Made the image_display widget draw mouse cross-hairs when the user holds shift.

dlib/gui_widgets/widgets.cpp

@@ -6012,7 +6012,8 @@ namespace dlib
         part_width(15), // width part circles are drawn on the screen
         overlay_editing_enabled(true),
         highlight_timer(*this, &image_display::timer_event_unhighlight_rect),
-        highlighted_rect(std::numeric_limits<unsigned long>::max())
+        highlighted_rect(std::numeric_limits<unsigned long>::max()),
+        holding_shift_key(false)
     { 
         enable_mouse_drag();
 
@@ -6250,16 +6251,26 @@ namespace dlib
         const point origin(total_rect().tl_corner());
         
         // draw the image on the screen
+        const double scale = zoom_out_scale/(double)zoom_in_scale;
         const rectangle img_area = total_rect().intersect(area);
         for (long row = img_area.top(); row <= img_area.bottom(); ++row)
         {
+            const long rc = row-c.top();
+            const long rimg = (row-origin.y())*scale;
             for (long col = img_area.left(); col <= img_area.right(); ++col)
             {
-                assign_pixel(c[row-c.top()][col-c.left()], 
-                             img[(row-origin.y())*zoom_out_scale/zoom_in_scale][(col-origin.x())*zoom_out_scale/zoom_in_scale]);
+                assign_pixel(c[rc][col-c.left()], 
+                             img[rimg][(col-origin.x())*scale]);
             }
         }
 
+        // draw the mouse cross-hairs
+        if (holding_shift_key && total_rect().contains(lastx,lasty) )
+        {
+            draw_line(c, point(lastx,-10000), point(lastx,100000),rgb_pixel(255,255,0), area);
+            draw_line(c, point(-10000,lasty), point(100000,lasty),rgb_pixel(255,255,0), area);
+        }
+
         // now draw all the overlay rectangles
         for (unsigned long i = 0; i < overlay_rects.size(); ++i)
         {
@@ -6393,6 +6404,20 @@ namespace dlib
     {
         scrollable_region::on_keydown(key,is_printable, state);
 
+        if (!is_printable && key==base_window::KEY_SHIFT)
+        {
+            if (!holding_shift_key)
+            {
+                holding_shift_key = true;
+                parent.invalidate_rectangle(rect);
+            }
+        }
+        else if (holding_shift_key)
+        {
+            holding_shift_key = false;
+            parent.invalidate_rectangle(rect);
+        }
+
         if (!is_printable && !hidden && enabled && rect_is_selected && 
             (key == base_window::KEY_BACKSPACE || key == base_window::KEY_DELETE))
         {
@@ -6459,6 +6484,16 @@ namespace dlib
     {
         scrollable_region::on_mouse_down(btn, state, x, y, is_double_click);
 
+        if (state&base_window::SHIFT)
+        {
+            holding_shift_key = true;
+        }
+        else if (holding_shift_key)
+        {
+            holding_shift_key = false;
+            parent.invalidate_rectangle(rect);
+        }
+
         if (rect.contains(x,y) == false || hidden || !enabled)
             return;
 
@@ -6757,6 +6792,16 @@ namespace dlib
     {
         scrollable_region::on_mouse_up(btn,state,x,y);
 
+        if (state&base_window::SHIFT)
+        {
+            holding_shift_key = true;
+        }
+        else if (holding_shift_key)
+        {
+            holding_shift_key = false;
+            parent.invalidate_rectangle(rect);
+        }
+
         if (drawing_rect && btn == base_window::LEFT && (state&base_window::SHIFT) &&
             !hidden && enabled)
         {
@@ -6817,6 +6862,17 @@ namespace dlib
     {
         scrollable_region::on_mouse_move(state,x,y);
 
+        if (enabled && !hidden)
+        {
+            if (holding_shift_key)
+                parent.invalidate_rectangle(rect);
+
+            if (state&base_window::SHIFT)
+                holding_shift_key = true;
+            else if (holding_shift_key)
+                holding_shift_key = false;
+        }
+
         if (drawing_rect)
         {
             if ((state&base_window::LEFT) && (state&base_window::SHIFT) && !hidden && enabled)

dlib/gui_widgets/widgets.h

@@ -3593,6 +3593,7 @@ namespace dlib
         bool overlay_editing_enabled;
         timer<image_display> highlight_timer;
         unsigned long highlighted_rect;
+        bool holding_shift_key;
 
         bool moving_overlay;
         unsigned long moving_rect;