Viewports: added per-viewport FramebufferScale, Platform_GetWindowFramebufferScale() + Backends: GLFW, SDL2, SDL3, Apple: added support. (#1065, #1542, #1676, #1786, #2826, #3757, #5081, #5580, #5592, #6465, #7273, #7779 etc.)

)

Metal backend is not in charge of writing to DpiScale/FramebufferScale (tho it was a neat workaround).

Author: ocornut

backends/imgui_impl_glfw.cpp

@@ -31,7 +31,8 @@
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
 //  2025-XX-XX: Platform: Added support for multiple windows via the ImGuiPlatformIO interface.
-//  2025-04-26: Disable multi-viewports under Wayland. (#8587)
+//  2025-05-15: [Docking] Add Platform_GetWindowFramebufferScale() handler, to allow varying Retina display density on multiple monitors.
+//  2025-04-26: [Docking] Disable multi-viewports under Wayland. (#8587)
 //  2025-03-10: Map GLFW_KEY_WORLD_1 and GLFW_KEY_WORLD_2 into ImGuiKey_Oem102.
 //  2025-03-03: Fixed clipboard handler assertion when using GLFW <= 3.2.1 compiled with asserts enabled.
 //  2025-02-21: [Docking] Update monitors and work areas information every frame, as the later may change regardless of monitor changes. (#8415)
@@ -970,20 +971,26 @@ static void ImGui_ImplGlfw_UpdateMonitors()
     }
 }
 
+static void ImGui_ImplGlfw_GetWindowSizeAndFramebufferScale(GLFWwindow* window, ImVec2* out_size, ImVec2* out_framebuffer_scale)
+{
+    int w, h;
+    int display_w, display_h;
+    glfwGetWindowSize(window, &w, &h);
+    glfwGetFramebufferSize(window, &display_w, &display_h);
+    if (out_size != nullptr)
+        *out_size = ImVec2((float)w, (float)h);
+    if (out_framebuffer_scale != nullptr)
+        *out_framebuffer_scale = (w > 0 && h > 0) ? ImVec2((float)display_w / (float)w, (float)display_h / (float)h) : ImVec2(1.0f, 1.0f);
+}
+
 void ImGui_ImplGlfw_NewFrame()
 {
     ImGuiIO& io = ImGui::GetIO();
     ImGui_ImplGlfw_Data* bd = ImGui_ImplGlfw_GetBackendData();
     IM_ASSERT(bd != nullptr && "Context or backend not initialized! Did you call ImGui_ImplGlfw_InitForXXX()?");
 
-    // Setup display size (every frame to accommodate for window resizing)
-    int w, h;
-    int display_w, display_h;
-    glfwGetWindowSize(bd->Window, &w, &h);
-    glfwGetFramebufferSize(bd->Window, &display_w, &display_h);
-    io.DisplaySize = ImVec2((float)w, (float)h);
-    if (w > 0 && h > 0)
-        io.DisplayFramebufferScale = ImVec2((float)display_w / (float)w, (float)display_h / (float)h);
+    // Setup main viewport size (every frame to accommodate for window resizing)
+    ImGui_ImplGlfw_GetWindowSizeAndFramebufferScale(bd->Window, &io.DisplaySize, &io.DisplayFramebufferScale);
     ImGui_ImplGlfw_UpdateMonitors();
 
     // Setup time step
@@ -1283,6 +1290,14 @@ static void ImGui_ImplGlfw_SetWindowSize(ImGuiViewport* viewport, ImVec2 size)
     glfwSetWindowSize(vd->Window, (int)size.x, (int)size.y);
 }
 
