See through walls shader

Hi All I am developing a surveying application on a 3d model, finished developing the measuring module but am having a problem that the scntext added on the scnode root cannot see through the 3d model. I am using the SCNBillboardConstraint contraint to towards the current camera.

So is there a way for the scntext to see through or overwrite the model like this? Thank

See through walls shader
 
 
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