Zero-centering of stored PDF values is not well documented and unclear
In waLBerla, the LBM PDF values are either stored regularly, i.e. text-book like, or "centered around 0", i.e. just storing the deviation from the corresponding lattice weight. The decision is done based on the lattice model with the compressible flag: true leads to regular PDF values, whereas false (=incompresisble) leads to centered PDF values. The seemingly only place where this is noted is in a comment in src/lbm/field/PdfField.h but it is a very important aspect to realize when implementing own algorithms etc.
This should be documented better in some central place or a tutorial (and this issue is supposed to be a temporary documentation of this behavior).
Furthermore, it should be evaluated whether or not this centering might also make sense for the compressible case, as the only reason for centering seems to be floating point accuracy. Or is there another reason not to do it?
For a clearer self-documentation, one should add a flag to the lattice model like 'zero-centered' and this flag should be used instead of the compressible flag to check for the centering and its implications.