Migration-guide-OTBv8.md

@@ -11,6 +11,24 @@ API. With the version 8 of OTB, it is time to remove this dependency, whose
 development cycle is difficult to follow. Until OTB v7, only a
 small portion of OSSIM was used anyway.
 
+### How OSSIM was used until OTB v7
+
+OSSIM comes with a complete framework to read and write metadata from
+image files. It is able to read the metadata and store them in an
+object callel KeywordList. The OTB is then in charge of channeling
+this object through the processing pipeline so every filter can access
+them. When one needs to output an image file, OSSIM takes the
+KeywordList object and write it's content to the file. OSSIM use the
+GEOM file format for the metadata. 
+
+OSSIM also comes with a framework for sensor modelling. It includes a
+factory that instantiate the correct model according the to content of
+the metadata KeywordList. It is the able to perform forward and invers
+transform. Depending on the sensor, the model will be based on RPCs,
+or other sensor specific model.
+
+### How we replaced OSSIM in OTB v8
+
 The two main work for this version 8 have been:
 
 - A complete refactoring of the metadata management. We replaced OSSIM
@@ -21,7 +39,8 @@ The two main work for this version 8 have been:
   OSSIM's RPC model by GDAL's. We implemented our own SAR model. A
   whole new SensorModel factory as developed.
 
-A new DEM Handler was also developed.
+A new DEM Handler was also developed, based on what GDAL proposes.
+
 
 ## The new ImageMetadata object