Backends: Win32: Added ImGui_ImplWin32_EnableAlphaCompositing() helper. (#2766, #3447)

Pragma linking with dwmapi.lib (Vista-era, ~9 kb). MinGW users will need to link with -ldwmapi.

backends/imgui_impl_win32.cpp

@@ -18,6 +18,10 @@
 #endif
 #include <windows.h>
 #include <tchar.h>
+#include <dwmapi.h>
+
+// Configuration flags to add in your imconfig.h file:
+//#define IMGUI_IMPL_WIN32_DISABLE_GAMEPAD              // Disable gamepad support (this used to be meaningful before <1.81) but we know load XInput dynamically so the option is less relevant now.
 
 // Using XInput for gamepad (will load DLL dynamically)
 #ifndef IMGUI_IMPL_WIN32_DISABLE_GAMEPAD
@@ -28,6 +32,7 @@ typedef DWORD (WINAPI *PFN_XInputGetState)(DWORD, XINPUT_STATE*);
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2021-02-18: Added ImGui_ImplWin32_EnableAlphaCompositing(). Non Visual Studio users will need to link with dwmapi.lib (MinGW/gcc: use -ldwmapi).
 //  2021-02-17: Fixed ImGui_ImplWin32_EnableDpiAwareness() attempting to get SetProcessDpiAwareness from shcore.dll on Windows 8 whereas it is only supported on Windows 8.1.
 //  2021-01-25: Inputs: Dynamically loading XInput DLL.
 //  2020-12-04: Misc: Fixed setting of io.DisplaySize to invalid/uninitialized data when after hwnd has been closed.
@@ -457,7 +462,7 @@ void ImGui_ImplWin32_EnableDpiAwareness()
 }
 
 #if defined(_MSC_VER) && !defined(NOGDI)
-#pragma comment(lib, "gdi32")   // Link with gdi32.lib for GetDeviceCaps()
+#pragma comment(lib, "gdi32")   // Link with gdi32.lib for GetDeviceCaps(). MinGW will require linking with '-lgdi32'
 #endif
 
 float ImGui_ImplWin32_GetDpiScaleForMonitor(void* monitor)
@@ -490,3 +495,43 @@ float ImGui_ImplWin32_GetDpiScaleForHwnd(void* hwnd)
 }
 
 //---------------------------------------------------------------------------------------------------------
+// Transparency related helpers (optional)
+//--------------------------------------------------------------------------------------------------------
+
+#if defined(_MSC_VER)
+#pragma comment(lib, "dwmapi")  // Link with dwmapi.lib. MinGW will require linking with '-ldwmapi'
+#endif
+
+// [experimental]
+// Borrowed from GLFW's function updateFramebufferTransparency() in src/win32_window.c
+// (the Dwm* functions are Vista era functions but we are borrowing logic from GLFW)
+void ImGui_ImplWin32_EnableAlphaCompositing(void* hwnd)
+{
+    if (!IsWindowsVistaOrGreater())
+        return;
+
+    BOOL composition;
+    if (FAILED(::DwmIsCompositionEnabled(&composition)) || !composition)
+        return;
+
+    BOOL opaque;
+    DWORD color;
+    if (IsWindows8OrGreater() || (SUCCEEDED(::DwmGetColorizationColor(&color, &opaque)) && !opaque))
+    {
+        HRGN region = ::CreateRectRgn(0, 0, -1, -1);
+        DWM_BLURBEHIND bb = {};
+        bb.dwFlags = DWM_BB_ENABLE | DWM_BB_BLURREGION;
+        bb.hRgnBlur = region;
+        bb.fEnable = TRUE;
+        ::DwmEnableBlurBehindWindow((HWND)hwnd, &bb);
+        ::DeleteObject(region);
+    }
+    else
+    {
+        DWM_BLURBEHIND bb = {};
+        bb.dwFlags = DWM_BB_ENABLE;
+        ::DwmEnableBlurBehindWindow((HWND)hwnd, &bb);
+    }
+}
+
+//---------------------------------------------------------------------------------------------------------

backends/imgui_impl_win32.h

@@ -18,10 +18,6 @@ IMGUI_IMPL_API bool     ImGui_ImplWin32_Init(void* hwnd);
 IMGUI_IMPL_API void     ImGui_ImplWin32_Shutdown();
 IMGUI_IMPL_API void     ImGui_ImplWin32_NewFrame();
 
-// Configuration
-// - Disable gamepad support
-//#define IMGUI_IMPL_WIN32_DISABLE_GAMEPAD
-
 // Win32 message handler your application need to call.
 // - Intentionally commented out in a '#if 0' block to avoid dragging dependencies on <windows.h> from this helper.
 // - You should COPY the line below into your .cpp code to forward declare the function and then you can call it.
@@ -38,3 +34,8 @@ extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hWnd, UINT msg
 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
+
+// Transparency related helpers (optional) [experimental]
+// - Use to enable alpha compositing transparency with the desktop.
+// - Use together with e.g. clearing your framebuffer with zero-alpha.
+IMGUI_IMPL_API void     ImGui_ImplWin32_EnableAlphaCompositing(void* hwnd);   // HWND hwnd

docs/CHANGELOG.txt

@@ -39,12 +39,15 @@ Breaking Changes:
 
 - Style: renamed rarely used style.CircleSegmentMaxError (old default = 1.60f)
   to style.CircleTessellationMaxError (new default = 0.30f) as its meaning changed. (#3808) [@thedmd]
+- Backends: Win32: Pragma linking with dwmapi.lib (Vista-era, ~9 kb). MinGW users will need to link with -ldwmapi.
 
 Other Changes:
 
 - ImDrawList: AddCircle, AddCircleFilled(): Tweaked default segment count calculation to honor MaxError
   with more accuracy. Made default segment count always even for better looking result. (#3808) [@thedmd]
 - ImDrawList: AddCircle, AddCircleFilled(): New default for style.
+- Backends: Win32: Added ImGui_ImplWin32_EnableAlphaCompositing() to facilitate experimenting with 
+  alpha compositing and transparent windows. (#2766, #3447 etc.).
 - Backends: OpenGL, Vulkan, DX9, DX10, DX11, DX12, Metal, WebGPU, Allegro: Rework blending equation to
   preserve alpha in output buffer (using SrcBlendAlpha = ONE, DstBlendAlpha = ONE_MINUS_SRC_ALPHA consistently
   accross all backends), facilitating compositing of the output buffer with another buffer.