[osg-users] bounding box of culled geometry

[OpenSceneGraph Users]

Hello,

I am using osg::Drawable::getBoundingBox() to retrieve the bounding box around the whole geometry. Is there a similar short way to get the equivalent box for just the culled geometry or do I need to write my own procedure?

Best regards,
Renzo

[OpenSceneGraph Users]

Hi Renzo,

On Wed, 14 Oct 2020 at 12:00, OpenSceneGraph Users <osg-...@lists.openscenegraph.org> wrote:

I am using osg::Drawable::getBoundingBox() to retrieve the bounding box around the whole geometry. Is there a similar short way to get the equivalent box for just the culled geometry or do I need to write my own procedure?

Culling isn't related to the nodes directly, but to a traversersal of the scene graph that compares the bounding shere's of internal nodes of the scene graph and the bounding boxes of the drawable leaves.  Culling can happen at any point in the hierarchy - it can cull right up at the top node in the scene graph if the scene is outside the view frustum.

Within the cull traversal there is no tracking of culled or non culling bounding boxes, a cull test is done and the bool result used directly.  There isn't a "bounding box" or "bounding sphere" for culled or non culled objects.  All the cull traversal cares about is building a list of drawables leaves that pass the view frustum test, there is no tracking of any objects that are culled - this would be a crazy waste of CPU resources.

I have to ask, why do you want a bounding box of culled objects - what is the high level problem you are trying to solve?  It may well be that you are asking the wrong question thinking about the task you have in the wrong way.

Robert.

 

[OpenSceneGraph Users]

Hi Robert,

thanks for pointing out how cull traversal operates.

Yes, it might be I am searching in the wrong direction. I need to get the minimum and the maximum Z coordinate of the geometry vertices in order to apply a color range. This is quite easy to do for the whole geometry but I would know if there is a way just for the vertices inside the view frustum.

Regards,
Renzo

[OpenSceneGraph Users]

Hi Renzo,
Maybe a node visitor with z-near and z-far as parameter is a possible approach.
Werner

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[OpenSceneGraph Users]

Hi Werner,

I do not know if I have found the code that you are suggesting. I see that CullVisitor could help to find the near and far plane, however with Z I meant the world vertical axis, not the axis of the view frustum.

Regards,
Renzo

On 15-Oct-20 11:25 AM, OpenSceneGraph Users wrote:

Hi Renzo,
Maybe a node visitor with z-near and z-far as parameter is a possible approach.
Werner

On 15. Oktober 2020 09:46:57 MESZ, OpenSceneGraph Users <osg-...@lists.openscenegraph.org> wrote:

Hi Robert,

On 14-Oct-20 3:56 PM, OpenSceneGraph Users wrote:

Hi Renzo,

On Wed, 14 Oct 2020 at 12:00, OpenSceneGraph Users <osg-...@lists.openscenegraph.org> wrote:

I am using osg::Drawable::getBoundingBox() to retrieve the bounding box around the whole geometry. Is there a similar short way to get the equivalent box for just the culled geometry or do I need to write my own procedure?

Culling isn't related to the nodes directly, but to a traversersal of the scene graph that compares the bounding shere's of internal nodes of the scene graph and the bounding boxes of the drawable leaves.  Culling can happen at any point in the hierarchy - it can cull right up at the top node in the scene graph if the scene is outside the view frustum.

Within the cull traversal there is no tracking of culled or non culling bounding boxes, a cull test is done and the bool result used directly.  There isn't a "bounding box" or "bounding sphere" for culled or non culled objects.  All the cull traversal cares about is building a list of drawables leaves that pass the view frustum test, there is no tracking of any objects that are culled - this would be a crazy waste of CPU resources.

I have to ask, why do you want a bounding box of culled objects - what is the high level problem you are trying to solve?  It may well be that you are asking the wrong question thinking about the task you have in the wrong way.

Robert.

thanks for pointing out how cull traversal operates.

Yes, it might be I am searching in the wrong direction. I need to get the minimum and the maximum Z coordinate of the geometry vertices in order to apply a color range. This is quite easy to do for the whole geometry but I would know if there is a way just for the vertices inside the view frustum.

Regards,
Renzo

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http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org

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[OpenSceneGraph Users]

Hi Renzo,

You may look at osgShadow\MinimalCullBoundsShadowMap.cpp. It scans RenderBins and compute bounds of culled drawables to optimize shadow map resolution. 

Cheers,

Wojtek Lewandowski

[OpenSceneGraph Users]

Hi Wojtek,

it looks interesting. I do not have a shadowed scene but I will try it, thanks.

Regards,
Renzo

[OpenSceneGraph Users]

Hi Renzo,

On Thu, 15 Oct 2020 at 09:05, OpenSceneGraph Users <osg-...@lists.openscenegraph.org> wrote:

thanks for pointing out how cull traversal operates.

Yes, it might be I am searching in the wrong direction. I need to get the minimum and the maximum Z coordinate of the geometry vertices in order to apply a color range. This is quite easy to do for the whole geometry but I would know if there is a way just for the vertices inside the view frustum.

The CullVisitor maintains the near and far values used in the view frustum so after the cull traversal you should be able query the settings.  See CullVisitor::getCalculatedNearPlane() and getCalculatedFarPlane()

Robert.