Viewport, DPI: Now using DpiScale from the ImGuiPlatformMonitor array instead of calling Platform_GetWindowDpiScale() before the platform window creation. Might even tentatively see if things work out without Platform_GetWindowDpiScale. (#1676)

examples/imgui_impl_win32.cpp

@@ -366,7 +366,7 @@ void ImGui_ImplWin32_EnableDpiAwareness()
 float ImGui_ImplWin32_GetDpiScaleForMonitor(void* monitor)
 {
     UINT xdpi = 96, ydpi = 96;
-    if (IsWindows8Point1OrGreater())
+    if (::IsWindows8Point1OrGreater())
     {
         static HINSTANCE shcore_dll = ::LoadLibraryA("shcore.dll"); // Reference counted per-process
         if (PFN_GetDpiForMonitor GetDpiForMonitorFn = (PFN_GetDpiForMonitor)::GetProcAddress(shcore_dll, "GetDpiForMonitor"))
@@ -389,13 +389,6 @@ float ImGui_ImplWin32_GetDpiScaleForHwnd(void* hwnd)
     return ImGui_ImplWin32_GetDpiScaleForMonitor(monitor);
 }
 
-float ImGui_ImplWin32_GetDpiScaleForRect(int x1, int y1, int x2, int y2)
-{
-    RECT viewport_rect = { (LONG)x1, (LONG)y1, (LONG)x2, (LONG)y2 };
-    HMONITOR monitor = ::MonitorFromRect(&viewport_rect, MONITOR_DEFAULTTONEAREST);
-    return ImGui_ImplWin32_GetDpiScaleForMonitor(monitor);
-}
-
 //--------------------------------------------------------------------------------------------------------
 // IME (Input Method Editor) basic support for e.g. Asian language users
 //--------------------------------------------------------------------------------------------------------
@@ -589,13 +582,8 @@ static void ImGui_ImplWin32_SetWindowAlpha(ImGuiViewport* viewport, float alpha)
 static float ImGui_ImplWin32_GetWindowDpiScale(ImGuiViewport* viewport)
 {
     ImGuiViewportDataWin32* data = (ImGuiViewportDataWin32*)viewport->PlatformUserData;
-    if (data && data->Hwnd)
-        return ImGui_ImplWin32_GetDpiScaleForHwnd(data->Hwnd);
-
-    // The first frame a viewport is created we don't have a window yet
-    return ImGui_ImplWin32_GetDpiScaleForRect(
-        (int)(viewport->Pos.x), (int)(viewport->Pos.y),
-        (int)(viewport->Pos.x + viewport->Size.x), (int)(viewport->Pos.y + viewport->Size.y));
+    IM_ASSERT(data->Hwnd != 0);
+    return ImGui_ImplWin32_GetDpiScaleForHwnd(data->Hwnd);
 }
 
 // FIXME-DPI: Testing DPI related ideas
@@ -706,7 +694,7 @@ static void ImGui_ImplWin32_InitPlatformInterface()
     platform_io.Platform_GetWindowMinimized = ImGui_ImplWin32_GetWindowMinimized;
     platform_io.Platform_SetWindowTitle = ImGui_ImplWin32_SetWindowTitle;
     platform_io.Platform_SetWindowAlpha = ImGui_ImplWin32_SetWindowAlpha;
-    platform_io.Platform_GetWindowDpiScale = ImGui_ImplWin32_GetWindowDpiScale;
+    platform_io.Platform_GetWindowDpiScale = ImGui_ImplWin32_GetWindowDpiScale; // FIXME-DPI
     platform_io.Platform_OnChangedViewport = ImGui_ImplWin32_OnChangedViewport; // FIXME-DPI
 #if HAS_WIN32_IME
     platform_io.Platform_SetImeInputPos = ImGui_ImplWin32_SetImeInputPos;

examples/imgui_impl_win32.h

@@ -16,10 +16,9 @@ IMGUI_IMPL_API void     ImGui_ImplWin32_Shutdown();
 IMGUI_IMPL_API void     ImGui_ImplWin32_NewFrame();
 
 // DPI-related helpers (which run and compile without requiring 8.1 or 10, neither Windows version, neither associated SDK)
-IMGUI_API void        ImGui_ImplWin32_EnableDpiAwareness();
-IMGUI_API float       ImGui_ImplWin32_GetDpiScaleForHwnd(void* hwnd);       // HWND hwnd
-IMGUI_API float       ImGui_ImplWin32_GetDpiScaleForMonitor(void* monitor); // HMONITOR monitor
-IMGUI_API float       ImGui_ImplWin32_GetDpiScaleForRect(int x1, int y1, int x2, int y2);
+IMGUI_IMPL_API void     ImGui_ImplWin32_EnableDpiAwareness();
+IMGUI_IMPL_API float    ImGui_ImplWin32_GetDpiScaleForHwnd(void* hwnd);       // HWND hwnd
+IMGUI_IMPL_API float    ImGui_ImplWin32_GetDpiScaleForMonitor(void* monitor); // HMONITOR monitor
 
