Windows regions and transparency

Question

I have a CDHTMLDialog in a BHO that I want to be partially transparent, in the sense that the transparent area changes according to the logic of the dialog. I got it to become transparent visually (using SetLayeredWindowAttributes), but it is critical to make this region truly transparent, because otherwise when I click on the transparent region my clicks do not reach the IE window which is below the transparent part of my dialog. I temporarily fix this by constantly resizing my dialog according to the size of the active part of the dialog, but I can't keep up with this forever...

I think the solution has something to do with what windows calls "regions" (http://msdn.microsoft.com/en-us/library/dd162915%28VS.85%29.aspx) but I'm not exactly sure how to work with them. Can anyone point me in the right direction?

Answer 1

I found the way to make an entire window transparent and click-through here:

http://www.codeproject.com/KB/wtl/transparent.aspx

But it's not useful for my case where I only want the transparent part of my window (transparent by HTML/CSS definitions) to be click-through...

Update: Apparently, the clicks are supposed to go through the transparent parts (see http://jalaj.net/2007/02/05/form-with-a-hole/), but in my CDHTMLDialog they don't. My best guess is that a sub-window of the BHO catches my clicks, but I don't really think that makes much sense...

Answer 2

I don't think you want to make parts of your window transparent, what you want to do is (I think) set the window region (like you mention). Read the MSDN on SetWindowRgn() - basically you define a GDI object of type HRGN (if you're using MFC, CRgn) which described a surface of a certain shape, and eventually with parts cut out. Windows then considers only the 'region' that you set on a window as the part of the window to use. Basically it's how you make non-rectangular windows. A 'region' isn't a 'transparent' part of a window, it's a way to discard areas of a window, in a way.