Backends: SDL2: added ImGui_ImplSDL2_GetDpiScaleForDisplay(), ImGui_ImplSDL2_GetContentScaleForWindow() helpers.

ocornut · ocornut · commit 9da3e6696abc · 2025-06-11T16:12:41.000+02:00
diff --git a/backends/imgui_impl_sdl2.cpp b/backends/imgui_impl_sdl2.cpp
@@ -21,6 +21,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2025-06-11: Added ImGui_ImplSDL2_GetContentScaleForWindow(SDL_Window* window) and ImGui_ImplSDL2_GetContentScaleForDisplay(int display_index) helper to facilitate making DPI-aware apps.
 //  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.
 //  2025-03-10: When dealing with OEM keys, use scancodes instead of translated keycodes to choose ImGuiKey values. (#7136, #7201, #7206, #7306, #7670, #7672, #8468)
@@ -702,6 +703,26 @@ static void ImGui_ImplSDL2_UpdateMouseCursor()
     }
 }
 
+// - On Windows the process needs to be marked DPI-aware!! SDL2 doesn't do it by default. You can call ::SetProcessDPIAware() or call ImGui_ImplWin32_EnableDpiAwareness() from Win32 backend.
+// - Apple platforms use FramebufferScale so we always return 1.0f.
+// - Some accessibility applications are declaring virtual monitors with a DPI of 0.0f, see #7902. We preserve this value for caller to handle.
+float ImGui_ImplSDL2_GetContentScaleForWindow(SDL_Window* window)
+{
+    return ImGui_ImplSDL2_GetContentScaleForDisplay(SDL_GetWindowDisplayIndex(window));
+}
+
+float ImGui_ImplSDL2_GetContentScaleForDisplay(int display_index)
+{
+#if SDL_HAS_PER_MONITOR_DPI
+#ifndef __APPLE__
+    float dpi = 0.0f;
+    if (SDL_GetDisplayDPI(display_index, &dpi, nullptr, nullptr) == 0)
+        return dpi / 96.0f;
+#endif
+#endif
+    return 1.0f;
+}
+
 static void ImGui_ImplSDL2_CloseGamepads()
 {
     ImGui_ImplSDL2_Data* bd = ImGui_ImplSDL2_GetBackendData();
diff --git a/backends/imgui_impl_sdl2.h b/backends/imgui_impl_sdl2.h
@@ -38,6 +38,10 @@ IMGUI_IMPL_API void     ImGui_ImplSDL2_Shutdown();
 IMGUI_IMPL_API void     ImGui_ImplSDL2_NewFrame();
 IMGUI_IMPL_API bool     ImGui_ImplSDL2_ProcessEvent(const SDL_Event* event);
 
+// DPI-related helpers (optional)
+IMGUI_IMPL_API float    ImGui_ImplSDL2_GetContentScaleForWindow(SDL_Window* window);
+IMGUI_IMPL_API float    ImGui_ImplSDL2_GetContentScaleForDisplay(int display_index);
+
 // Gamepad selection automatically starts in AutoFirst mode, picking first available SDL_Gamepad. You may override this.
 // When using manual mode, caller is responsible for opening/closing gamepad.
 enum ImGui_ImplSDL2_GamepadMode { ImGui_ImplSDL2_GamepadMode_AutoFirst, ImGui_ImplSDL2_GamepadMode_AutoAll, ImGui_ImplSDL2_GamepadMode_Manual };