 // Handler for Win32 messages, update mouse/keyboard data.
 // You may or not need this for your implementation, but it can serve as reference for handling inputs.

imgui.cpp

@@ -7376,8 +7376,10 @@ static void ImGui::UpdateViewports()
 
         // Update DPI scale
         float new_dpi_scale;
-        if (g.PlatformIO.Platform_GetWindowDpiScale)
+        if (g.PlatformIO.Platform_GetWindowDpiScale && platform_funcs_available)
             new_dpi_scale = g.PlatformIO.Platform_GetWindowDpiScale(viewport);
+        else if (viewport->PlatformMonitor != -1)
+            new_dpi_scale = g.PlatformIO.Monitors[viewport->PlatformMonitor].DpiScale;
         else
             new_dpi_scale = (viewport->DpiScale != 0.0f) ? viewport->DpiScale : 1.0f;
         if (viewport->DpiScale != 0.0f && new_dpi_scale != viewport->DpiScale)
@@ -7490,10 +7492,10 @@ ImGuiViewportP* ImGui::AddUpdateViewport(ImGuiWindow* window, ImGuiID id, const
         g.DrawListSharedData.ClipRectFullscreen.z = ImMax(g.DrawListSharedData.ClipRectFullscreen.z, viewport->Pos.x + viewport->Size.x);
         g.DrawListSharedData.ClipRectFullscreen.w = ImMax(g.DrawListSharedData.ClipRectFullscreen.w, viewport->Pos.y + viewport->Size.y);
 
-        // Request an initial DpiScale before the OS platform window creation
+        // Store initial DpiScale before the OS platform window creation, based on expected monitor data.
         // This is so we can select an appropriate font size on the first frame of our window lifetime
-        if (g.PlatformIO.Platform_GetWindowDpiScale)
-            viewport->DpiScale = g.PlatformIO.Platform_GetWindowDpiScale(viewport);
+        if (viewport->PlatformMonitor != -1)
+            viewport->DpiScale = g.PlatformIO.Monitors[viewport->PlatformMonitor].DpiScale;
     }
 
     viewport->Window = window;

imgui.h

@@ -2124,13 +2124,13 @@ struct ImFont
 // - if you are new to dear imgui and trying to integrate it into your engine, you should probably ignore this for now.
 //-----------------------------------------------------------------------------
 
-// (Optional) Represent the bounds of each connected monitor/display
-// Dear ImGui only uses this to clamp the position of popups and tooltips so they don't straddle multiple monitors.
+// (Optional) Represent the bounds of each connected monitor/display and their DPI
+// This is used for: multiple DPI support + clamping the position of popups and tooltips so they don't straddle multiple monitors.
 struct ImGuiPlatformMonitor
 {
     ImVec2  MainPos, MainSize;  // Coordinates of the area displayed on this monitor (Min = upper left, Max = bottom right)
     ImVec2  WorkPos, WorkSize;  // (Optional) Coordinates without task bars / side bars / menu bars. imgui uses this to avoid positioning popups/tooltips inside this region.
-    float   DpiScale;
+    float   DpiScale;           // 1.0f = 96 DPI
     ImGuiPlatformMonitor() { MainPos = MainSize = WorkPos = WorkSize = ImVec2(0,0); DpiScale = 1.0f; }
 };
 
@@ -2165,7 +2165,7 @@ struct ImGuiPlatformIO
     void    (*Platform_SetWindowAlpha)(ImGuiViewport* vp, float alpha);     // (Optional) Setup window transparency
     void    (*Platform_RenderWindow)(ImGuiViewport* vp, void* render_arg);  // (Optional) Setup for render (platform side)
     void    (*Platform_SwapBuffers)(ImGuiViewport* vp, void* render_arg);   // (Optional) Call Present/SwapBuffers (platform side)
-    float   (*Platform_GetWindowDpiScale)(ImGuiViewport* vp);               // (Optional) [BETA] (FIXME-DPI) DPI handling: Return DPI scale for this viewport. 1.0f = 96 DPI. IMPORTANT: this will be called _before_ the window is created, in which case the implementation is expected to use the viewport->Pos/Size fields to estimate DPI value.
+    float   (*Platform_GetWindowDpiScale)(ImGuiViewport* vp);               // (Optional) [BETA] (FIXME-DPI) DPI handling: Return DPI scale for this viewport. 1.0f = 96 DPI.
     void    (*Platform_OnChangedViewport)(ImGuiViewport* vp);               // (Optional) [BETA] (FIXME-DPI) DPI handling: Called during Begin() every time the viewport we are outputting into changes, so back-end has a chance to swap fonts to adjust style.
     void    (*Platform_SetImeInputPos)(ImGuiViewport* vp, ImVec2 pos);      // (Optional) Set IME (Input Method Editor, e.g. for Asian languages) input position, so text preview appears over the imgui input box.
     int     (*Platform_CreateVkSurface)(ImGuiViewport* vp, ImU64 vk_inst, const void* vk_allocators, ImU64* out_vk_surface); // (Optional) For Renderer to call into Platform code