Some notes about the coordinate systems
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* 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)
  describe the 3D geometry of a slice.

* The tag "Patient Position" (0018,5100) could be used to convert from
  patient coordinates to gantry coordinates, but this would be useful
  to device manufacturers, whereas Stone primarly deals with users of
  modalities.

* The "Image Position Patient" gives the 3D coordinates of the CENTER
  of the top-left voxel of slice.

* In 2D compositors, the origin of a texture corresponds to the CORNER
  of the top-left pixel of the texture (*not* to the center of this
  pixel). Roughly speaking, the operation "floor()" must be applied to
  move from canvas coordinates (floating-point numbers) to pixel
  coordinates in the image buffer (integer numbers).

* The classes deriving from "IVolumeSlicer" must pay to attention to
  convert from centers of 3D voxels to bounding boxes of 2D pixels.