Add fl_remove_scale()/fl_restore_scale() to transiently draw without scaling factor.

This new API is a response to this message in fltk.general :
Can custom box type functions handle their own high-DPI screen scaling?

FL/Fl_Device.H

@@ -81,7 +81,9 @@ public:
   /** \brief Returns the graphics driver of this drawing surface. */
   inline Fl_Graphics_Driver *driver() {return pGraphicsDriver; };
   /** The current drawing surface.
-   In other words, the Fl_Surface_Device object that currently receives all graphics requests */
+   In other words, the Fl_Surface_Device object that currently receives all graphics requests
+   \note It's possible to transiently remove the GUI scaling factor in force in the current
+   drawing surface with \ref fl_remove_scale(). */
   static inline Fl_Surface_Device *surface() {
     return surface_ ? surface_ : default_surface();
   };

FL/Fl_Graphics_Driver.H

@@ -364,6 +364,8 @@ public:
   virtual void font_name(int num, const char *name);
   // Defaut implementation may be enough
   virtual void overlay_rect(int x, int y, int w , int h);
+  virtual float remove_scale();
+  virtual void restore_scale(float);
 };
 
 #ifndef FL_DOXYGEN
@@ -486,6 +488,8 @@ protected:
   void transformed_vertex(double xf, double yf);
   virtual void transformed_vertex0(float x, float y);
   void vertex(double x, double y);
+  virtual float remove_scale();
+  virtual void restore_scale(float);
 };
 #endif // FL_DOXYGEN
 

FL/fl_draw.H

@@ -687,6 +687,11 @@ FL_EXPORT const char *fl_local_to_mac_roman(const char *t, int n=-1);
 
 /** \addtogroup  fl_drawings
     @{ */
+
+FL_EXPORT float fl_remove_scale();
+
+FL_EXPORT void fl_restore_scale(float s);
+
 /**
   Draws a nul-terminated UTF-8 string starting at the given \p x, \p y location.
 

src/Fl_Graphics_Driver.cxx

@@ -626,6 +626,9 @@ void Fl_Graphics_Driver::overlay_rect(int x, int y, int w , int h) {
   loop(x, y, x+w-1, y, x+w-1, y+h-1, x, y+h-1);
 }
 
+float Fl_Graphics_Driver::remove_scale() { return 1.f;}
+
+void Fl_Graphics_Driver::restore_scale(float) { }
 
 /**
  \}
@@ -976,4 +979,20 @@ void Fl_Scalable_Graphics_Driver::draw_image_mono_unscaled(Fl_Draw_Image_Cb cb,
 
 void Fl_Scalable_Graphics_Driver::transformed_vertex0(float x, float y) {}
 
+float Fl_Scalable_Graphics_Driver::remove_scale() {
+  float s = scale();
+  if (s != 1.f) {
+    push_no_clip();
+    scale(1.f);
+  }
+  return s;
+}
+
+void Fl_Scalable_Graphics_Driver::restore_scale(float s) {
+  if (s != 1.f) {
+    scale(s);
+    pop_clip();
+  }
+}
+
 #endif

src/drivers/Quartz/Fl_Quartz_Graphics_Driver.H

@@ -188,6 +188,8 @@ protected:
   void descriptor_init(const char* name, Fl_Fontsize Size, Fl_Quartz_Font_Descriptor *d);
 #endif
   virtual void overlay_rect(int x, int y, int w , int h);
+  virtual float remove_scale();
+  virtual void restore_scale(float);
 };
 
 class Fl_Quartz_Printer_Graphics_Driver : public Fl_Quartz_Graphics_Driver {

src/drivers/Quartz/Fl_Quartz_Graphics_Driver.cxx

@@ -17,6 +17,7 @@
 #include <config.h>
 #include "Fl_Quartz_Graphics_Driver.H"
 #include "../Darwin/Fl_Darwin_System_Driver.H"
+#include "../../Fl_Screen_Driver.H"
 #include <FL/platform.H>
 #include <FL/fl_draw.H>
 #include <FL/Fl_Image_Surface.H>
@@ -158,3 +159,19 @@ void Fl_Quartz_Graphics_Driver::cache_size(Fl_Image *img, int &width, int &heigh
   width *= 2 * scale();
   height *= 2 * scale();
 }
+
+float Fl_Quartz_Graphics_Driver::remove_scale() {
+  float s = scale();
+  if (s != 1.f && Fl_Display_Device::display_device()->is_current()) {
+    Fl::screen_driver()->scale(0, 1.f);
+    CGContextScaleCTM(gc_, 1/s, 1/s);
+  }
+  return s;
+}
+
+void Fl_Quartz_Graphics_Driver::restore_scale(float s) {
+  if (s != 1.f && Fl_Display_Device::display_device()->is_current()) {
+    Fl::screen_driver()->scale(0, s);
+    CGContextScaleCTM(gc_, s, s);
+  }
+}

src/fl_draw.cxx

@@ -458,3 +458,20 @@ int fl_height(int font, int size) {
     fl_font(tf,ts);                       // restore
     return(height);
 }
+
+/** Removes any GUI scaling factor in subsequent drawing operations.
+ This must be matched by a later call to fl_restore_scale().
+ This function can be used to transiently perform drawing operations
+ that are not rescaled by the current value of the GUI scaling factor.
+ The resulting drawing context has no clipping region.
+ \return The GUI scaling factor value that was applied when the function started.
+ */
+float fl_remove_scale() {
+  return fl_graphics_driver->remove_scale();
+}
+
+/** Restores the GUI scaling factor and the clipping region in subsequent drawing operations.
+ \param s Value returned by a previous call to fl_remove_scale(). */
+void fl_restore_scale(float s) {
+  fl_graphics_driver->restore_scale(s);
+}