfield]

On 11 February 2017 at 16:56, sduclos <sduclos.cairo at gmail.com> wrote:
> Perhaps using Vulkan as a backend to OpenGL ES2 (ex.: ANGLE)
> would not be as involving as rewriting OSG (outch!)

Vulkan is very different to OpenGL/OpenGL ES.  To make the most of
Vulkan, to give all the flexibility and performance benefits you need
to build the scene graph with this in mind.

The OSG just can't deliver this, it's an OpenGL scene graph by design,
it works with and around OpenGL capabilities/limitations.  If you
attempted to graft Vulkan support you'd have to limit the
implementation, if you attempted to use a Vulkan backend hidden by a
OpenGL/ES layer on top of Vulkan you'd loose the benefit of Vulkan,
you'd be better off just using the OpenGL or OpenGL driver directly.

I think it's crucial to grasp that Vulkan is VERY different to
OpenGL/ES.  The threading and performance differences in Vulkan will
change the way we go about implementing graphics.  For instance right
now the API overhead in OpenGL/ES is so high we have to do lots to
work batching graphics operations to get best performance. hiding the
API overhead.  We also are forced to dispatch data into the OpenGL/ES
fifo single threaded.  For a scene graph to make the most of the new
capabilities you have to be able thread preparation of the graphics
data, and now that the API overhead is reduced we open the door to
doing less batching and returning to a more fine grained scene graph
in memory that better maps to what is convenient to implement for the
graphics application rather than try to workaround them doing batching
even when it hurts other parts of our application development.  With a
potentially fine grained scene graph we also need to avoid CPU
overheads associated with that scene graph, otherwise will put a
performance bottleneck in the application that prevents us seeing all
the potential of the graphics hardware.

We simple can't achieve the full potential by grafting Vulkan into/or
adapting it.  We have to understand Vulkan and build around it,

Another consideration is that C++ has moved on, a new scene graph can
adopt C++11, 14, 17, which due to backwards compatibility issues the
OSG can't adopt these recent versions of C++. Perhaps it 5 years it
might be able to do, but not right now if we want to carry the
majority of the community with us.

Robert.