+static ImVec2 ImGui_ImplGlfw_GetWindowFramebufferScale(ImGuiViewport* viewport)
+{
+    ImGui_ImplGlfw_ViewportData* vd = (ImGui_ImplGlfw_ViewportData*)viewport->PlatformUserData;
+    ImVec2 framebuffer_scale;
+    ImGui_ImplGlfw_GetWindowSizeAndFramebufferScale(vd->Window, nullptr, &framebuffer_scale);
+    return framebuffer_scale;
+}
+
 static void ImGui_ImplGlfw_SetWindowTitle(ImGuiViewport* viewport, const char* title)
 {
     ImGui_ImplGlfw_ViewportData* vd = (ImGui_ImplGlfw_ViewportData*)viewport->PlatformUserData;
@@ -1381,6 +1396,7 @@ static void ImGui_ImplGlfw_InitMultiViewportSupport()
     platform_io.Platform_GetWindowPos = ImGui_ImplGlfw_GetWindowPos;
     platform_io.Platform_SetWindowSize = ImGui_ImplGlfw_SetWindowSize;
     platform_io.Platform_GetWindowSize = ImGui_ImplGlfw_GetWindowSize;
+    platform_io.Platform_GetWindowFramebufferScale = ImGui_ImplGlfw_GetWindowFramebufferScale;
     platform_io.Platform_SetWindowFocus = ImGui_ImplGlfw_SetWindowFocus;
     platform_io.Platform_GetWindowFocus = ImGui_ImplGlfw_GetWindowFocus;
     platform_io.Platform_GetWindowMinimized = ImGui_ImplGlfw_GetWindowMinimized;

backends/imgui_impl_metal.mm

@@ -485,13 +485,12 @@ static void ImGui_ImplMetal_RenderWindow(ImGuiViewport* viewport, void*)
     }
     data->FirstFrame = false;
 
-    viewport->DpiScale = (float)window.backingScaleFactor;
-    if (data->MetalLayer.contentsScale != viewport->DpiScale)
+    float fb_scale = (float)window.backingScaleFactor;
+    if (data->MetalLayer.contentsScale != fb_scale)
     {
-        data->MetalLayer.contentsScale = viewport->DpiScale;
-        data->MetalLayer.drawableSize = MakeScaledSize(window.frame.size, viewport->DpiScale);
+        data->MetalLayer.contentsScale = fb_scale;
+        data->MetalLayer.drawableSize = MakeScaledSize(window.frame.size, fb_scale);
     }
-    viewport->DrawData->FramebufferScale = ImVec2(viewport->DpiScale, viewport->DpiScale);
 #endif
 
     id <CAMetalDrawable> drawable = [data->MetalLayer nextDrawable];

backends/imgui_impl_osx.mm

@@ -35,6 +35,7 @@
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
 //  2025-XX-XX: Added support for multiple windows via the ImGuiPlatformIO interface.
+//  2025-05-15: [Docking] Add Platform_GetWindowFramebufferScale() handler, to allow varying Retina display density on multiple monitors.
 //  2025-03-21: Fill gamepad inputs and set ImGuiBackendFlags_HasGamepad regardless of ImGuiConfigFlags_NavEnableGamepad being set.
 //  2025-01-20: Removed notification observer when shutting down. (#8331)
 //  2024-08-22: moved some OS/backend related function pointers from ImGuiIO to ImGuiPlatformIO:
@@ -1021,6 +1022,14 @@ static void ImGui_ImplOSX_SetWindowSize(ImGuiViewport* viewport, ImVec2 size)
     [window setFrame:rect display:YES];
 }
 
+static ImVec2 ImGui_ImplOSX_GetWindowFramebufferScale(ImGuiViewport* viewport)
+{
+    ImGui_ImplOSX_ViewportData* vd = (ImGui_ImplOSX_ViewportData*)viewport->PlatformUserData;
+    NSWindow* window = vd->Window;
+    const float fb_scale = (float)[window backingScaleFactor];
+    return ImVec2(fb_scale, fb_scale);
+}
+
 static void ImGui_ImplOSX_SetWindowFocus(ImGuiViewport* viewport)
 {
     ImGui_ImplOSX_Data* bd = ImGui_ImplOSX_GetBackendData();
@@ -1110,6 +1119,7 @@ static void ImGui_ImplOSX_InitMultiViewportSupport()
     platform_io.Platform_GetWindowPos = ImGui_ImplOSX_GetWindowPos;
     platform_io.Platform_SetWindowSize = ImGui_ImplOSX_SetWindowSize;
     platform_io.Platform_GetWindowSize = ImGui_ImplOSX_GetWindowSize;
+    platform_io.Platform_GetWindowFramebufferScale = ImGui_ImplOSX_GetWindowFramebufferScale;
     platform_io.Platform_SetWindowFocus = ImGui_ImplOSX_SetWindowFocus;
     platform_io.Platform_GetWindowFocus = ImGui_ImplOSX_GetWindowFocus;
     platform_io.Platform_GetWindowMinimized = ImGui_ImplOSX_GetWindowMinimized;

