How to render to an "offscreen" CWnd

If I have a CWnd object that is either not visible or that is clipped on the screen by another window, if I try to get access to the CWnd's DC for the purpose of copying the CWnd's image data to a bitmap using StretchBlt(), the areas of the CWnd that are not actually drawn and visible on the screen appear in the result as either the other window or black.

Is there a way to, well, create an "offscreen window" - a full blown CWnd that has its own bitmap/screen data - such that if I draw stuff to it and then try to StretchBlt() from it, the data is intact, even if part or all of the window is not visible on the screen?

Any help would be greatly appreciated.
pcommonsAsked:
Who is Participating?
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

DanRollinsCommented:
Image data is generally not preserved.  If one window covers up part of another and is later moved, then the partly-covered window is asked to redraw itself.

You can trigger that process by sending a WM_PAINT message to the window.  If you supply a prepared DC to pass in (it must match what the window expects and the clipping region must be correect), theen the window will draw to the HBITMAP in the DC and you can do something with that.

Check out the documentation for the WM_PRINTCLIENT message... the basic purpose of that message is to make a window draw itself into a caller-supplied DC.

Note that some WM_PAINT handlers are "too smart"  They see that the window is minimized or partly off-screen and will do thier own clipping of the output.   In such cases, there is no real way to get a bitmap of the screen image without forcing the window to be at least temporarily fully visible).

-- Dan
0

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
AlexFMCommented:
If this CWnd object is yours, draw in in using memory bitmap and DC. When you need to get the image from this window, just get it from memory DC. This will work also when windiw is invisible.
0
pcommonsAuthor Commented:
The parent CWnd object is mine, but there are child CWnds that are "not" (e.g. 3rd party controls). Is there a way to tell the CWnd (and all its children) to render into a memory bitmap - so I can have the CWnd and all its children respond to the WM_PAINT messages - and they get rendered into the memory bitmap?
0
Cloud Class® Course: Microsoft Office 2010

This course will introduce you to the interfaces and features of Microsoft Office 2010 Word, Excel, PowerPoint, Outlook, and Access. You will learn about the features that are shared between all products in the Office suite, as well as the new features that are product specific.

DanRollinsCommented:
Try using WM_PRINTCLIENT.  It sometimes works.
0
pcommonsAuthor Commented:
Nope, some of the 3rd party controls I'm rendering don't support that. That's why I was trying to come up with a way like this.
0
vijay_visanaCommented:
0
pcommonsAuthor Commented:
Well, I'm not sure I got the answer I was hoping for, but I appreciate everyone's comments as they were very insightful into understanding the problem.
0
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
System Programming

From novice to tech pro — start learning today.

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.