Windows Desktop Transparency — Reference Demos

Minimal, self-contained Win32 programs that demonstrate how window transparency works on Windows 10/11 under DWM (Desktop Window Manager) composition.

These demos focus on the window setup side of transparency. With a properly configured window, APIs like DirectComposition or supported rendering APIs like Vulkan can present translucent content into it, with per-pixel alpha blending against underlying windows or the desktop.

Keywords

Windows transparent window, Win32 per-pixel alpha, DWM blur behind, DwmEnableBlurBehindWindow, WS_EX_LAYERED, WS_EX_NOREDIRECTIONBITMAP, layered window, alpha compositing, Vulkan transparent swapchain, VK_COMPOSITE_ALPHA_PRE_MULTIPLIED_BIT_KHR, pre-multiplied alpha, GLFW GLFW_TRANSPARENT_FRAMEBUFFER bug, DirectComposition alternative, translucent window C++, redirection surface, DWM composition, Windows 10, Windows 11, Mica, Acrylic, DWMWA_SYSTEMBACKDROP_TYPE.

Background

On modern Windows, every window has a DWM redirection surface — a backing bitmap that DWM composites onto the desktop. Transparency against content underneath the window depends on getting the alpha channel of this surface (and anything layered on top) right.

There are several mechanisms that interact:

Mechanism	What it does
hbrBackground brush	Paints the client area on WM_ERASEBKGND. A BLACK_BRUSH writes zero alpha, making the redirection surface fully transparent.
DwmEnableBlurBehindWindow	Tells DWM to use the alpha channel of the redirection surface for per-pixel compositing. Without this, DWM treats the window as opaque regardless of alpha values.
WS_EX_LAYERED	Legacy whole-window alpha (uniform opacity). Simple but no per-pixel control.
WS_EX_NOREDIRECTIONBITMAP	Removes the redirection surface entirely. The window has no GDI backing. Caution: this breaks any rendering path that blits into the redirection surface (e.g., GDI, some Vulkan present paths).
DWMWA_SYSTEMBACKDROP_TYPE	Win11-only API for system backdrop effects (Mica, Acrylic).

The recommended setup for Vulkan/D3D transparency

1. Use BLACK_BRUSH as the window class background brush (zeroes alpha in the redirection surface).
2. Call DwmEnableBlurBehindWindow with DWM_BB_ENABLE | DWM_BB_BLURREGION and a full-window region (CreateRectRgn(0, 0, -1, -1)).
3. Create your swapchain with pre-multiplied alpha compositing (e.g., VK_COMPOSITE_ALPHA_PRE_MULTIPLIED_BIT_KHR for Vulkan).

Note that graphics driver implementations may choose to activate DwmEnableBlurBehindWindow automatically for a window if a non-opaque composite mode is requested -- I still recommend calling it anyway since you should not count on that.

Do not use WS_EX_NOREDIRECTIONBITMAP

The Vulkan specification does not define the mechanism by which swapchain images are presented to the desktop compositor. Some implementations may rely on the window's redirection surface for presentation. Removing it with WS_EX_NOREDIRECTIONBITMAP can cause swapchain content to not appear at all, resulting in a fully transparent window. The recommended setup above (BLACK_BRUSH + DwmEnableBlurBehindWindow) achieves proper transparency without interfering with any presentation path.

GLFW note

GLFW's GLFW_TRANSPARENT_FRAMEBUFFER hint calls DwmEnableBlurBehindWindow internally, which is correct. However, as of GLFW 3.4, it does not properly clear the redirection surface to zero alpha — the surface may start with undefined alpha values, causing incorrect compositing.

This problem is demonstrated here and a fix was submitted as PR #2815. Until merged, consider avoiding GLFW for window creation in scenarios where you need per-pixel translucency. I recommend using the reference code in 05_perpixel_alpha.cpp instead.

Test programs

Each program demonstrates a specific aspect of window transparency, building progressively from a basic opaque window to per-pixel alpha compositing with Vulkan.

Program	Description
01_basic_opaque	Basic opaque window with red background. Baseline reference.
02_popup_no_decorations	Same but with WS_POPUP style (no title bar / borders).
03_win11_backdrop	Win11 system backdrop API (DWMWA_SYSTEMBACKDROP_TYPE).
04_layered_uniform_alpha	Whole-window alpha via WS_EX_LAYERED + SetLayeredWindowAttributes. Uniform opacity applied to the entire window including decorations. Simple, but no per-pixel control — not sufficient for Vulkan/D3D compositeAlpha transparency.
05_perpixel_alpha	Per-pixel translucent client area. Uses BLACK_BRUSH + DwmEnableBlurBehindWindow. This is the recommended approach — it enables per-pixel alpha compositing in DWM and ensures the redirection surface is fully transparent. Required foundation for Vulkan VK_COMPOSITE_ALPHA_PRE_MULTIPLIED_BIT_KHR.
06_perpixel_alpha_patblt	Variant of 05: uses NULL background brush but manually clears to zero alpha in WM_PAINT via PatBlt(BLACKNESS). Equivalent result, more control over when the clear happens.
07_vulkan_perpixel_alpha	05 + Vulkan swapchain. Builds on 05's window setup and adds a minimal Vulkan swapchain with VK_COMPOSITE_ALPHA_PRE_MULTIPLIED_BIT_KHR, clearing to 50% transparent black. Both prerequisites are visible: per-pixel compositing is enabled (via DwmEnableBlurBehindWindow), and the redirection surface is transparent (via BLACK_BRUSH). The complete, minimal reference for Vulkan transparency on Windows.
08_glfw_transparency_bug	GLFW window with GLFW_TRANSPARENT_FRAMEBUFFER. Demonstrates the GLFW transparency bug: GLFW enables per-pixel compositing via DwmEnableBlurBehindWindow but does not clear the redirection surface to zero alpha, so undefined alpha values cause incorrect compositing.

Building

Requires CMake 3.20+ and a C++17 compiler. GLFW 3.4 is fetched automatically. The Vulkan test requires the Vulkan SDK to be installed.

cmake -S . -B build
cmake --build build

All programs exit on Escape.