backends/imgui_impl_sdl2.cpp

@@ -26,6 +26,7 @@
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
 //  2025-XX-XX: Platform: Added support for multiple windows via the ImGuiPlatformIO interface.
+//  2025-05-15: [Docking] Add Platform_GetWindowFramebufferScale() handler, to allow varying Retina display density on multiple monitors.
 //  2025-04-09: [Docking] Revert update monitors and work areas information every frame. Only do it on Windows. (#8415, #8558)
 //  2025-04-09: Don't attempt to call SDL_CaptureMouse() on drivers where we don't call SDL_GetGlobalMouseState(). (#8561)
 //  2025-03-21: Fill gamepad inputs and set ImGuiBackendFlags_HasGamepad regardless of ImGuiConfigFlags_NavEnableGamepad being set.
@@ -939,29 +940,35 @@ static void ImGui_ImplSDL2_UpdateMonitors()
     }
 }
 
-void ImGui_ImplSDL2_NewFrame()
+static void ImGui_ImplSDL2_GetWindowSizeAndFramebufferScale(SDL_Window* window, SDL_Renderer* renderer, ImVec2* out_size, ImVec2* out_framebuffer_scale)
 {
-    ImGui_ImplSDL2_Data* bd = ImGui_ImplSDL2_GetBackendData();
-    IM_ASSERT(bd != nullptr && "Context or backend not initialized! Did you call ImGui_ImplSDL2_Init()?");
-    ImGuiIO& io = ImGui::GetIO();
-
-    // Setup display size (every frame to accommodate for window resizing)
     int w, h;
     int display_w, display_h;
-    SDL_GetWindowSize(bd->Window, &w, &h);
-    if (SDL_GetWindowFlags(bd->Window) & SDL_WINDOW_MINIMIZED)
+    SDL_GetWindowSize(window, &w, &h);
+    if (SDL_GetWindowFlags(window) & SDL_WINDOW_MINIMIZED)
         w = h = 0;
-    if (bd->Renderer != nullptr)
-        SDL_GetRendererOutputSize(bd->Renderer, &display_w, &display_h);
+    if (renderer != nullptr)
+        SDL_GetRendererOutputSize(renderer, &display_w, &display_h);
 #if SDL_HAS_VULKAN
-    else if (SDL_GetWindowFlags(bd->Window) & SDL_WINDOW_VULKAN)
-        SDL_Vulkan_GetDrawableSize(bd->Window, &display_w, &display_h);
+    else if (SDL_GetWindowFlags(window) & SDL_WINDOW_VULKAN)
+        SDL_Vulkan_GetDrawableSize(window, &display_w, &display_h);
 #endif
     else
-        SDL_GL_GetDrawableSize(bd->Window, &display_w, &display_h);
-    io.DisplaySize = ImVec2((float)w, (float)h);
-    if (w > 0 && h > 0)
-        io.DisplayFramebufferScale = ImVec2((float)display_w / w, (float)display_h / h);
+        SDL_GL_GetDrawableSize(window, &display_w, &display_h);
+    if (out_size != nullptr)
+        *out_size = ImVec2((float)w, (float)h);
+    if (out_framebuffer_scale != nullptr)
+        *out_framebuffer_scale = (w > 0 && h > 0) ? ImVec2((float)display_w / w, (float)display_h / h) : ImVec2(1.0f, 1.0f);
+}
+
+void ImGui_ImplSDL2_NewFrame()
+{
+    ImGui_ImplSDL2_Data* bd = ImGui_ImplSDL2_GetBackendData();
+    IM_ASSERT(bd != nullptr && "Context or backend not initialized! Did you call ImGui_ImplSDL2_Init()?");
+    ImGuiIO& io = ImGui::GetIO();
+
+    // Setup main viewport size (every frame to accommodate for window resizing)
+    ImGui_ImplSDL2_GetWindowSizeAndFramebufferScale(bd->Window, bd->Renderer, &io.DisplaySize, &io.DisplayFramebufferScale);
 
