From 89449280dc6be1d0d54359d48fc74d6c05f0c0c0 Mon Sep 17 00:00:00 2001
From: ManoloFLTK <41016272+ManoloFLTK@users.noreply.github.com>
Date: Thu, 26 Mar 2020 06:29:25 +0100
Subject: [PATCH] Doc: more details about initialisation of display scale
 factors.

---
 documentation/src/drawing.dox  |  5 ++++-
 documentation/src/osissues.dox | 14 ++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/documentation/src/drawing.dox b/documentation/src/drawing.dox
index 6213d47ee..f40cbb9c1 100644
--- a/documentation/src/drawing.dox
+++ b/documentation/src/drawing.dox
@@ -70,7 +70,10 @@ to two pixels.
 </ul>
 
 At application start time, FLTK attempts to detect the adequate scale factor value for
-each screen of the system. If this attempt fails, it's possible to set the
+each screen of the system. Here is how that's done under the \ref osissues_x_scaling "X11"
+and \ref osissues_windows_scaling "Windows" platforms.
+If the resulting scale factor is not satisfactory, and also under the macOS platform,
+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 screen at run time
diff --git a/documentation/src/osissues.dox b/documentation/src/osissues.dox
index c391d28c0..d70118d69 100644
--- a/documentation/src/osissues.dox
+++ b/documentation/src/osissues.dox
@@ -474,6 +474,12 @@ the class "fltk" is used (e.g. <tt>fltk.background</tt>). If no
 match is found, a global search is done (e.g.
 <tt>*background</tt>).
 
+\subsection osissues_x_scaling Dislay Scaling Factor
+
+FLTK uses the value of the Xft.dpi resource divided by 96.
+to initialize the display scaling factor. That is also
+what is done by the gnome and KDE desktops.
+
 \section osissues_win32 The Windows Interface
 
 The Windows interface provides access to the Windows GDI
@@ -577,6 +583,14 @@ DrawSomething(fl_gc, ..., fl_brush());
 It may also be useful to refer to Fl_Window::current()
 to get the window's size or position.
 
+\subsection osissues_windows_scaling Dislay Scaling Factor
+
+FLTK uses the value given by function GetDpiForMonitor() divided by 96.
+to initialize the scaling factor of each display in the system.
+This matches the value of
+"Change the size of text, apps and other items" found in section
+"System" subsection "Display" of Windows settings.
+
 \subsection osissues_icon_windows Setting the Icon of a Window
 
 FLTK currently supports setting a window's icon *before* it