A question about transmission occurring

[DWin233]

I'm reading the source code to learn VMC and have a little doubt.

In the device function reflectray(...), when judging whether to enter the background medium, is there an implicit assumption that the upper medium is not the background medium? If so, I would like to understand why this assumption is made?

if (((nuvox->sv.isupper) ? nuvox->sv.isupper : nuvox->sv.lower) == 0) { /*transmit to background medium*/
    return 1;
}

[fangq]

@DWin233, this if-branch is related to our SVMC algorithm implemented by @ShijieYan. In the SVMC volume data structure, one voxel can contain two labels - thus split-voxel - separated by an sub-voxel interface. the sv.isupper flag tells whether the current photon is located in the upper-label portion or the lower-label portion if the voxel does contain two labels.

the above statement is simply retrieving the "current" medium label and test if it is 0 - if in a single-medium voxel, sv.isupper will never be 1, therefore, sv.lower will always be used; if in a two-medium voxel, depends on sv.isupper, we retrieve the corresponding label.