Capture visual output of a DirectX application - even in background?

Question

I need to capture the visual output (like a screenshot) of a DirectX window. Currently, I use this approach.
But, when the window is in background, it captures whatever is in front of it.

I see that DirectX windows render even when minimized or in background, so this should be possible.
But, how? (It also needs to be fast, and it needs to work on Windows XP too, unfortunately...)

Edit: I am very busy these days... Don't worry, I'll put the bounty back if it expires.

Answer 1

To capture Direct3D windows that are in the background (or moved off screen), I believe you have the following options:

1. Inject and hook Direct3D within the target application via the link you have already posted or this more up-to-date example (EasyHook can be difficult to get setup but it does work really well) - you can always ask for help about getting it working. I have used that technique for capturing in a number of games without issues (most recently for an ambilight-clone project). The problem with this approach is your concern about game protection causing bans, however FRAPs also uses hooking to achieve this, so perhaps your concerns are exaggerated? I guess gamers being banned for a screen shot is an expensive way of finding out.

2. For windowed applications on Vista/Win 7 - you could inject and hook the DWM and make your capture requests through its shared surface. I have had this working on Vista, but have not finished getting it working on Windows 7, here is an example of it working for Windows 7 http://www.youtube.com/watch?v=G75WKeXqXkc. The main problem with this approach is the use of undocumented API's which could mean your application breaks without any warning upon a windows patch release - also you would have to redo the technique for each new major Windows flavour. This also does not address your need to capture in Windows XP.

3. Also within the DWM, there is a thumbnail API. This has limitations depending on what your trying to do. There is some information on this API along with other DWM API's here http://blogs.msdn.com/b/greg_schechter/archive/2006/09/14/753605.aspx

4. There are other techniques for intercepting the Direct3D calls without using EasyHook, such as substituting the various DLL's with wrappers. You will find various other game hooking/interception techniques here: http://www.gamedeception.net/

5. Simply bring the Direct3D application to the foreground (which I guess is undesirable in your situation) - this wouldn't work for off-screen windows unless you also move the window.

6. Unfortunately the only solution for Windows XP that I can think of is intercepting the Direct3D API in some form.

Just a clarification on Direct3D rendering while minimised. During my fairly limited testing on this matter I have found this to be application dependant; it is generally not recommended that rendering take place while the application is minimized (also this reference), it does continue to render while in the background however.

UPDATED: provided additional link to more up-to-date injection example for point 1.

Answer 2

A quick google and i found this Code Project which relates to Windows XP. I dont know if you can apply this knowledge to Windows Vista and 7??

http://www.codeproject.com/Articles/5051/Various-methods-for-capturing-the-screen

EDIT:

I found this article as well:

http://www.codeproject.com/Articles/20651/Capturing-Minimized-Window-A-Kid-s-Trick

This links off from Justins blog post here from the comments. It seems he was working on this with someone (i see thats your link about).

http://spazzarama.com/2009/02/07/screencapture-with-direct3d/

Answer 3

The code that you linked to (from spazzarama), which you said you were using in your project, captures the front buffer of your DirectX device. Have you tried capturing the back buffer instead? Going from the code on your linked site, you would change line 90 from

device.GetFrontBufferData(0, surface);

to

Surface backbuffer = device.GetBackBuffer(0, 0, BackBufferType.Mono);
SurfaceLoader.Save("Screenshot.bmp", ImageFileFormat.Bmp, backbuffer);

This would also involve removing lines 96-98 in your linked example. The backbuffer might be generated without the obstructing window.

EDIT

Nevermind all of that. I just realized that your linked sample code is using the window handle to define a region of the screen, and not actually doing anything with the DirectX window. Your sample code won't work around the obstruction because your region is already drawn with the other window in front of it by the time you access it.

Your best bet to salvage the application is probably to bring the DirectX window to the top of the screen before running the code to capture the image. You can use the Wind32API BringWindowToTop function to do that (http://msdn.microsoft.com/en-us/library/ms632673%28VS.85%29.aspx).