Slogan / News Ticker / Scrolling Text in WPF


Oh, I used to write so called intros with scrolling text slogans at the bottom on the Commodore Amiga. The hardware was pretty much unalterable and you could easily work with the Blitter and Copper. The vertical blank beam on the monitor was my timer. These days are over. No more Assembler. And who cares about high and low byte these days?

I have to admit that using C# in combination with WPF is not the best approach for moving text around. You should directly talk to graphic cards for such. Anyway, it started with a short benchmark test and ended up with this example program. If you program properly, then you can make WPF do the job without any flickering.

You might wonder why my approach is quite indirect this time. I am not simply rendering text to the screen and then change its position. This program is slightly more complex. The reason is that I am planning a follow-up with more complex movements soon. I am planning to show you how to scroll text along the screen in a sinus curve while the curve itself behaves like a wave. Maybe I will add some topsy-turvy stuff as well. Hehe, it is not written yet.

This post is providing the basics to get prepared for the next step. Today, the text simply moves from the right to the left. And no worries about the end of the text. It is dealt with properly.
You can change the speed with the slider while the text field returns the calculation time in milliseconds. You can see that we still have some timing leeway. There are no issues with our frequency, which is 20 updates per second. The screen itself obviously updates more often. Its frequency has little to do with the one for showing new BitmapImages. Our thread loop takes less than 20 milliseconds.

How does the program work?

  • A loop is running on an independent thread. This loop executes each time right after our WPF update. This well chosen moment avoids timing conflicts efficiently. The next BitmapImage is ready to be shown before the next WPF GUI update.
  • At the beginning of the program we convert the entire text to vectors. The orientation of all vectors is vertical. Each x-value can consist of several vertical vectors. For instance the character “E” has 1 or 3 vertical vectors. The first vector is a straight vertical black line from the top to the bottom. When you are in the middle of the character, then you have three black and two white areas. We only convert the black parts. Imagine a vertical cut right through the middle from the top to the bottom.
  • The conversion from “character to bitmap to vector” is NOT conducted in real-time. This is done during the program initialization as mentioned above. Only then we can run at full steam afterwards.
  • We only change the precalculated vectors and render the bitmap in real-time – nothing else.
  • These vectors are then drawn to bitmaps. These bitmaps are not dependent on the Dispatcher thread. We are still running at maximum speed. Before we can change the source for the Image control, we have to convert the Bitmap class to an ImageSource.
  • This ImageSource is then assigned to the Image control during a DispatcherTimer event.

Once again the program was kept to a minimum length. Maybe I should have added some more classes. I hope I found the right length for this example program in the end.

This code was not straight forward. There had to be a learning curve … as usual. First, I tried some direct WPF binding. The timing was nearly out of control. So I gave up after a while. But I had some nice blog ideas in the meantime. We should enter the 3D world at some point, don’t you think so?
The 3D approach would most likely the best approach for our scrolling text as well. I expect the efficiency to be extreme. There are so many games with millions of mesh triangles rendered within milliseconds – this cannot be bad stuff. We could even use 3D features for showing 2D text.

This will surely require some profound research on my side. Don’t expect anything anytime soon. I am definitely not omniscient.

<Window x:Class="Ticker.MainWindow"
  <StackPanel x:Name="MyStackPanel"
    <DockPanel LastChildFill="True">
      <Slider Width="300"  DockPanel.Dock="Right" 
              Minimum="0.5" Maximum="8.0"
              ValueChanged="Slider_ValueChanged" />
      <TextBox Name="Info"  DockPanel.Dock="Right"/>
    <Image Name="MyImage" 
           Width="{Binding ActualWidth, ElementName=MyStackPanel}"/>
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Drawing;
using System.Threading;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media.Imaging;
using System.Windows.Threading;

namespace Ticker {

  public partial class MainWindow : Window {

    private Thread _Thread;
    private TextEngine _TextEngine = new TextEngine();
    private float _WidthInPixels;
    private double _Speed = 1.0; // number of pixels to shift per iteration
    private AutoResetEvent _AutoResetEvent = new AutoResetEvent(true);
    private BitmapImage _BitmapImage = null;
    private string _ElapsedTime = string.Empty;

    public MainWindow() {
    } //  constructor

    private void Window_Loaded(object sender, EventArgs e) {
      DataContext = this;

      _Thread = new Thread(Loop);
      _Thread.Name = "MainLoop";
      _Thread.IsBackground = true;
      _Thread.Priority = ThreadPriority.AboveNormal;