     // Update monitors
 #ifdef WIN32
@@ -1147,6 +1154,15 @@ static void ImGui_ImplSDL2_SetWindowSize(ImGuiViewport* viewport, ImVec2 size)
     SDL_SetWindowSize(vd->Window, (int)size.x, (int)size.y);
 }
 
+static ImVec2 ImGui_ImplSDL2_GetWindowFramebufferScale(ImGuiViewport* viewport)
+{
+    // FIXME: SDL_Renderer does not support multi-viewport.
+    ImGui_ImplSDL2_ViewportData* vd = (ImGui_ImplSDL2_ViewportData*)viewport->PlatformUserData;
+    ImVec2 framebuffer_scale;
+    ImGui_ImplSDL2_GetWindowSizeAndFramebufferScale(vd->Window, nullptr, nullptr, &framebuffer_scale);
+    return framebuffer_scale;
+}
+
 static void ImGui_ImplSDL2_SetWindowTitle(ImGuiViewport* viewport, const char* title)
 {
     ImGui_ImplSDL2_ViewportData* vd = (ImGui_ImplSDL2_ViewportData*)viewport->PlatformUserData;
@@ -1220,6 +1236,7 @@ static void ImGui_ImplSDL2_InitMultiViewportSupport(SDL_Window* window, void* sd
     platform_io.Platform_GetWindowPos = ImGui_ImplSDL2_GetWindowPos;
     platform_io.Platform_SetWindowSize = ImGui_ImplSDL2_SetWindowSize;
     platform_io.Platform_GetWindowSize = ImGui_ImplSDL2_GetWindowSize;
+    platform_io.Platform_GetWindowFramebufferScale = ImGui_ImplSDL2_GetWindowFramebufferScale;
     platform_io.Platform_SetWindowFocus = ImGui_ImplSDL2_SetWindowFocus;
     platform_io.Platform_GetWindowFocus = ImGui_ImplSDL2_GetWindowFocus;
     platform_io.Platform_GetWindowMinimized = ImGui_ImplSDL2_GetWindowMinimized;

backends/imgui_impl_sdl3.cpp

@@ -24,6 +24,7 @@
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
 //  2025-XX-XX: Platform: Added support for multiple windows via the ImGuiPlatformIO interface.
+//  2025-05-15: [Docking] Add Platform_GetWindowFramebufferScale() handler, to allow varying Retina display density on multiple monitors.
 //  2025-05-06: [Docking] macOS: fixed secondary viewports not appearing on other monitors before of parenting.
 //  2025-04-09: [Docking] Revert update monitors and work areas information every frame. Only do it on Windows. (#8415, #8558)
 //  2025-04-22: IME: honor ImGuiPlatformImeData->WantTextInput as an alternative way to call SDL_StartTextInput(), without IME being necessarily visible.
@@ -901,22 +902,28 @@ static void ImGui_ImplSDL3_UpdateMonitors()
     SDL_free(displays);
 }
 
+static void ImGui_ImplSDL3_GetWindowSizeAndFramebufferScale(SDL_Window* window, ImVec2* out_size, ImVec2* out_framebuffer_scale)
+{
+    int w, h;
+    int display_w, display_h;
+    SDL_GetWindowSize(window, &w, &h);
+    if (SDL_GetWindowFlags(window) & SDL_WINDOW_MINIMIZED)
+        w = h = 0;
+    SDL_GetWindowSizeInPixels(window, &display_w, &display_h);
+    if (out_size != nullptr)
+        *out_size = ImVec2((float)w, (float)h);
+    if (out_framebuffer_scale != nullptr)
+        *out_framebuffer_scale = (w > 0 && h > 0) ? ImVec2((float)display_w / w, (float)display_h / h) : ImVec2(1.0f, 1.0f);
+}
+
 void ImGui_ImplSDL3_NewFrame()
 {
     ImGui_ImplSDL3_Data* bd = ImGui_ImplSDL3_GetBackendData();
     IM_ASSERT(bd != nullptr && "Context or backend not initialized! Did you call ImGui_ImplSDL3_Init()?");
     ImGuiIO& io = ImGui::GetIO();
 
