[osg-users] Concerns about quads, quad strips, polygons

Robert Osfield robert.osfield at gmail.com
Thu Nov 12 11:27:57 PST 2009


HI Paul,

This particular code is mainly there for just experimenting with
trying to get a range of 3D models I have rendered under GLES, and
also to get more of the NodeKits working quickly under GLES.
Performance wise the best solution is for us to move the adaption of
primitive mode to support ones within the Optimizer, and then remove
completely the #if #else blocks.  We also need to migrate the plugins
and NodeKit across from using primitive modes that aren't supported.

W.r.t Instanced vs non Instanced, the best performance solution would
be to have separate PrimtiveSet subclasses from the two types then the
subclass can call the appropriate method.

Robert.

On Thu, Nov 12, 2009 at 5:32 PM, Paul Martz <pmartz at skew-matrix.com> wrote:
> Hi Robert -- After looking at your recent check in of support for quads,
> quad strips, and polygons in GL ES 1 and 2, I have some performance
> concerns.
>
> Consider this code snippet for DrawArrays::draw()...
>
> void DrawArrays::draw(State& state, bool) const
> {
> #if defined(OSG_GLES1_AVAILABLE) || defined(OSG_GLES2_AVAILABLE)
>    GLenum mode = _mode;
>    if (_mode==GL_QUADS)
>    {
>        state.drawQuads(_first, _count, _numInstances);
>        return;
>    }
>    else if (mode==GL_POLYGON)
>    {
>        mode = GL_TRIANGLE_FAN;
>    }
>    else if (mode==GL_QUAD_STRIP)
>    {
>        mode = GL_TRIANGLE_STRIP;
>    }
>
>    if (_numInstances>=1) state.glDrawArraysInstanced(mode,_first,_count,
> _numInstances);
>    else glDrawArrays(mode,_first,_count);
> #else
>    if (_numInstances>=1) state.glDrawArraysInstanced(_mode,_first,_count,
> _numInstances);
>    else glDrawArrays(_mode,_first,_count);
> #endif
> }
>
> Note that if you're not on GL ES 1 or 2, you would take the #else path,
> which would result in the status quo performance. However, if you're on GL
> ES 1 or 2, you now have the cost of a cascading if / else if / else if
> inflicted per PrimitiveSet during draw -- even if you are using a supported
> primitive type such as triangles.
>
> For GL 3, the easy route would be for me to add GL3 to the #if and take this
> same code path, but I don't want to handicap performance on GL 3. So I'm
> hoping you might consider a different approach.
>
> Could we, for example, ignore these unsupported primitive types during draw
> for GL ES 1, 2, and GL3, and require an OSG app running on those platforms
> to use a NodeVisitor to convert their scene graph over to supported
> primitive types? This way, the PrimitiveSet ::draw routines could be
> restored to their previous lean versions, resulting in optimal performance.
>
> Apps that choose not to use the NodeVisitor would generate GL errors in this
> case, and PrimitiveSets with unsupported modes would simply not render.
> Presumably they'd see the error output and take actions to run the necessary
> NodeVisitor to resolve the issue.
>
> (If we wanted even better performance, we could probably eliminate the
> number of instances check and just always use the instanced variant, as the
> behavior for numInstances=1 is identical to using the non-instanced
> variant.)
>
> Thanks for giving this some thought.
>   -Paul
>
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