      TimeSpan lInterval = new TimeSpan(0, 0, 0, 0, 50); // run each 50 ms
      EventHandler lHandler = new EventHandler(OnTime);
      DispatcherTimer lDispatcherTimer = new DispatcherTimer(lInterval, DispatcherPriority.Send, lHandler, this.Dispatcher);
    } //

    private void MyImage_SizeChanged(object sender, SizeChangedEventArgs e) {
      StackPanel lStackPanel = sender as StackPanel;
      if (lStackPanel == null) return;
      using (var lGraphics = Graphics.FromHwnd(IntPtr.Zero)) {
        _WidthInPixels = (float)(e.NewSize.Width * lGraphics.DpiX / 96.0);
    } //

    public void OnTime(object XSender, EventArgs e) {
      BitmapImage lBitmapImage = _BitmapImage;
      if (lBitmapImage == null) return;

      MyImage.Source = lBitmapImage;
      Info.Text = _ElapsedTime;
    } //

    private void Loop() {
      float lEnd = 0f;
      int lSectionFrom = 0;
      int lSectionTo = 0;
      Stopwatch lStopwatch = new Stopwatch();

      while (true) {

        float lWidthInPixel = _WidthInPixels; // copy the value to avoid changes during the calculation
        if (lWidthInPixel <= 0.0) continue;
        List<Line> lSection = _TextEngine.getVectorSection(ref lSectionFrom, ref lSectionTo, lEnd, lWidthInPixel);

        // This value determines the speed. 
        // Even numbers give better results due to the rounding error nature of bitmaps. 
        // Odd numbers create jitter. Luckily humans have bad eyes, they cannot perceive it.
        lEnd += (float)_Speed;

        if (lSection == null) {
          // end reached, reset text
          lSectionFrom = 0;
          lSectionTo = 0;
          lEnd = 0f;
        else {
          Bitmap lBitmap = _TextEngine.VectorsToBitmap(lSection, lWidthInPixel);
          _BitmapImage = _TextEngine.BitmapToImageSource(lBitmap);

        _ElapsedTime = lStopwatch.ElapsedMilliseconds.ToString("#,##0");
    } //

    private void Slider_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e) {
      _Speed = e.NewValue;
    } //

  } // class
} // namespace
using System.Collections.Generic;
using System.Drawing;
using System.IO;
using System.Windows.Media.Imaging;

namespace Ticker {
  public class TextEngine {

    private const string cText = @"Die Gedanken sind frei, wer kann sie erraten, sie fliegen vorbei wie nächtliche Schatten. Kein Mensch kann sie wissen, kein Jäger erschießen mit Pulver und Blei: Die Gedanken sind frei! Ich denke was ich will und was mich beglücket, doch alles in der Still', und wie es sich schicket. Mein Wunsch und Begehren kann niemand verwehren, es bleibet dabei: Die Gedanken sind frei! Und sperrt man mich ein im finsteren Kerker, das alles sind rein vergebliche Werke. Denn meine Gedanken zerreißen die Schranken und Mauern entzwei: Die Gedanken sind frei! Drum will ich auf immer den Sorgen entsagen und will mich auch nimmer mit Grillen mehr plagen. Man kann ja im Herzen stets lachen und scherzen und denken dabei: Die Gedanken sind frei! Ich liebe den Wein, mein Mädchen vor allen, sie tut mir allein am besten gefallen. Ich sitz nicht alleine bei meinem Glas Weine, mein Mädchen dabei: Die Gedanken sind frei!";
    private List<Line> _TextAsVectorChain = new List<Line>();

    public TextEngine() {
      // convert the entire text to vectors
      float lPosition = 0;
      Dictionary<char, List<Line>> lVectorCache = new Dictionary<char, List<Line>>();
      char[] lChars = cText.ToCharArray();

      foreach (char lChar in lChars) {
        if (lChar == ' ') lPosition += 10; // distance for an empty space character
        else {
          List<Line> lOneCharVectors;

          if (!lVectorCache.TryGetValue(lChar, out lOneCharVectors)) {
            Bitmap lBitmap = CharToBitmap(lChar);
            lOneCharVectors = BitmapToVectors(lBitmap);
            lVectorCache.Add(lChar, lOneCharVectors);

          float lNewPosition = lPosition;
          foreach (Line lLine in lOneCharVectors) {
            Line lClone = lLine.Clone();
            lClone.X += lPosition;
            lNewPosition = lClone.X;
          lPosition = lNewPosition + 4; // 4 == space between two characters

