Troubleshoot why PrintWindow is blank

Question

I am trying to capture screenshot of inactive window with PrintWindow. It works correctly for calculator and for capturing Google Chrome, but for some other applications, li

Answer 1

The documentation for PrintWindow provides information on its implementation:

The application that owns the window referenced by hWnd processes the PrintWindow call and renders the image in the device context that is referenced by hdcBlt. The application receives a WM_PRINT message or, if the PW_PRINTCLIENT flag is specified, a WM_PRINTCLIENT message. For more information, see WM_PRINT and WM_PRINTCLIENT.

Whether or not PrintWindow returns a window's content is subject to the window procedure handling the WM_PRINT or WM_PRINTCLIENT message^[1] appropriately. If a window doesn't handle either of those messages, it will not render anything into the provided device context.

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^[1]Standard window implementations provide a message handler for WM_PRINT/WM_PRINTCLIENT through DefWindowProc. Custom window class implementations need to provide their own.

Answer 2

Every window has a default implementation for WM_PRINT, you will use it if you call PrintWindow() and don't use the PW_CLIENTONLY flag. Provided by the default window procedure, DefWindowProc(). It is pretty straight-forward, it creates a memory DC and sends WM_PAINT. So what you get in the bitmap is the same you get on the screen. Everybody happy.

But that only works for windows that actually use WM_PAINT to render their content. The chips are down when it doesn't, most any game actually renders with DirectX and does so at a high rate, not relying on WM_PAINT messages. A program that uses a layered window with alpha transparency doesn't either, you can typically recognize them from a fancy blended border.

What such a program should do is write their own message handler for WM_PRINT/CLIENT and render their surface into the device context. Necessary because the default implementation doesn't know beans about DirectX surfaces.

Games just don't do this, that's extra code they have to write that just about nobody ever actually uses. You'll inevitably end up with an empty bitmap. Nothing you can do about it of course.