VTK SetInputConnection Vs SetInputData

SetInputConnection is for a filter to connect to another filter.

bFilter.SetInputConnection(aFilter.GetOutputPort())

SetInputData is for a filter to connect to a dataset

bFilter.SetInputData(aFilter.GetOutput())

The difference in behavior is that SetInputConnection setup the auto pipeline update mechanism, a render or update() down in the pipeline will trigger all preceding filters to update

reader.SetFileName('name.stl')

aFilter.SetInputConnection(reader.GetOutputPort())
// aFilter do some setting

bFilter.SetInputConnection(aFilter.GetOutputPort())
// bFilter do some setting

bFilter.Update()

SetInputData however need manual update() in each stage for the whole pipeline to work.

reader.SetFileName('name.stl')
reader.Update()

aFilter.SetInputData(reader.GetOutput())
// aFilter do some setting
aFilter.Update()

bFilter.SetInputData(aFilter.GetOutput())
// bFilter do some setting

bFilter.Update()

If any update() misses, you will get empty result error.