--- a/OrthancStone/Resources/Documentation/Coordinates.txt	Tue May 18 09:57:14 2021 +0200
+++ b/OrthancStone/Resources/Documentation/Coordinates.txt	Wed May 19 18:59:45 2021 +0200
@@ -2,6 +2,38 @@
 Some notes about the coordinate systems
 =======================================
 
+Different systems
+-----------------
+
+* "World" coordinates refer to (x,y,z) positions, typically expressed
+  in millimeters (according to the DICOM conventions), which allows to
+  define volumes and 3D cutting planes.
+
+  As explained below, those coordinates actually correspond to
+  "patient" coordinates if rendering DICOM images.
+
+  The "world" coordinates are *not* used by the Stone Web viewer,
+  because the latter is a 2D application.
+
+* "Scene" coordinates refer to (x,y) positions, typically expressed in
+  millimeters (as a byproduct of world coordinates), which allows to
+  define the 2D geometric elements that are part of the scene, and
+  Y-axis pointing downward.
+
+* "Canvas" coordinates refer to (x,y) subpixel positions that will be
+  blitted by a compositor onto a drawing canvas. The units are
+  floating-point pixels, with (0,0) corresponding to the center of the
+  display surface, and Y-axis pointing downward. The "canvas"
+  coordinates modify the "scene" coordinates according the zoom/pan
+  specified by the user.
+
+* "Pixels" coordinates refer to standard integer (x,y) coordinates of
+  the surface in memory. (0,0) is the top-left pixel.
+
+
+Relations between 2D and 3D
+---------------------------
+
 * The 3D coordinates are expressed in the PATIENT system, *not* the
   "gantry" coordinates. As a consequence, the tags "Image Position
   Patient" (0020,0032) and "Image Orientation Patient" (0020,0037)