[vtk-developers] Temporal Redesign

[John Biddiscombe]

I made a few changes to TemporalShiftScale and SnapToTimeStep last week 
which had unfortunate repercussions for the TemporalFractal test.

I've spent some time going over the executives and various classes which 
make use of TemporalData and I would like to make some changes to the 
whole temporal framework...

Problems : The main problem with the temporal design as it stands is 
that it is very difficult to know when a filter has got the right input 
or output when temporal looping is involved. The problem is largely 
caused by the TemporalDataSetAlgorithm Family. These classes export a 
TemporalDataSet directly, and it isn't possible to know what kind of 
datatype they have inside the collection.

Consider the case of the failing TemporalFractal Test.
A TemporalFractal class exports TemporalDataSet. Currently, this can 
only be connected to another class which takes TemporalData as input. 
Even if it only exports a single timestep, which may be uniform grid, or 
rectilinear, or even a hierarchy of either (Multiblock etc)

We would like the Executive to take temporal data and loop it for non 
temporal filters, but we can't connect a non temporal filter to it and 
get it right- because when the filters are connected together and first 
executed, the RequestDataObject pass for the filter accepting the 
TemporalDataSet doesn't know what type of output to create -  we fudged 
around this by simply exporting a temporal dataset too (which is why 
snap and shift scale were TemporalAlgorithms (and they should not be, 
they do not change the type of data passing through them). There are 
several bugs existing 
(http://public.kitware.com/mantis/view.php?id=6662) which all stem from 
the problem that the executive needs to take a single dataset from a 
temporal collection and pass it into a non temporal filter - currently 
it just doesn't do it and we see the output of a filter change to 
temporal within the paraview gui and various other side effects once 
temporal stuff is initiated.

I'd am going to remove the whole TemporalDataSetAlgorithm family and 
instead have filters which declare themselves as (say) 
UniformGrid/Image/PolyData algorithms (or even multiblock), but export a 
new information key which says "I support temporal" if they are capable 
of delivering multiple time steps. Filters which require temporal 
collectios of type X, will add a key requesting time.

The advantages of this are
a) Filters will export a defined 'type', which other filters and the gui 
will be aware of, so temporal filters exporting polydata will be 
distinct from temporal filters exporting imagedata and so on. The user 
can then also write a filter which requires N Polydata and not any other 
type, currently the pipeline connects all temporal classes without checks.
b) The checks in the executive will be simpler. A filter needs temporal 
data, but the upstream pipeline exports simple, so loop. When a filter 
needs simple, but upstream has declared time support, we take the i'th 
dataset on iteration i (or more correctly, the dataset with timestamp=t).
c) Filters which support temporal activity (exporting more than one 
dataset) as in b) above can have their output dataobjects replaced by 
the executive and fill the N steps as before. We replace the code in the 
existing CheckDataObject() method which currently is incorrect (temporal 
IsA(composite) always returns true and causes untold trouble) and solve 
most of the outstanding temporal problems in one place (almost). 
Effectively we have, if filter supports temporal output and more than 
one steps were requested. change it - . If filter requires temporal and 
only one step exists, create temporal - the rest of the time, stop 
messing about. The current dataset swapping logic is a mess.
d) existing filters(readers mainly) can be updated to support temporal 
natively with a pretty simple strategy. Add the temporal support flag, 
and in request data, generate N steps if required, and place them in the 
output. Much easier than the current option of creating a new class 
derived from TemporalDataSetAlgorithm and instantiating a reader inside 
it. (I'm talking here about generating multiple time steps from an 
existing reader - the executive can loop for you, but if you have a 
sophisticated file format, reading N at once can be desirable...)

The changes the users will notice are
vtkTemporalDataSetAlgorithm is removed, instead, filters which can 
/supply /multiple steps place an extra key TEMPORAL_OUTPUT_SUPPORTED in 
FillOutputInformation
Filters which /require /multiple timesteps will place 
TEMPORAL_INPUT_REQUIRED during the FillInputInformation call. (Here' I 
think we might enhance the paraview GUI to allow a temporal input domain 
so that you can only instantiate temporal filters when connected to 
filters which are capable of supplying it). Filters such as temporal 
interpolator, become dataobject algorithms, likewise temporalcache.

I believe this will solve a great many problems, though I have not 
considered the effect on TEMPORAL_FAST_PATH so please get back to me if 
this is going to be a problem.

I will be working on this on the back burner until the new year no 
doubt, when I hope I have everything in order. The changes I have 
outlined above are of course subject to change as I encounter problems 
I've not considered. Please get in touch if you have strong feelings 
about this.

JB



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