Integrating new gradient support I noticed that the Visual3d_Layer class has been extended to contain multiple items called Visual3d_LayerItem. Exploring this further I realized that this was a way to offer an extensible way to support multiple user-defined objects. One of these is a color scale in a 3D view defined as V3d_ColorScaleLayerItem subclass.
Experimenting with this new concept I went on to redefine the way how the CAD Exchanger logo was implemented. It is a part of the over-layer, one which draws 'on top' of objects in a main scene. See the screenshot below:
Logo displayed in front of a 3D model
So I subclassed Visual3d_LayerItem and redefined its RedrawLayerPrs() method to do the actual work. Check this archive to see the full code.
Here is how you can add this object into the layer:
//create an overlayer
Handle(V3d_View) aView = ...;
Handle(Visual3d_Layer) anOverLayer = new Visual3d_Layer (aView->Viewer()->Viewer(), Aspect_TOL_OVERLAY, Standard_True /*aSizeDependant*/);
//create a texture layer item
const Handle(Visual3d_Layer)& aLayer = aView->Viewer()->Viewer()->OverLayer();
Handle(QOOcc_TextureLayerItem) aTexture = new QOOcc_TextureLayerItem (QImage (":/viewlogo.png"), aView, aLayer.operator->());
This way you may add as many items as you want. The only downside (or assumption if you will), which is likely in place is that all items have to be drawn with the same coordinate system that needs to be supported for the layer (using Visual3d_Layer::SetOrtho()). So RedrawLayerPrs() needs to be written with a single convention in mind, and you seem unable to work with individual settings. It would be great to hear whether this assumption is correct or not – from both the OCC folks and anyone having practical experience.