更改0度旋转的位置？

Question

I have a canvas with some objects on it, and I can rotate them based on some number entered where 0 is upward on the screen, 90 is to the right, 180 downward, and 270 to the left. Is there anyway to change where the 0 degrees is? Right now I'm assuming the canvas background image has north as straight ahead (it's a map), but if I have an image where 0 degrees north is to the left, I can't rotate my objects properly unless I can change where the original 0 degrees is. I was thinking I could have the user select where 0 degrees is, but it doesn't seem like I can retrieve the necessary information just by the user selecting a point. Any ideas?

Example: I have a map. 0 degrees is to the left (where 270 would be). The user knows this, but the program doesn't. Program thinks 0 degrees is still north. So if the user wanted to display his heading, he would have to add the offset of the two 0 degrees to his heading. Not sure how to get that offset though.

1st idea: I'm thinking if I get the center point of the canvas, and have the user click a point where 0 is, and get the angle the center point WOULD have to rotate to in order to be facing the new 0.

Answer 1

if you have viewmodel, then bind it to computed property:

XAML:

<RotateTransform Angle="{Binding FinalRotationAngle}" />

C#:

private double _northOrientationAngle;
private double _mapRotationAngle;


public double MapRotationAngle
{
    get { return _mapRotationAngle; }
    set
    {
        _mapRotationAngle= value;
        OnPropertyChanged("MapRotationAngle");
        OnPropertyChanged("FinalRotationAngle");
    }
}

public double NorthOrientationAngle
{
    get { return _northOrientationAngle; }
    set
    {
        _northOrientationAngle= value;
        OnPropertyChanged("NorthOrientationAngle");
        OnPropertyChanged("FinalRotationAngle");
    }
}

public double FinalRotationAngle
{
    get { return NorthOrientationAngle + MapRotationAngle; }
}

Now you have configurable north orientation as well as user defined orientation.

if you have custom control witout mvvm you can achieve the same using DependecyProperties and PropertyChanged callback

Answer 2

How about:

private double neutralPosition;

public void SetNeutralPosition(double position)
{
    this.neutralPosition = position;
}

public double Rotate(double angle)
{
    return this.neutralPosition + angle;
}

So the idea is: you use SetNeutralPosition to set where your "0" is relative to the REAL 0 degrees position. Then you can use the Rotate method (with your relative degree of rotation) to get the "real" rotation.