WPF: Hit testing is slow.

Question

1.00/5 (2 votes)

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I create a game in WPF.
My goal is to create ellipses at random free positions of a canvas.
I need to exclude positions overlaid by created ellipses and create new ellipses on free random positions.
I tried VisualTreeHelper.HitTest and Geometry FillContains, but they are slow.

How can i do it in WPF?

Code (fixed timer):

C#

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        this.Loaded += MainWindow_Loaded;
    }

    public Canvas canvas;
    Random random = new Random();
    int add = 5;

    Timer timerEllipses;

    void MainWindow_Loaded(object sender, RoutedEventArgs e)
    {
        canvas = new Canvas();
        canvas.Background = Brushes.Black;
        this.Content = canvas;

        timerEllipses = new Timer();
        timerEllipses.Interval = 1;
        timerEllipses.Start();

        Timer timer = new Timer();
        timer.Interval = 2000;
        timer.Elapsed += timer_Elapsed;
        timer.Start();
    }

    void timer_Elapsed(object sender, ElapsedEventArgs e)
    {
        this.Dispatcher.Invoke((Action)(() =>
        {
            int add = this.add;

            int x = random.Next((int)canvas.ActualWidth);
            int y = random.Next((int)canvas.ActualHeight);

            for (int i1 = 0; i1 < canvas.ActualWidth; i1++)
                for (int i2 = 0; i2 < canvas.ActualHeight; i2++)
                    VisualTreeHelper.HitTest(canvas, new Point(i1, i2)); // Slows down

            Ellipse ellipse = new Ellipse();
            ellipse.Width = 1;
            ellipse.Height = 1;
            ellipse.Stroke = Brushes.White;
            canvas.Children.Add(ellipse);

            timerEllipses.Elapsed += delegate
            {
                this.Dispatcher.Invoke((Action)(() =>
                {
                    ellipse.Width += add;
                    ellipse.Height += add;

                    ellipse.Margin = new Thickness(x - (ellipse.Width / 2), y - (ellipse.Height / 2), 0, 0);

                }));
            };
        }));
    }
}

You can see how a separate thread doesn't help in my code when i hit test the canvas to get free points.
Code (separate thread):

C#

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        this.Loaded += MainWindow_Loaded;
    }

    public Canvas canvas;
    Random random = new Random();

    Ellipse ellipse;
    int add = 5;
    double x = 0;
    double y = 0;

    Stopwatch stopwatch = new Stopwatch();
    public double elapsed = 10;

    void MainWindow_Loaded(object sender, RoutedEventArgs e)
    {
        canvas = new Canvas();
        canvas.Background = Brushes.Black;
        this.Content = canvas;

        ellipse = new Ellipse();
        ellipse.Width = 1;
        ellipse.Height = 1;
        ellipse.Stroke = Brushes.White;
        canvas.Children.Add(ellipse);

        new System.Threading.Thread(Thread1).Start();
    }

    void Thread1()
    {
        stopwatch.Start();

        while (true)
        {
            if (stopwatch.Elapsed.TotalMilliseconds >= elapsed)
            {
                stopwatch.Stop();

                this.Dispatcher.Invoke((Action)(() =>
                {
                    for (int i1 = 0; i1 < canvas.ActualWidth; i1++)
                        for (int i2 = 0; i2 < canvas.ActualHeight; i2++)
                            VisualTreeHelper.HitTest(canvas, new Point(i1, i2)); // Slows down

                    ellipse.Width += add;
                    ellipse.Height += add;

                    ellipse.Margin = new Thickness(x - (ellipse.Width / 2), y - (ellipse.Height / 2), 0, 0);
                }));

                stopwatch.Restart();
            }
        }
    }
}

Posted 21-Aug-15 3:07am

Ziya1995

Updated 23-Aug-15 21:07pm

v8

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Comments

[no name] 21-Aug-15 10:41am

Only for ellipses? Than it should be easy. But you have to dive a "little bit" into math.

