Updated cesium integration (markdown)

cesium-integration.md

@@ -8,9 +8,13 @@ See example with OpenLayers 2 and Google Earth plugin: http://plan.epfl.ch/?lang
 
 Question: How to get the same control feeling when manipulating the Ol3 view / the Cesium view?
 
+2D view can be in local projection or in Spherical Mercator.
+
 ## Smooth transition between 2d and 3d
 Idea: A la google map, share the bottom line and tilt. Forward and Backward.
 
+2D view can be in local projection or in Spherical Mercator.
+
 Some thoughts about the camera movement: https://gist.github.com/tonio/750dbc843ef95601c4be (might be outdated with the experience of the following POCs)
 
 Ongoing work:
@@ -74,6 +78,10 @@ For now, we have:
 # Architecture
 This integration is developed in an open source layer built ontop of ol3js as ol3js is not bound to a specific 3D globe. This layer should allow to provide a common and efficient way to have ol3js and cesium working closely together.
 
+As 2D view can be in local projection or in Spherical Mercator and Cesium is always in WGS84, it might be needed to hook a reprojection tool (proj4js) in order to reproject the vector data between the views.
+
+In case 2 different prohections, the backend should support the different projections. IE a WMS layer displayed in a local projection in 2D should also be avilable in WGS84 for integration in Cesium
+
 ## Technical issues
 * How to keep both ol3js and cesium synchronized?
  * Apply vector transformations (diff from current state) each time a context change?