commit d1c7caec3bf726ac137566ced91d067591a71e7e
Author:     ManoloFLTK <41016272+ManoloFLTK@users.noreply.github.com>
AuthorDate: Wed Dec 2 14:39:37 2020 +0100
Commit:     ManoloFLTK <41016272+ManoloFLTK@users.noreply.github.com>
CommitDate: Wed Dec 2 14:39:49 2020 +0100

    Improve Fl_Graphics_Driver::cache_size() when GUI is scaled.
    
    The change lets this function enlarge the size only when strictly necessary
    for image tiling.

 src/Fl_Graphics_Driver.cxx                     | 17 +++++++++--------
 src/drivers/WinAPI/Fl_WinAPI_Screen_Driver.cxx |  2 +-
 src/drivers/X11/Fl_X11_Screen_Driver.cxx       |  2 +-
 3 files changed, 11 insertions(+), 10 deletions(-)

diff --git src/Fl_Graphics_Driver.cxx src/Fl_Graphics_Driver.cxx
index e156ae2..5b33f7b 100644
--- src/Fl_Graphics_Driver.cxx
+++ src/Fl_Graphics_Driver.cxx
@@ -203,14 +203,15 @@ unsigned Fl_Graphics_Driver::font_desc_size() {
  scale() and in slightly modifying that to help support tiled images. */
 void Fl_Graphics_Driver::cache_size(Fl_Image *img, int &width, int &height)
 {
-  if ( int(scale()) == scale() ) {
-    width  = width * scale();
-    height = height * scale();
-  } else {
-    width  = (width+1) * scale();
-    height = (height+1) * scale();
-  }
-  img->cache_size_(width, height);
+  // Image tiling may require to convert the floating value of width * scale() or
+  // height * scale() to a larger integer value to avoid undrawn space between adjacent images.
+  float s = scale(), fs = width * s;
+  width = (fs - int(fs) < 0.001 ? int(fs) :
+           int((width+1) * s));
+  fs = height * s;
+  height = (fs - int(fs) < 0.001 ? int(fs) :
+           int((height+1) * s));
+  if (img) img->cache_size_(width, height);
 }
 
 /** Draws an Fl_Pixmap object using this graphics driver.
diff --git src/drivers/WinAPI/Fl_WinAPI_Screen_Driver.cxx src/drivers/WinAPI/Fl_WinAPI_Screen_Driver.cxx
index 659113c..998d4f9 100644
--- src/drivers/WinAPI/Fl_WinAPI_Screen_Driver.cxx
+++ src/drivers/WinAPI/Fl_WinAPI_Screen_Driver.cxx
@@ -503,7 +503,7 @@ Fl_WinAPI_Screen_Driver::read_win_rectangle(
   int ws, hs;
   if (int(s) == s) { ws = w * s; hs = h * s;}
   else {
-    ws = (w+1)*s; // matches what Fl_Graphics_Driver::cache_size() does
+    ws = (w+1)*s; // approximates what Fl_Graphics_Driver::cache_size() does
     hs = (h+1)*s;
     if (ws < 1) ws = 1;
     if (hs < 1) hs = 1;
diff --git src/drivers/X11/Fl_X11_Screen_Driver.cxx src/drivers/X11/Fl_X11_Screen_Driver.cxx
index 539fdfd..ec3a6e0 100644
--- src/drivers/X11/Fl_X11_Screen_Driver.cxx
+++ src/drivers/X11/Fl_X11_Screen_Driver.cxx
@@ -800,7 +800,7 @@ Fl_RGB_Image *Fl_X11_Screen_Driver::read_win_rectangle(int X, int Y, int w, int
       sh = screens[ns].height;
     }
     if (win && !allow_outside && int(s) != s) {
-      ws = (w+1) * s; // matches what Fl_Graphics_Driver::cache_size() does
+      ws = (w+1) * s; // approximates what Fl_Graphics_Driver::cache_size() does
       hs = (h+1) * s;
       if (Xs + ws >= int(win->w()*s)) ws = win->w()*s - Xs -1;
       if (Ys + hs >= int(win->h()*s)) hs = win->h()*s - Ys -1;