diff --git a/documentation/src/drawing.dox b/documentation/src/drawing.dox
index d983c89e8..40e970f94 100644
--- a/documentation/src/drawing.dox
+++ b/documentation/src/drawing.dox
@@ -37,18 +37,70 @@ Fl_Surface_Device::pop_current().
 
 \subsection ssect_DrawingUnit What Drawing Unit do FLTK drawing functions use?
 
-When drawing to the display or to instances of Fl_Copy_Surface and Fl_Image_Surface, 
-the unit of drawing functions corresponds
-generally to one pixel. The so-called 'retina' displays of some recent
-Apple computers are an exception to this rule: one drawing unit corresponds
-to the width or the height of 2 display pixels on a retina display.
+Before version 1.4, all graphical quantities used by FLTK are in pixel units:
+a window of width 500 units is 500-pixel wide, a line of length 10 units is
+10-pixel long, lines of text written using a 14-point font are 14 pixels below
+each other. This organization is not sufficient to support GUI apps that can be drawn on
+displays of varying pixel density, especially on High-DPI displays, because
+widgets become very small and text becomes unreadable.
 
-When drawing to surfaces that are instances of Fl_Paged_Device
-derived classes (i.e., Fl_Printer or Fl_PostScript_File_Device), the drawing unit
-is initially one point, that is, 1/72 of an inch. But this unit is changed
-after calls to Fl_Paged_Device::scale().
+FLTK version 1.4 introduces a new feature, a display-specific <b>scale factor</b> which is
+a float number with a typical value in the 1-2.5 range and is used as follows: any graphical
+element with an FLTK value of \e v units is drawn on the display with \e v * \e scale units.
+Thus, a window with width 500 units is 500*scale-pixel wide, a line of length 10 units is
+10*scale-pixel long, lines of text written using a 14-point font are 14*scale pixels below
+each other. Consider a system with two displays, one with regular DPI and one with
+a twice higher DPI. If the first display's scale factor is set to 1 and that of the
+second display to 2, the GUI of any FLTK app appears equally sized on the two displays.
 
+FLTK uses several units to measure graphical elements:
+<ul><li>All data visible by the public API (e.g., window widths, line lengths, font sizes,
+clipping regions) are in <b>FLTK units</b> which are both system- and DPI-independent.
+<li>Just before drawing to a display, the library internally multiplies all quantities
+expressed in FLTK units by the current value of the scale factor
+for the display in use and obtains quantities in <b>drawing units</b>.
+The current scale factor value, for an Fl_Window named \e window, is given by
+\code
+int nscreen = window->driver()->screen_num(); // the screen where window is mapped
+float s = Fl::screen_driver()->scale(nscreen); // this screen's scale factor
+\endcode
+One drawing unit generally corresponds to one display pixel...
+<li>...but not on Mac OS X and for retina displays, where drawing units correspond
+to two pixels.
+</ul>
 
+At application start time, FLTK attempts to detect the adequate scale factor value for
+each display of the system. If this attempt fails, it's possible to set the
+<tt>FLTK_SCALING_FACTOR</tt> environmental variable to the desired numerical value
+(e.g., 1.75) and any FLTK app will start scaled with that value. Furthermore,
+it's possible to change the scale factor value of any display at run time
+with ctrl/+/-/0/ keystrokes which enlarge, shrink, and reset, respectively,
+the GUI of all FLTK windows on a display.
+Under Mac OS X, the corresponding GUI scaling shortcuts are cmd/+/-/0/.
+
+When drawing images to a display whose scale value is > 1, the image pixel data
+are mapped to the larger number of pixels present in
+a display area sized (in FLTK units) as the image. This operation is done internally
+by the library using diverse scaling methods that vary with the image type
+(Fl_Pixmap, Fl_Bitmap or Fl_RGB_Image) and the platform.
+A situation of special interest arises when drawing Fl_Shared_Image's. The
+Fl_Shared_Image::scale() member function allows to set the image drawing size
+(in FLTK units) independently of its pixel size. Image pixels are mapped to the
+pixels of the image drawing area on the display which is determined by the scale
+factor value. Therefore, the image data can fill the drawing area of the display at
+full physical resolution even for high scale values if the original image is large enough.
+
+The Fl_Image_Surface class is intended to create an Fl_RGB_Image from a series
+of FLTK drawing operations. The Fl_Image_Surface constructor allows to control
+whether the size in pixels of the resulting image matches the FLTK units used when
+performing drawing operations, or matches the number of pixels corresponding to
+these FLTK units given the current scale factor value. The first result is obtained
+with <tt>new Fl_Image_Surface(w, h)</tt>, the second with
+<tt>new Fl_Image_Surface(w, h, 1)</tt>.
+
+When drawing to Fl_Printer or Fl_PostScript_File_Device, the drawing unit
+is initially one point, that is, 1/72 of an inch. This unit is changed
+by calls to Fl_Paged_Device::scale().
 
 \section sect_DrawingFunctions Drawing Functions