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Fast drawing lots of rectangles one at a time in WPF

Asked 2010-12-15T07:49:07.087
13

My application is fed data from an external device. After each data point, there is a short electronic dead time (of about 10µs) in which no other data point can arrive, which my application should use to process and display the data on screen in a scatter plot. My most important goal is to not exceed this electronic dead time. How would one approach this problem in a WPF based application, and what would be a way to benchmark different methods?

Things I've tried are:

  • Creating a Rectangle in a Canvas for every arriving data point. This is too slow by a factor of 10.
  • The same approach, but drawing DrawingVisuals in a custom control. Better, but still a little too slow. Adding visual/logical children to the tree may have too much overhead.
  • A UserControl where all data points are stored in an array and displayed in the OnRender method. Here I have to draw every point again on each call to OnRender. This method therefore slows down over time, which is undesireable. Is there a way to tell OnRender not to clear the screen on each pass, so that I could paint incrementally?
  • Displaying each point as a pixel in a WriteableBitmap. This seems to work, but I 've not found a way to determine, if invalidating part of the Bitmap does not add a few very long wait times ocassionally (when the Image is actually refreshed on screen). Any Ideas for measuring this?

Edit:

In the comments, the point of buffering data and displaying it at a slower rate has been raised. The problem with that approach is, that at some point I have to process the buffer. Doing that during the measurement introduces a long time during which my system is busy and new events would be discarded. Therefore dealing with every point individually, but for good, would be more desireable. Using 10 µs to trigger the display for every

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1 Answer

11

Using a WriteableBitmap will be the fastest approach. For testing you could pre-allocate an array and use a Stopwatch to sample timings as you go about rendering, you can then analyse the timings to get some idea of performance.

One overriding issue you have is with garbage collection. This will unfortunately introduce potential for the exact kind of performance issues you describe i.e. occasional stalling whilst GC is carried out. You could experiment with low latency GC to mitigate this.

Update

Here is an example of using low latency GC:

http://blogs.microsoft.co.il/blogs/sasha/archive/2008/08/10/low-latency-gc-in-net-3-5.aspx

You could leverage this to ensure that there are no garbage collections during your "dead time" i.e. rendering time.

Update 2

As I mentioned in my comment a while ago - are you batching updates to your WritableBitmap?

Your device update frequency is too high to able to sustain writing to the bitmap for each device update - I think there are 10k-100k updates per second. Try and update your bitmap on a more sensible frequency (e.g. 60 or 25 times per second), as the overhead of forcing a bitmap render will dominate performance at 10k-100k updates per second. Write to a buffer when you receive device updates, then periodically transfer this buffer to the WritableBitmap. You could use a timer for this, or do it every n device updates. In this way you will batch your updates and vastly reduce WritableBitmap render overhead.

Update 3

Ok, it sounds like you are updating the WritableBitmap 10k-100k times per second - this isn't feasible. Please try a frame\batch based mechanism as de

answered 2010-12-15T08:06:35.223

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