-    // Setup display size (every frame to accommodate for window resizing)
-    int w, h;
-    int display_w, display_h;
-    SDL_GetWindowSize(bd->Window, &w, &h);
-    if (SDL_GetWindowFlags(bd->Window) & SDL_WINDOW_MINIMIZED)
-        w = h = 0;
-    SDL_GetWindowSizeInPixels(bd->Window, &display_w, &display_h);
-    io.DisplaySize = ImVec2((float)w, (float)h);
-    if (w > 0 && h > 0)
-        io.DisplayFramebufferScale = ImVec2((float)display_w / w, (float)display_h / h);
+    // Setup main viewport size (every frame to accommodate for window resizing)
+    ImGui_ImplSDL3_GetWindowSizeAndFramebufferScale(bd->Window, &io.DisplaySize, &io.DisplayFramebufferScale);
 
     // Update monitors
 #ifdef WIN32
@@ -1117,6 +1124,14 @@ static void ImGui_ImplSDL3_SetWindowSize(ImGuiViewport* viewport, ImVec2 size)
     SDL_SetWindowSize(vd->Window, (int)size.x, (int)size.y);
 }
 
+static ImVec2 ImGui_ImplSDL3_GetWindowFramebufferScale(ImGuiViewport* viewport)
+{
+    ImGui_ImplSDL3_ViewportData* vd = (ImGui_ImplSDL3_ViewportData*)viewport->PlatformUserData;
+    ImVec2 framebuffer_scale;
+    ImGui_ImplSDL3_GetWindowSizeAndFramebufferScale(vd->Window, nullptr, &framebuffer_scale);
+    return framebuffer_scale;
+}
+
 static void ImGui_ImplSDL3_SetWindowTitle(ImGuiViewport* viewport, const char* title)
 {
     ImGui_ImplSDL3_ViewportData* vd = (ImGui_ImplSDL3_ViewportData*)viewport->PlatformUserData;
@@ -1187,6 +1202,7 @@ static void ImGui_ImplSDL3_InitMultiViewportSupport(SDL_Window* window, void* sd
     platform_io.Platform_GetWindowPos = ImGui_ImplSDL3_GetWindowPos;
     platform_io.Platform_SetWindowSize = ImGui_ImplSDL3_SetWindowSize;
     platform_io.Platform_GetWindowSize = ImGui_ImplSDL3_GetWindowSize;
+    platform_io.Platform_GetWindowFramebufferScale = ImGui_ImplSDL3_GetWindowFramebufferScale;
     platform_io.Platform_SetWindowFocus = ImGui_ImplSDL3_SetWindowFocus;
     platform_io.Platform_GetWindowFocus = ImGui_ImplSDL3_GetWindowFocus;
     platform_io.Platform_GetWindowMinimized = ImGui_ImplSDL3_GetWindowMinimized;

docs/CHANGELOG.txt

@@ -129,12 +129,20 @@ Other changes:
 
 Docking+Viewports Branch:
 
