Where does the formula for HU to density come from?

[eigenvivek]

I came across this function in the Volume code and was wondering where the piecewise linear formula came from? A quick lit review turned up a lot of different formulas to convert HU to density, so I was wondering if this function is specific to particular scanner / radiation dose. Thank you!

https://github.com/arcadelab/deepdrr/blob/2c45632e6b93f6b043afe3a5a792971637aab35b/deepdrr/vol/volume.py#L219-L232