Unlock Dynamic Cone Heights in osgEarth: A Comprehensive Guide

Are you struggling to dynamically update the height of your cones in osgEarth using osgEarth::ModelNode? Many developers encounter this issue, where changes to the underlying osg::Cone geometry don't reflect visually despite calling update methods. This guide provides a clear, step-by-step solution to ensure your cone heights update correctly.

The Challenge: Why dirtyDisplayList() Isn't Enough

Often, the first approach is to call dirtyDisplayList() on the osg::ShapeDrawable after modifying the osg::Cone's height. However, this doesn't always trigger a visual update within the osgEarth::ModelNode pipeline. The problem lies in how osgEarth manages object updates, particularly when spatial referencing is involved.

Root Cause: Spatial Reference Management

osgEarth::ModelNode handles the spatial referencing and transformation of your model within the Earth's coordinate system. Changes to the underlying geometry may not automatically propagate through this spatial reference system.

Solution: Force a Scene Graph Update

To ensure the cone's height updates correctly, you need to force a more comprehensive update of the scene graph. Here's the recommended approach:

1. Invalidate the Bounding Volume: Since osgEarth::ModelNode relies on bounding volumes for rendering optimizations and frustum culling, invalidating the bounding volume forces a recalculation based on the new cone dimensions. Use _modelNode->dirtyBound();.

2. **Mark StateSet as Dirty::**Sometimes the stateset needs to be marked dirty for the changes to propagate correctly. You can use _modelNode->getOrCreateStateSet()->setRenderingCacheDirty();

3. Request a redraw. osgEarth::GeoTransform has a dirty() method, calling this together with requestRedraw() on the viewer should allow to trigger a redraw of the modified element.

Code Example: Implementing the Update

Here's the corrected updateCone function, incorporating the necessary updates:

void Cone::updateCone(osgEarth::GeoPoint gp, double height, double r, osg::Quat quat) {
    _height = height;
    _cone->setHeight(_height);
    _cone->setRadius(r); // Update radius, if applicable
    _coneDrawable->dirtyDisplayList();
    _modelNode->dirtyBound();
    _modelNode->getOrCreateStateSet()->setRenderingCacheDirty();
    _modelNode->setPosition(gp);
    // Optionally, force a redraw of the view
    if (_modelNode->getGeoTransform()) {
        _modelNode->getGeoTransform()->dirty();
    }
    if(auto view = _modelNode->getView()){
        view->requestRedraw();
    }
}

Explanation of changes

• Update Radius: If you're also changing the radius, include _cone->setRadius(r);

• Force Bounding Volume Recalculation: _modelNode->dirtyBound(); ensures that the osgEarth::ModelNode recalculates its bounding volume based on the updated cone geometry.

• Stateset marked dirty: _modelNode->getOrCreateStateSet()->setRenderingCacheDirty(); ensures the updated stateset will apply the correct rendering techniques to the model

• GeoTransform Dirty: A call to _modelNode->getGeoTransform()->dirty() will mark the geometry for an update

• Viewer Redraw: The call to view->requestRedraw() will force the viewer to redraw after the update.

Best Practices for Dynamic Updates

• Minimize Updates: Frequent updates can impact performance. Batch updates where possible.

• Optimize Geometry: Consider using simpler geometry or level-of-detail techniques for distant cones to improve rendering speed.

Conclusion

By understanding how osgEarth::ModelNode manages spatial referencing and incorporating the suggested update methods, you can achieve smooth, dynamic updates to your cone heights. This approach ensures that your changes are correctly reflected in the rendered scene, creating a more interactive and responsive experience. Remember to invalidate the bounding volume and request that the viewer redraw to guarantee the changes propagate through the scene graph.