![]() ![]() You can try to get the child window to paint into a system-memory bitmap, and then blit the bitmap into WPF. Child WindowsĪgain, because layered windows that use the application-provided content mode do not redirect GDI calls, child windows do not automatically get rendered into the bitmap that the application is going to use to represent its content. This is fine for many scenarios where the intent is to create a custom window shape, but it can be annoying for people who just want to “fade in” a normal window. No matter what your window styles may suggest, transparent WPF windows do not have any visible non-client area. WPF chose to avoid all of this complexity by expanding the client area to fill the entire window by handling the WM_NCCALCSIZE message. And because the bitmap WPF is going to provide will represent the entire window (including the non-client area), WPF would have to carefully handle the non-client messages. This distortion cannot simply be encoded into a bitmap anyway. Further, on Vista, the windowing system uses a complex shader to distort the background behind the non-client area. Because layered windows that use the application-provided content mode do not redirect GDI calls, the windowing system is not able to render into the bitmap that the application is using to represent its content. These parts are drawn by the windowing system to the DC of the window. This includes the title bar, the resize edges, the menu bar, the scroll bars, etc. The “non-client” area of a window is a general reference to the parts of the window that the windowing system normally renders for the application. This choice has a few important ramifications: Non-Client Area Per-pixel transparency naturally allows WPF’s anti-aliased rendering to work on the layered window too, which makes the edges look much nicer. WPF deeply supports per-pixel transparency in its rendering, so it was a natural fit. This was chosen primarily because it enabled the broadest range of features, and we didn’t want to confuse our API with a mix of modes and features. WPF chose to only support the Application-Provided Content mode of layered windows. But because the OS does not redirect any GDI painting calls, this mode does not support compatibility with existing code. This API allows the application to provide a bitmap that has a per-pixel alpha channel. The application is completely responsible for generating its content. If the application has fresh content, it is responsible for calling UpdateLayeredWindow again. The window does not receive WM_PAINT messages, since there is no need to ask the application to ever repaint itself. The bitmap for the window is allocated by the application, and is passed to the OS which makes a copy. This mode is determined by using the UpdateLayeredWindow API. This caused lots of visual artifacts on the screen. This caused DX to render directly to the screen, while the OS assumed the window content was in a bitmap. Note: on XP there was a bug where the redirection was not respected by DirectX. ![]() Instead, the SetLayeredWindowAttributes API lets you specify a color key and an opacity for the entire window. However, because GDI does not preserve the alpha channel in all of its APIs, this mode does not support per-pixel transparency. This mode provides an excellent level of compatibility with existing code. The window receives WM_PAINT messages like normal, though the frequency of these messages is reduced since the OS does not need to have the application repaint the window as it moves around. The bitmap for the window is allocated by the OS, and every attempt to render into a DC for the window is redirected to render into the bitmap. This mode is determined by using the SetLayeredWindowAttributes API. This is not a technique that can be used for child window transparency. Windows only supports the layered feature for top-level windows. ![]() The mode is determined by which API you use to update the window. In either case, the HWND is given the WS_EX_LAYERED extended style. There are two layered windows modes currently supported by the OS: System-Redirected Content and Application-Provided Content. Layered windows are represented by a bitmap, and the OS renders the bitmap whenever it needs to. Windows supports transparency at the HWND level through a feature called layered windows. This feature comes with a few issues, which I’ll discuss in this post. WPF can obviously render transparent elements within its own window, but it also supports rendering the entire window with per-pixel transparency. ![]()
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