vtk render window GetZbufferData method - vtk

Question

As I understand it, if I create a vtk render window, then I can add different renderers to it and for each renderer renders from a different perspective. No to actually render the scene I use the vtk render window method render() to render all renderers in parallel. Now there is a vtk render window method called GetZbufferData which apparently returns an array containing the zbuffer. So my question is, to which renderer does this zbuffer correspond to?
Thanks for any clarification.

Answer 1

If you have all renderers in the same window, then they will share the same framebuffer, so also the same z-buffer. So a simple answer to your question is "to all of them". To get the individual z-values, it depends on what you are exactly doing with the renderers.
If you are doing some kind of a "tiled view", you want to assign different viewports (vtkRenderer::SetViewport(), like here) to each of the renderers. Then you can access the z data for a given "tile" (renderer) by passing appropriate x,y coordinates to the GetZBufferData function. For example, to get the whole part of the z buffer that belongs to renderer ren1 of vtkRenderWindow renWin:
double x1 = ren1->GetViewport()[0] * (renWin->GetSize()[0] - 1);
double y1 = ren1->GetViewport()[1] * (renWin->GetSize()[1] - 1);
double x2 = ren1->GetViewport()[2] * (renWin->GetSize()[0] - 1);
double y2 = ren1->GetViewport()[3] * (renWin->GetSize()[1] - 1);
float *ren1Z = renWin->GetZbufferData(
static_cast<int>(x1),static_cast<int>(y1),static_cast<int>(x2),
static_cast<int>(y2));
If you have the same viewport, it would be more complicated. You can have a renderwindow with multiple "layers", by setting vtkRenderWindow::SetNumberOfLayers(int) and then you can assign each renderer to a different layer (vtkRenderer::SetLayer(0-based layer index)). The window then renders from layer 0 to the last layer over each other. If you are interested in getting only one specific renderer's z-data, you should get it if you have it render in the last layer. However, I am not sure if the z-buffer is cleaned in between individual renderer's renders, I would actually bet on that it is not, so you might also get some inconsistent mess.

Answer 2

I would like to complement tomj answer:
Any of the vtkRenderWindow::GetZbufferData() methods query the framebuffer for Z-values, which is contained in the vtkRenderWindow, but there is a slight remark:
You need to set this in your renderers: vtkRenderer::PreserveDepthBufferOn(). This is because as the documentation says:
"By default, the depth buffer is reset for each renderer.
If this flag is true, this renderer will use the existing depth buffer for its rendering."
So, that bring us to the vtkRenderers. There is a layering of vtkRenderers, which tells which "chain" or "precedence order" to make the drawing. Check the method vtkRenderer::SetLayer().
So, you first need to set up your layered vtkRenderers, attach them to the vtkRenderWindow, and then set up correctly if you want to preserve some depth buffers or not.
Notice that if the z-buffer has not been set (first draw of the first vtkRenderer), it will return 1.0. I'm still figuring out why, but currently that is the situation.