Could not find BettiNumbers module in source listed as a module as per the documentation

[jitinnair1]

Describe the bug

I wanted to use TTK to calculate Betti Numbers. I was going through the developer documentation and under Modules -> VTK I found a class called ttkBettiNumbers (screenshot below).

However, it is not available in the source files in the latest release. I also checked a few of the older releases but could not find it.

(I am unfamiliar with the Doxygen layout hierarchy so I may be looking at the wrong places perhaps?)

To Reproduce Steps to reproduce the behavior:

1. Download the latest release
2. Go to core/vtk
3. vtkBettiNumbersdirectory is not found

Expected behavior Find the ttkBettiNumbers module in the source files in core/vtk

Screenshots

Here is the link from where this screenshot is: https://topology-tool-kit.github.io/doc/html/group__vtk.html

System (please complete the following information):

• OS: Ubuntu
• Compiler GCC
• Installed TTK's optional dependencies None

Additional context I installed ttk from source and did not install the optional dependencies. While building I chose to not build the Paraview Plugins.

[julien-tierny]

Hi!

thanks for your issue.

Actually, there is indeed no BettiNumbers module in the official TTK release. This is a documentation bug (we program this module internally as an exercise for newcomers). This commit (https://github.com/topology-tool-kit/topology-tool-kit.github.io/commit/7cbac85f8618ee13d14f4d5f510973d2ef1f6761) should fix this.

If you want to compute Betti numbers, I'd suggest to proceed as follows:

• compute the persistence diagram of your dataset
• collect the bars for which the cell data array IsFinite is set to 0 (these are the essential homology classes)
• the cell data array PairType gives you the dimension p of the corresponding homology group.

Then, the number of p-dimensional "infinite" bars gives you the p-th Betti number.

I hope this helps.

Best,

[jitinnair1]

Thank you for clarifying and fixing this, and also for your suggestion on how Betti numbers could be calculated.