Overlaying UI mockup PNGs on vtkImageData without hurting picking

Support
1

Context

I’m prototyping a scientific visualization interface where the base render is real vtkImageData (volume slices), but I want to overlay UI chrome and annotation mockups that were generated as static PNGs in a browser tool. For reference, I used Seedream 5.0 Pro AI Image Generator purely as an external image source to sketch overlay concepts before touching VTK - no integration exists, it’s just PNG output I’m importing.

The actual VTK question

Once the PNG is in hand, what’s the more robust path for compositing:

  • A vtkImageActor in a separate renderer layer, or
  • A vtkImageSlice with its own mapper, positioned in display/overlay coordinates?

I need the overlay to stay screen-locked (like a HUD) rather than move with camera pan/zoom on the 3D layer beneath it. Is a second vtkRenderer with SetLayer() and a vtkCoordinate transform to VTK_DISPLAY the standard approach, or is there a cleaner idiom for this in current VTK?

Alpha and resizing

The mockups have transparent backgrounds. Does vtkPNGReader preserve the alpha channel automatically into the RGBA image data, or do I need to explicitly request 4 components? Also, when the render window resizes, how do people avoid blurring the overlay - is nearest-neighbor interpolation on the image property enough, or should the overlay be regenerated per-resize event?

Picking

Finally, since this overlay is purely cosmetic, how do I exclude it from vtkPropPicker or cell/point picking so it never gets selected accidentally during normal interaction with the underlying dataset?

Any pointers to relevant examples or classes would be appreciated.