-- Backends: Win32: Viewports: fixed an issue when closing a window from 
+- Viewports: added per-viewport FramebufferScale for Retina display multi-monitor support.
+  Backend must provide platform_io.platform_io.Platform_GetWindowFramebufferScale handler.
+  This effectively fixes clipping/rendering on multi-monitors with varying Retina scale.
+  (this per-se doesn't fix the font quality which requires setting RasterizerDensity
+  separately. More on this later as it should soon become automatic).
+  (#1065, #1542, #1676, #1786, #2826, #3757, #5081, #5580, #5592, #6465, #7273, #7779 etc.)
+- Backends: Win32: Viewports: fixed an issue when closing a window from
   the OS close button (with io.ConfigViewportsNoDecoration=false) while 
   user code is discarding the 'bool *p_open=false output' from Begin(). 
   Because we allowed the Win32 window to close early, Windows destroyed
   it and our imgui window became not visible even though user code was
   still submitting it.
+- Backends: GLFW, SDL2, SDL3, Apple: provide Platform_GetWindowFramebufferScale handler,
+  (#1065, #1542, #1676, #1786, #2826, #3757, #5081, #5580, #5592, #6465, #7273, #7779 etc.)
 - Backends: SDLGPU3 for SDL3: added multi-viewport support. (#8573) [@Lekoopapaul]
 - Backends: SDL2, SDL3: revert updating monitors and work areas info every
   frame. Only do it on Windows to detect task-bar resize until we get an

examples/example_apple_opengl2/main.mm

@@ -6,6 +6,8 @@
 // - Documentation        https://dearimgui.com/docs (same as your local docs/ folder).
 // - Introduction, links and more at the top of imgui.cpp
 
+// FIXME: Multi-viewports is not yet functional in this example. May need backend rework/coordination.
+
 #import <Cocoa/Cocoa.h>
 #import <OpenGL/gl.h>
 #import <OpenGL/glu.h>

imgui.cpp

@@ -5719,7 +5719,7 @@ static void InitViewportDrawData(ImGuiViewportP* viewport)
     draw_data->TotalVtxCount = draw_data->TotalIdxCount = 0;
     draw_data->DisplayPos = viewport->Pos;
     draw_data->DisplaySize = is_minimized ? ImVec2(0.0f, 0.0f) : viewport->Size;
-    draw_data->FramebufferScale = io.DisplayFramebufferScale; // FIXME-VIEWPORT: This may vary on a per-monitor/viewport basis?
+    draw_data->FramebufferScale = (viewport->FramebufferScale.x != 0.0f) ? viewport->FramebufferScale : io.DisplayFramebufferScale;
     draw_data->OwnerViewport = viewport;
 }
 
@@ -16015,6 +16015,8 @@ static void ImGui::UpdateViewportsNewFrame()
                     viewport->Pos = viewport->LastPlatformPos = g.PlatformIO.Platform_GetWindowPos(viewport);
                 if (viewport->PlatformRequestResize)
                     viewport->Size = viewport->LastPlatformSize = g.PlatformIO.Platform_GetWindowSize(viewport);
+                if (g.PlatformIO.Platform_GetWindowFramebufferScale != NULL)
+                    viewport->FramebufferScale = g.PlatformIO.Platform_GetWindowFramebufferScale(viewport);
             }
         }
 
@@ -22313,8 +22315,9 @@ void ImGui::DebugNodeViewport(ImGuiViewportP* viewport)
     if (open)
     {
         ImGuiWindowFlags flags = viewport->Flags;
-        BulletText("Main Pos: (%.0f,%.0f), Size: (%.0f,%.0f)\nWorkArea Inset Left: %.0f Top: %.0f, Right: %.0f, Bottom: %.0f\nMonitor: %d, DpiScale: %.0f%%",
+        BulletText("Main Pos: (%.0f,%.0f), Size: (%.0f,%.0f)\nFrameBufferScale: (%.2f,%.2f)\nWorkArea Inset Left: %.0f Top: %.0f, Right: %.0f, Bottom: %.0f\nMonitor: %d, DpiScale: %.0f%%",
             viewport->Pos.x, viewport->Pos.y, viewport->Size.x, viewport->Size.y,
+            viewport->FramebufferScale.x, viewport->FramebufferScale.y,
             viewport->WorkInsetMin.x, viewport->WorkInsetMin.y, viewport->WorkInsetMax.x, viewport->WorkInsetMax.y,
             viewport->PlatformMonitor, viewport->DpiScale * 100.0f);
         if (viewport->Idx > 0) { SameLine(); if (SmallButton("Reset Pos")) { viewport->Pos = ImVec2(200, 200); viewport->UpdateWorkRect(); if (viewport->Window) viewport->Window->Pos = viewport->Pos; } }