[osg-users] [About VertexArrayObject Branch] FeedBack and Insight

[Robert Osfield]

Hi Jannik,

On 17 August 2016 at 14:52, Jannik Heller <scrawl at baseoftrash.de> wrote:
> I have tested out the VAO branch on my Nvidia graphics card. As you said, VAO+VBO is slightly faster than VBO but not as fast as display lists. Would be curious on the performance results with AMD cards.
>
> Unfortunately, I am unable to use VAO for animated meshes because you've introduced a new meaning of the dataVariance flag. https://github.com/openscenegraph/OpenSceneGraph/blob/vertex_array_object/include/osg/Drawable#L523

I haven't introduced a new meaning to DataVariance, all I have done it
utilize it for knowing whether the VAO needs updating or not.  I have
done this to avoid the extra checks required against the individual
arrays to see if they are dirty or not - I have done it for
performance reasons as I want to keep the CPU overhead for draw
dispatch as low as possible.

> I do *not* set the dataVariance to DYNAMIC for all my animated meshes because the performance overhead of DYNAMIC is not acceptable for me. If Drawables are set to DYNAMIC, OSG would not break frame until all the draws are submitted. I manage the updates using a double buffering scheme that prevents the buffer from being modified while the draw thread is using it. So essentially I don't need the DYNAMIC flag, but now I'm forced to use it (and incur the performance overhead associated with it) because OSG won't update the arrays if I don't. :( Previously it was sufficient to dirty() the vertex arrays to get OSG to update things. Hopefully that can be fixed.

This is a sticky one.  Your mesh *is* dynamic, the correct way to
label the geometry is DYNAMIC.  In your usage case you are taking
advantage of the fact that in certain circumstances you ignore the
threading issues associated with dynamically updating whilst it's
being dispatched by the drawing thread, however, this is a dangerous
game to play that you have to be really careful to do without
introducing a race condition.  Whether you are just getting lucky in
your code or it's genuinely safe I can't say as I don't have the code.

In your case you are wanting a DataVariance of
DYNAMIC_BUT_DRAW_TRAVERSAL_PLEASE_DONT_TREAT_ME_AS_SUCH.

I guess the special case where modifying the data is safe would be
STATIC_ALLOCATION_DYNAMIC_UPDATE or something along those line.
Perhaps STREAM might be terminology that might fit this.

However, even with the STREAM case if you are updating in the update
or cull traversal while the draw thread is reading that same data you
could get corrupted data sent to OpenGL driver, here you really should
be double buffering.

I'll ponder on this issue, but for now strictly speaking the VAO
implementation is perfectly correct, requiring that the DataVariance
be DYNAMIC is a correct interpretation of what the DataVariance is.

Robert.