Sergey Alexandrovich Kryukov 21-Aug-15 10:50am

Why is it slow: many thousands ellipses? If not, I never spotted WPF was slow. Perhaps we would need to see how you made is slow...
Filling in Geometry can be slow, but why not coloring each ellipse individually?
—SA

Ziya1995 21-Aug-15 12:16pm

> Why is it slow: many thousands ellipses?
No, it gets slow after a few ellipses.

> Perhaps we would need to see how you made is slow...
I added the code.

Sergey Alexandrovich Kryukov 21-Aug-15 15:11pm

Then something is fundamentally wrong. Few ellipses is nothing at all, should be blazing fast. Again, using a timer is a bad idea, using a separate thread is much more reliable.
—SA

Ziya1995 22-Aug-15 3:51am

> Then something is fundamentally wrong. Few ellipses is nothing at all, should be blazing fast.
You don't understand, you even don't know what causes the problem, but i repeat:
"WPF: Hit testing is slow." - the topic name shows what is a cause of slow performance.
The code has a certain line commented "// Slows down" you can see as a cause of slow performance.

Solution 2 is no solution at all having nothing related to the problem.
Do you have a solution or not?

Sergey Alexandrovich Kryukov 22-Aug-15 5:43am

I explained the solution. Did you notice it?
—SA

Ziya1995 22-Aug-15 6:55am

> Did you notice it?
I noticed the solution and in my view it has nothing related to the problem.
No matter how i create Timer once or every time, the problem is caused by VisualTreeHelper.HitTest.

I said what causes the problem, but you say it is Timer.
If you don't see how i fixed Timer creating, you can fix it yourself and post the code, are you agree?

If you can't solve the problem, just let me know.
But if you have some potentiality, let's continue.

Sergey Alexandrovich Kryukov 22-Aug-15 21:49pm

No. If you don't want to get rid of timer, the whole "problem" hardly worth bothering. And the problem is really simple, WPF in such operation is blazing fast, any complains about performance of adding just few ellipses and hit test on them is just ridiculous. Do it in a simple way, and it will work.
—SA

Ziya1995 23-Aug-15 1:50am

> Do it in a simple way, and it will work.
I can't.

> And the problem is really simple
If it is easy, can you you post the code?

Sergey Alexandrovich Kryukov 23-Aug-15 11:48am

Because I don't want to spend more time on it. Remember, I'm not working for you. Every time I'm trying to help you, I regret about it. If you showed decent effort and gave me some hope, it would be a diffent story.
—SA

Ziya1995 23-Aug-15 12:30pm

Ok, you are right, i am wrong.
You have a solution, but don't want to post it, in this case don't help me, if you don't want - it is still up to you.

But you can still answer my questions, if you can and want it at the same time.

> If you showed decent effort and gave me some hope, it would be a diffent story.
Feels like i have a chance, if you want, you can let me know what i can do to show decent effort and you will post the solution you have.

> If you don't want to get rid of timer, the whole "problem" hardly worth bothering.
I don't want, thread loop increases CPU and i feel it can't solve the problem.

> WPF in such operation is blazing fast
I checked FillContains and Visual Hit Test and they are not fast.
FillContains was checked in a different parallel thread, it was better, but not enough.

Ziya1995 24-Aug-15 2:52am

I added a code where i do like you said, i use a separate thread to animate one ellipse, but VisualTreeHelper.HitTest slows down the app.

Alexander Dymshyts 24-Aug-15 4:27am

Dispatcher:Invoke runs on a main thread! Look for a async / await or Tasks in C#

Ziya1995 24-Aug-15 4:37am

You mean i should run UI in a parallel thread?
It is impossible in C# to run UI in a parallel thread.

If you are not agree, you can show your code where you run UI outside a main thread.

3 solutions

Solution 1

Possibly this Code Project article might interest you: "Solutions for WPF Performance Issue"[^].

Posted 21-Aug-15 6:00am

CPallini

Comments

Sergey Alexandrovich Kryukov 21-Aug-15 16:36pm

Carlo,

What performance are you talking about so seriously? Just take a closer look at the "code". This is so much of gibberish that it's hard to imagine that it can work at all. See also Solution 2.

