c# floating point inconsistencies

[roche72]

Help.

I have a simple floating point calculation and depending on what thread it gets executed on, I get a different result. Here are the pieces of code:

class OmniCamera
{
  private float m_PixelSize;
  private float m_ImageHeight;
 
  ...
 
  /// prints out the hex value of the floating point number.
  private static string HexString(float val)
  {
    byte[] bytes = BitConverter.GetBytes(val);
    string s = string.Empty;
    for (int i = bytes.Length - 1; i >= 0; i--)
    {
       s += bytes[i].ToString("X2");
    }
    return s;
  }  
  public bool WorldFromImage(Vector2 screenPosition, out Vector3 worldPoint)
  {
    worldPoint = new Vector3();
    ...
 
    worldPoint.Y = (screenPosition.Y * this.m_PixelSize) -  (this.m_ImageHeight / 2.0f);
 
    Debug.WriteLine("worldPoint.Y: " + HexString(worldPoint.Y) + 
                             " screenPosition.Y: " + HexString(screenPosition.Y) + 
                             " m_PixelSize: " + HexString(this.m_PixelSize) + 
                             " m_ImageHeight: " + HexString(this.m_ImageHeight));
    Debug.WriteLine("worldPoint.Y: " + worldPoint.Y.ToString("r") + 
                             " screenPosition.Y: " + screenPosition.Y.ToString("r") + 
                             " m_PixelSize: " + this.m_PixelSize.ToString("r") + 
                             " m_ImageHeight: " + this.m_ImageHeight.ToString("r"));
 
    ...
 
    return true;
  }
}
 

Here is the output of the code when worldFromImage(...) gets executed from the worker thread:

worldPoint.Y: BFAA07A7 screenPosition.Y: 43EB1D26 m_PixelSize: 3D0240B8 m_ImageHeight: 420240B8
worldPoint.Y: -1.32835853 screenPosition.Y: 470.227722 m_PixelSize: 0.0318 m_ImageHeight: 32.5632

Here is the output of the code when worldFromImage(...) gets executed from the main app thread:

worldPoint.Y: BFAA07A8 screenPosition.Y: 43EB1D26 m_PixelSize: 3D0240B8 m_ImageHeight: 420240B8
worldPoint.Y: -1.32835865 screenPosition.Y: 470.227722 m_PixelSize: 0.0318 m_ImageHeight: 32.5632


As you can see the right hand side of the equation has exactly the same values. But we get a different result from the different threads. I thought c# floating point followed the IEEE floating point standard, and I thought that guarenteed that floating point operations always returned the same value.

Any insight would be greatly appreciated. I would like to understand this behaviour.

-thanks
steve

[Plater]

Are you using Singles or Doubles, if you mix and match them, there will be conversion issues I think.

[roche72]

Are you using Singles or Doubles, if you mix and match them, there will be conversion issues I think.

Everything is singles (floats). But even if i was mixing and matching I would expect the same result each time through.

[Plater]

Yeah it *should* be the same, as long as its called with identical data.

Seems like this line is the issue yes?
worldPoint.Y = (screenPosition.Y * this.m_PixelSize) - (this.m_ImageHeight / 2.0f);

I did the following and always go the same number:

float m_PixelSize=13.24572F;
float m_ImageHeight=7.5757F;
float screenPosition = 3.4F;
for (int i = 0; i < 10; i++)
{
   float Y = (screenPosition * m_PixelSize) - (m_ImageHeight / 2.0f);
   System.Diagnostics.Debug.WriteLine(Y.ToString() + "=(" + screenPosition + "*" + m_PixelSize + ")-(" + m_ImageHeight + "/2.0F)");
}