winapi - 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, like games, it saves white area.

What could be the reasons for PrintWindow to fail and how to validate them?

EDIT: I want tool to report why window can not be captured

Answer 1

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.