Improve DDDigi doc

DDDigi/README.md

+![DDDigi](../doc/logo_small.png)
+
+# DDDigi: The digitization tools of DD4hep
+
+
+DDDigi is supposed to interprete the results of the detector simulation phase
+e.g. using the simulation toolket [DDG4](../DDG4) with its command line interface
+[ddsim](../DDG4/python/DDSim).
+This is the final phase of data processing to obtain data from the simulation 
+branch which are as close as possible to the response of the front-end electronics
+of a real experimental setup.
+
+The simulation phase ejects for all subdetectors and the individual 
+sensitive elements contained the energy deposits created by particles from a 
+primary interaction passing the sensitive volume.
+These data have to processed for taking into accound imperfections of the 
+apparatus such as 
+
+- uncertainties in cable lengths resulting in shifts of time of signal arriving.
+- uncertainties in the position of sensitive volumes.
+- dead or hot readout channels.
+- conversion of the final signal data into the equivalent of ADC counts.
+- ...
+
+In addition these effects should be computed in the presence of
+
+- multiple interactions in the same beam-crossing
+- spillover signal from previous or following interactions due to the
+  relaxation time of the apparatus.
+
+DDDigi addresses these issues in a very modular way and implements a multi-threaded approach
+to perform such data processing.
+To support multi-threading and to avoid any complication for users due to data races 
+the following basic assumptions were made:
+
+- The signals from disjunct subdetectors are generally independent
+  This means the detector response of any tracking subdetector is uncorrelated with
+  the detector response of e.g. a calorimeter.
+- Within a subdetector the response of disjunct units are uncorrelated. This means that 
+  e.g. hits on one layer of a layered tracking device are uncorrelated 
+  with the hits produced in another layer.
+- Within one such a layered device the response of individual sensors is tyically uncorrelated.
+
+These assumptions however are not strict, but rather require input from the designer of such
+a digitization application for a given appratus. These basic assumption only show the maximal
+level of parallelization possible when processing data signals.
+The parallelization can be configured for each subdetector at each level according to 
+boundary conditions e.g. given by cross-talk or the data volume arising from the
+energy deposits of the simulation.
+
+# DDDigi Components
+
+### DigiDDG4ROOT
+  Reader for ROOT files produced with DDG4. <br/>
+  Properties: <br/>
+    |**Property**|**Data type**|**Description**|
+    |:--- |:--- |:--- |
+    |`.input`      | vector<string>  | List of input files to be processed                             |
+    |`.tree`       | string          | Name of the main data tree. default: `EVENT`                    |
+    |`.containers` | vector<string>  | List of containers to be loaded to DDDigi.                      |
+    |              |                 | If empty => all.                                                |
+    |`.segment`    | string          | Name of the input segment. default: "inputs"                    |
+    |`.mask`       | integer         | Mask of this input source in the store. default: NO_MASK (0x0)  |
+    |`.rescan`     | boolean         | Rescan input sources for continuous execution. default: true    |
+    |`.keep_raw`   | boolean         | Keep raw input as opaque objects in the DDDigi store.           |
+
+  Functionality: self explaining
+
+### DigiAttenuate
+  Deposit attenuator for energy deposits according to decay time constant. <br/>
+  Properties: <br/>
+    |**Property**  |**Data type**    |**Description**                                                  |
+    |:---          |:---             |:---                                                             |
+    |`.input`      | vector<string>  | List of input files to be processed                             |
+
+
+![HORIZON2020](../doc/usermanuals/DD4hep/figures/AIDAinnova.png)
+=======
+![DDDigi](../doc/logo_small.png)
+
+# DDDigi: The digitization tools of DD4hep
+
+
+DDDigi is supposed to interprete the results of the detector simulation phase
+e.g. using the simulation toolket [DDG4](../DDG4) with its command line interface
+[ddsim](../DDG4/python/DDSim).
+This is the final phase of data processing to obtain data from the simulation 
+branch which are as close as possible to the response of the front-end electronics
+of a real experimental setup.
+
+The simulation phase ejects for all subdetectors and the individual 
+sensitive elements contained the energy deposits created by particles from a 
+primary interaction passing the sensitive volume.
+These data have to processed for taking into accound imperfections of the 
+apparatus such as 
+
+- uncertainties in cable lengths resulting in shifts of time of signal arriving.
+- uncertainties in the position of sensitive volumes.
+- dead or hot readout channels.
+- conversion of the final signal data into the equivalent of ADC counts.
+- ...
+
+In addition these effects should be computed in the presence of
+
+- multiple interactions in the same beam-crossing
+- spillover signal from previous or following interactions due to the
+  relaxation time of the apparatus.
+
+DDDigi addresses these issues in a very modular way and implements a multi-threaded approach
+to perform such data processing.
+To support multi-threading and to avoid any complication for users due to data races 
+the following basic assumptions were made:
+
+- The signals from disjunct subdetectors are generally independent
+  This means the detector response of any tracking subdetector is uncorrelated with
+  the detector response of e.g. a calorimeter.
+- Within a subdetector the response of disjunct units are uncorrelated. This means that 
+  e.g. hits on one layer of a layered tracking device are uncorrelated 
+  with the hits produced in another layer.
+- Within one such a layered device the response of individual sensors is tyically uncorrelated.
+
+These assumptions however are not strict, but rather require input from the designer of such
+a digitization application for a given appratus. These basic assumption only show the maximal
+level of parallelization possible when processing data signals.
+The parallelization can be configured for each subdetector at each level according to 
+boundary conditions e.g. given by cross-talk or the data volume arising from the
+energy deposits of the simulation.
+
+# DDDigi Components
+
+### DigiDDG4ROOT
+
+  Reader for ROOT files produced with DDG4. <br/>
+  Properties: <br/>
+    |**Property**|**Data type**|**Description**|
+    |:--- |:--- |:--- |
+    |`.input`      | vector<string>  | List of input files to be processed                             |
+    |`.tree`       | string          | Name of the main data tree. default: `EVENT`                    |
+    |`.containers` | vector<string>  | List of containers to be loaded to DDDigi.                      |
+    |              |                 | If empty => all.                                                |
+    |`.segment`    | string          | Name of the input segment. default: "inputs"                    |
+    |`.mask`       | integer         | Mask of this input source in the store. default: NO_MASK (0x0)  |
+    |`.rescan`     | boolean         | Rescan input sources for continuous execution. default: true    |
+    |`.keep_raw`   | boolean         | Keep raw input as opaque objects in the DDDigi store.           |
+
+  Functionality: self explaining
+
+### DigiAttenuate
+
+  Deposit attenuator for energy deposits according to decay time constant. <br/>
+  Properties: <br/>
+    |**Property**  |**Data type**    |**Description**                                                  |
+    |:---          |:---             |:---                                                             |
+    |`.input`      | vector<string>  | List of input files to be processed                             |
+
+
+![HORIZON2020](../doc/usermanuals/DD4hep/figures/AIDAinnova.png)
+