WPF custom drawing and transparency - how to optimize?

Question

I have a WPF application with a custom windows style (AllowsTransparency="True" WindowStyle="None"). This window has a custom Peak Meter control which uses WriteableBitmap to draw data from DirectSound audio input.

I am using a bitmap with all levels drawn as colored rectangles (red/yellow/green/red) and when the audio arrives I just draw a black rectangle over my level meter form the top. It works fine on my machine (Windows 7, Pentium 4 single core). But it works bad on a laptop with Windows XP SP3 and integrated video.

I know that transparency effects have some issues on DirectX 9, but I have read the problem should be fixed in SP3. Still the call to _writeableBitmap.AddDirtyRect takes 30-40% CPU on XP and sometimes it causes dropouts in audio (if USB audio is used, and those dropouts occur not in my application but somewhere in DirectSound<->drivers subsystem).

On Windows 7 the same app takes no more than 3% CPU and no audio dropouts noticed (but the CPU is actually weaker on the Win7 PC than on the laptop with XP).

I tried not to use AddDirtyRect but just draw a WPF Rectangle element over the Image with leds and set the height of the rectangle when new audio level arrives. What a surprise! Somehow changing the height of a Rectangle element takes noticeably less resources than calling _writeableBitmap.AddDirtyRect for 100x20 pixel rectangle! Now on XP it took just 10-20% instead of 30-40% with AddDirtyRect.

But when I removed transparency effect from the window, finally also XP (and even on VirtualBox) went down to 2-6% CPU. Obviously transparency makes it really hard to redraw 20x100 rectangle 10 times per second.

I could live with no transparency in my application, but the problem is - the design uses rounded corner windows and I need to cutoff the area around them. But as soon as I set AllowsTransparency="False", the window shows the background color behind.

So the main question is - how do I make the contents of the window to clip away the background of the window so the corners stay round without using XP-heavy transparency?

I remember that I could do than even on a C++ Windows application just by setting a custom window region and not using any transparency (that was on a Windows 98 machine). Can the same thing be done on WPF?

Or maybe there is some trick how to make WriteableBitmap to use less resources on XP?

JustAMartin JustAMartin · Accepted Answer · 2011-03-06T09:57:07

As I did not find anything better, I used the old way: SetWindowRgn API.

It was a bit complicated because my application uses some animated slide-out parts which protrude out of the main window so I had to sync WPF animation with SetWindowRgn calls to make slide-out effect look as clean as possible. The result is not ideal but acceptable. And the main thing - no more high CPU and no audio dropouts.

WPF custom drawing and transparency - how to optimize?

1 Answers