[Bf-viewport] OpenGL low level shader API proposal

[Brecht Van Lommel]

I've updated the proposal now with GPUShaderInstance and more example code:
http://wiki.blender.org/index.php/Dev:2.8/Source/OpenGL/Shaders

On Sat, Dec 5, 2015 at 6:18 PM, Antony Riakiotakis <kalast at gmail.com> wrote:
> Every time the relevant data changes. Light position was a bad example of
> that, let's say transform is a better example here. I don't think we need
> data to be immutable on the GPU, just cached.
>
> Keep in mind that when rebinding a shader, we are required to rebind all the
> uniform state. This is not as bad with uniform buffer objects but for GL 2.1
> it makes sense to have a way to bind all data.

Looking towards Vulkan, imagine if you draw once with one set of
uniform values, and another time with other uniform values. Then you
need to allocate two buffers with uniforms, and keep those around
until the command buffer has finished rendering. You can't just have a
single buffer and modify that because it would affect both draw
operations.

If you make these things mutable, then every time you change the data
we (or the OpenGL driver) need to make a copy of the data to avoid
that kind of interference. I rather avoid doing that under the hood,
it's one of the sources of inefficiency in the OpenGL API design. If
we make things immutable we avoid that problem, and it will make
multithreading easier as well.

As I understand it, you can prepare the next frame on the CPU while
the previous frame is still drawing on the GPU, for efficiency those
computations can overlap. This means that under the hood your data has
to be effectively immutable on the GPU anyway, otherwise rendering the
two frames will interfere.

For sorting it's also easy if you can just compare pointers instead of
the actual state.

Regards,
Brecht.