    } // constructor

    // Convert a bitmap to an ImageSource.
    // We can then display the result in the WPF Image element.
    public BitmapImage BitmapToImageSource(Bitmap xBitmap) {
      using (MemoryStream lMemoryStream = new MemoryStream()) {
        xBitmap.Save(lMemoryStream, System.Drawing.Imaging.ImageFormat.Bmp);
        lMemoryStream.Position = 0;
        BitmapImage lBitmapImage = new BitmapImage();
        lBitmapImage.StreamSource = lMemoryStream;
        lBitmapImage.CacheOption = BitmapCacheOption.OnLoad;
        return lBitmapImage;
    } //

    // draw a single character into a bitmap
    private Font _Font = null;
    private Bitmap _Bitmap = null;
    public Bitmap CharToBitmap(char xChar) {
      if (_Font == null) {
        _Font = new Font("Arial", 40.0f, System.Drawing.FontStyle.Bold, GraphicsUnit.Pixel);
        _Bitmap = new Bitmap(60, 70);
      using (Graphics lGraphics = Graphics.FromImage(_Bitmap)) {
        lGraphics.DrawString(xChar.ToString(), _Font, Brushes.Black, 0f, 0f);
      return _Bitmap;
    } //

    // Replicate the characters now by reading the vectors and drawing lines.
    Pen lPen = new Pen(Color.Black, 2f);
    public Bitmap VectorsToBitmap(List<Line> xLines, float xBitmapWidth) {
      if (_Font == null) { _Font = new Font("Arial", 40.0f, System.Drawing.FontStyle.Bold, GraphicsUnit.Pixel); }
      Bitmap lBitmap = new Bitmap((int)xBitmapWidth, 60);
      using (Graphics lGraphics = Graphics.FromImage(lBitmap)) {
        foreach (Line lLine in xLines) {
          lGraphics.DrawLine(lPen, lLine.X, lLine.Y1, lLine.X, lLine.Y2);
      return lBitmap;
    } //

    // Convert a single character to vectors.
    private List<Line> BitmapToVectors(Bitmap xBitmap) {
      int lXCoordinateOfFirstPixel = -1;
      List<Line> lList = new List<Line>();

      for (int x = 0, lWidth = xBitmap.Width; x < lWidth; x++) {
        Line lVector = null;
        for (int y = 0, lHeight = xBitmap.Height; y < lHeight; y++) {
          Color lColor = xBitmap.GetPixel(x, y);
          bool lIsWhite = lColor.B == 255;
          if (lIsWhite) {
            if (lVector != null) {
              lVector = null;
          else {
            if (lVector == null) {
              if (lXCoordinateOfFirstPixel < 0) lXCoordinateOfFirstPixel = x;  // to always start at zero for our vectors
              lVector = new Line { X = x - lXCoordinateOfFirstPixel, Y1 = y, Y2 = y };
            else lVector.Y2 = y;

      return lList;
    } //

    // The text was converted to vectors.
    // Now we cut out the sequence we need for the display.
    internal List<Line> getVectorSection(ref int xSectionFrom, ref int xSectionTo, float xEnd, float xWidth) {
      int lCount = _TextAsVectorChain.Count;
      float lStart = xEnd - xWidth;

      // find the right section
      do {
        if (xSectionTo >= lCount) { xSectionTo = lCount - 1; break; }
        if (xEnd < _TextAsVectorChain[xSectionTo].X) break;
      } while (true);

      do {
        if (lStart < 0) break; // to allow empty spaces at the beginning of the slogan
        if (xSectionFrom >= lCount) return null;
        if (lStart < _TextAsVectorChain[xSectionFrom].X) break;
      } while (true);

      // clone that section
      List<Line> lList = new List<Line>();
      for (int x = xSectionFrom; x <= xSectionTo; x++) {
        Line lClone = _TextAsVectorChain[x].Clone();
        lClone.X -= lStart; // adjust the X-axis

      return lList;
    } //

  } // class
} // namespace
namespace Ticker {
  public class Line {
    public float X { get; set; }
    public float Y1 { get; set; }
    public float Y2 { get; set; }

    public Line Clone() {
      Line lLine = new Line();
      lLine.X = X;
      lLine.Y1 = Y1;
      lLine.Y2 = Y2;
      return lLine;
    } //

  } // class
} // namespace

About Bastian M.K. Ohta

Happiness only real when shared.

Posted on March 26, 2015, in Advanced, C#, DataBinding and tagged , , , , , , , , , , . Bookmark the permalink. 1 Comment.

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