—SA

CPallini 21-Aug-15 16:42pm

There was no code, if I remember well.

Sergey Alexandrovich Kryukov 21-Aug-15 18:24pm

I see; this is v.3 now...
—SA

Solution 3

do some things:
1. don't subscribe on Timer.Elapsed events many times. Throw away subscription and second Dispatcher.BeginInvoke in event handler;
2. don't test every screen point as you do now. I think, you only need to hit test single random point. And if this place is not empty, the second one. Not whole canvas.
Yes, hittesting is expensive, especially if you do it every time for every pixel
3. try to use DispatcherTimer instead of Timer. In this case you won't need BeginInvoke in event handler at all

Posted 25-Aug-15 13:28pm

Irina Pykhova

Updated 25-Aug-15 13:31pm

v2

Comments

Ziya1995 26-Aug-15 5:13am

> don't test every screen point as you do now.
But i need to do so.

> Yes, hittesting is expensive, especially if you do it every time for every pixel
I consider it as impossible to do in WPF.

Irina Pykhova 26-Aug-15 7:22am

no need at all. I can imagine several ways to find empty place without that. For example, look at this: http://stackoverflow.com/questions/10352555/how-can-i-find-uielements-in-a-rectangle-in-wpf
The idea is to suggest some place for new ellipse (your approach with random points) and check whether it is intersecting with existing ellipses. You can start from checking rectangle bounds, it would be not very accurate for ellipses, but will give you the starting point for further improvements. The number of ellipses should be much smaller than number of pixels in canvas, so it will be more effective.
In my opinion, your code from the first post will work bad on any platform. It is not related to WPF

Add a Solution

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Sergey Alexandrovich Kryukov · Accepted Answer · 2015-08-21T09:18:00

Solution 2

Ahhh! I see. How can I ansee it? You are doing awful thing in principle: under Invoke, on each timer event, you create a new timer, set it up to fire other events, and other crazy thing. This is insane. Don't do anything like that. The problem is amazingly simple, even with a timer. I don't even want to discuss it. Just throw out it all and write something reasonable.

For example, thread solution will be the simplest. Just create a thread with a loop and some Thread.Sleep. Create this thread only once and use it during all the runtime. In each loop, do all the calculation and do under Invoke only one thing: pure UI manipulation: creation of ellipse and adding it to canvas (removal from canvas, anything like that). This is not simple, just extremely simple.

—SA

Posted 21-Aug-15 9:18am

Sergey Alexandrovich Kryukov

Comments

CPallini 21-Aug-15 16:50pm

5.I agree, the posted code is wrong. However, it looks many people is disappointed by WPF performance.

Sergey Alexandrovich Kryukov 21-Aug-15 18:25pm

Thank you Carlo. Maybe I've done not too much of WPF development, never face performance problems.
—SA

Ziya1995 22-Aug-15 2:53am

My code shows the line which slows down the app and it is noted by the comment: "// Slows down". If you remove this line, you can see no problem, but with the line you get slow performance.

> on each timer event, you create a new timer
You said the problem is Timer, i fixed it, but no difference, it works same well until i add the line VisualTreeHelper.HitTest.

> thread solution will be the simplest
Thread is no option, it increases CPU usage.

Do you have solution or not?

Sergey Alexandrovich Kryukov 22-Aug-15 5:44am

How did you "fix it"?
—SA

Ziya1995 22-Aug-15 6:53am

I updated the code where i create Timer once.
Is it ok?

Sergey Alexandrovich Kryukov 22-Aug-15 21:46pm

Everything with timer is not quite OK, but let's see... You have only one timer and created it only once. Do you still have the problem?
—SA

Ziya1995 23-Aug-15 1:52am

> Do you still have the problem?
Yes, i do.
Are you gonna post the right version of my code, where it will work fast as you say?

Ziya1995 24-Aug-15 2:35am

> Thread.Sleep
It is much more inaccurate than Timers.Timer.

> do under Invoke only one thing: pure UI manipulation
Visual Hit Test is UI manipulation and it is slow.

WPF: Hit testing is slow.

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WPF: Hit testing is slow.

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