Partial transparency with C# .NET 3.5 WinForms?

I think I would call this expected behavior, actually. What I would do is render my background to an in-memory bitmap and, in the paint event, copy that to the form (basic double-buffering).

I think I would call this expected behavior, actually. What I would do is render my background to an in-memory bitmap and, in the paint event, copy that to the form (basic double-buffering). If I'm way off base, could you post a screenshot?

I don't know that I'm imagining what you're describing correctly. EDIT: I'm wondering about your use of OnPaintBackground... pre-. NET, if you were doing double-buffering you'd catch and ignore the WM_ERASKBKGND message (to prevent flicker), render your image to an offscreen buffer, and copy from the buffer to the screen on WM_PAINT.

So, try changing from the OnPaintBackground to OnPaint. I haven't done too much of this kind of thing in . NET, but I had a pretty good handle on it before; I just don't know if it'll translate well or not!

EDIT 2: Marc, the more I think about what you're trying to do, the more problems appear. I was going to suggest creating a background thread dedicated to capturing the screen and rendering it darkened; however, in order to remove your own form you'd have to set the visibility to false which would create other problems.... If you're unwilling to give up, I would suggest creating two windows and "binding" them together. Create a semi-opaque window (by setting opacity) for your background window, and create a second "normal" window for the foreground.

Use SetWindowRgn on the foreground window to cut away the background and position them on top of each other. Good luck!

I gave it a try (created a new Bitmap, created a Graphics from the image, filled it the same way as above, and draw it to the screen using the paint event's Graphics) and it had the same issue. It's not a bad idea though, is there any other way of drawing the image to the screen I could try? – Marc Charbonneau May 17 '09 at 5:11 I've updated my answer with something else to try; if you have any code you can send, it's the same username at gmail and I'll see if I can be of any help.

– overslacked May 17 '09 at 5:59 Thanks, I'll give the double windows option a try and see if I can't make that work. – Marc Charbonneau May 18 '09 at 13:15 I ended up creating a second transparent background window and now I'm in business. Thanks for all the help!

– Marc Charbonneau May 20 '09 at 2:25.

Is Graphics. CompositingMode set to CompositingMode. SourceCopy?

That should cause painting the background twice to be equivalent to painting it once, since it will replace the existing alpha/color data instead of compositing over it.

Good suggestion, but when I set it to SourceCopy it kills any transparency and just draws an opaque black background. – Marc Charbonneau May 17 '09 at 1:09 1 I'm not certain there's any way to get the behavior you desire with pure WinForms. You could use P/Invoke to access the layered window APIs directly and set per-pixel alpha transparency for your window, but I'm not sure it's worth it.

Have you considered using TransparencyKey to make the non-UI parts of your form show through, and then putting a partially opaque window behind it to create the desired effect? – Kevin Gadd May 17 '09 at 7:48.

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