3.3. Playing back filter data

A SpecTcl filter is a mechanism that allows you to transform a raw data set into a decoded subset of the data. By decoded subset we mean that:

• Only events that satisfy the filter's gate are written to the filtered data set.

• A subset of the parameters decoded from each event are written to the data set.

• The structure of the raw event is lost.

Since the structure of the raw event is lost, and since any parameter can be written to the output file, the event processing pipeline used to analyze data from a raw event file is not useful in analyzing filter files:

• There is no raw event to decode into raw parameters.

• Parameters computed by later stages of the pipeline can already be present in the event file.

When analyzing filter files you must therefore:

• Register an initial stage of the event analysis pipeline that can read the filter file.

• Register subsequent stages that compute parameters that are not already present in the filter file. You could think of analysis as performing an initial broad cut on the data and then subsequently only producing physically meaningful parameter on this subset. The event processor that computes these parameters would be in the pipeline for analyzing filtered data.

In this section, we're going to look at how to set up the first element of an analysis pipeline that can analyze filter data. In the next section, we'll look at an extension to SpecTcl that allows you to dynamically switch SpecTcl between the analysis of raw data and filtered data.

SpecTcl provides an event processor that can decode any filter file. The parameters in the filter file are decoded into parameters of the name they had when they were written.. If a parameter is found in the filter file, but is not defined in SpecTcl when the filter is read back, it is ignored without reporting an error.

The filter event processor element must be registered as the first element of the SpecTcl event processing pipeline. To use it we must:

• #include the filter file decoder event processor into MySpecTclApp.cpp. This header is named FilterEventProcessor.h and defines the class CFilterEventProcessor.

• Replace the event processing pipeline definition in CreateAnalysisPipeline so that it contains an instance of a CFilterEventProcessor class.

Note that it does no harm to leave our raw event processors linked in with SpecTcl. Towards the top of MySpecTclApp.cpp:

//////////////
#include <config.h>
#include "MySpecTclApp.h"
#include "EventProcessor.h"
#include "TCLAnalyzer.h"
#include <Event.h>
#include <TreeParameter.h>
#include "MyEventProcessor.h"                // <- this line can stay in
#include <ilterEventProcessor.h>       // <- add this line
            

Make CreateAnalysisPipeline look like this:

void
CMySpecTclApp::CreateAnalysisPipeline(CAnalyzer& rAnalyzer)
{
  RegisterEventProcessor(*(new CFilterEventProcessor), "Filter-unpacker");


}