Remove-OSSIM.md

@@ -422,6 +422,72 @@ Note that RPC transforms will use GDAL DEM functions internally instead of using
 
 ### Re-implement RPC model
 
+#### What will change with the new model
+
+##### Description of the current model
+
+The current RPC model is based on OSSIM SensorModel mechanism. OTB has
+the classes `otb::SensorModelBase`, `otb::ForwardSensorModel` and
+`otb::InverseSensorModel` to wrap OSSIM mechanism. When provided with
+the metadata (keywordList), it uses OSSIM's sensor model factory to
+instantiate the sensor model corresponding to the sensor described by
+the metadata. It can then use this model to process the transformation
+from image to coordinates (ForwardSendorModel) or from coordinates to
+image (InverseSensorModel). 
+
+It is interesting to note that OTB doesn't know what kind of model is
+used. It could be a RPC model, a SAR model, or a sensor specific
+modeling. OSSIM takes care of this point, and OTB only provides
+the metadata and calls the *TransformPoint* function of the OSSIM model.
+
+This classes are mainly used by the filter
+`otb::GenericRSTransform`. The purpose of this filter is to transform
+the coordinate from any map projection or sensor model to any map
+projection or sensor model. It chains an inverse transformation with a
+forward transformation, and sets the correct parameters to those
+transformations. This filter is used by composite filters to process
+orthorectifications or reprojections.
+
+##### Description of the new model
+
+As we remove OSSIM, OTB needs to take over the modeling of the
+sensors. So we choose to separate the different kind of models, with a
+base class `otb::SensorTransformBase`, and the inheriting classes
+`otb::RPCTransformBase` or `otb::SARTransformBase` (and the
+possibility to add other type of model, if needed). 
+
+We decided to use GDAL's RPC implementation to manage the RPC
+transformations (see [this discussion](#Syntheses) ). To do so, we
+implemented a wrapper around GDAL's RPC:
+`otb::GDALRPCTransformer`. This wrapper is instantiated by the
+`otb::RPCTransformBase`, and provides a *TransformPoint* function, as
+OSSIM did.
+
+The `otb::GenericRSTransform` filter then needs to be modified to
+select the model corresponding to the metadata, since OSSIM is not
+there to do it anymore.
+
+The RPC transformer works with the new metadata framework, not the
+KeywordList. This means every time the old SensorModel is used, we
+need to be sure we change it with the correct model (RPC or SAR), and
+provide the ImageMetadata instead of the KeywordList. It is easy to
+find all the calls to SensorModel, because the compiler will complain
+that it is not available anymore.
+
+##### Validation strategy
+
+The different tests for the orthorectification filters and application
+are good validation test for this new framework. But in case of error,
+they won't help much to find the source of the problem.
+
+The tests for the `otb::SensorModel` classes were removed years ago,
+because the they depended on the precision in OSSIM, that could change
+with OSSIM's version. This was fine at this time, because the values
+were validated in OSSIM, and the OTB part was tested in the
+filters. But we are introducing big changes in the mechanism, and
+tests closer to the changes are helpful to track down eventual
+regressions.
+
 #### DEM and RPC exclusion
 
 The new DEM Handler uses GDAL's DEM functionalities. The new RPC