将偏移应用于缩放以保留特定点

Question

我目前正在努力使用WPF画布和缩放。我希望能够围绕特定点缩放画布（缩放后给定点应保持在同一屏幕位置）。

使用简单的 ScaleTransform 完成缩放。翻译存储在 Point 对象中，并应用于画布中的所有形状。这意味着我必须在应用缩放后偏移所有形状（修改转换点）。为简单起见，这里是一个简化的代码

class CanvasManager
{
    Point mTranslation; // holds the translation
    Canvas mCanvas;
    ScaleTransform mCanvasScale; // linked to mCanvas

    // Constructor omitted

    public void Scale(double factor, Point pt)
    {
        // retrieve the previous scaling factor
        double oldfactor = mCanvasScale.ScaleX;
        //apply the new factor
        mCanvasScale.ScaleX = factor;
        mCanvasScale.ScaleY = factor;
        // now offset so Point pt gets back at its original position
        mTranslation.X += ???;
        mTranslation.Y += ???;
    }
}

点 pt 来自相对于画布测量的鼠标事件，包括缩放。 （当比例为2时，相对屏幕x坐标为200，我得到400作为 pt 的值） Transform（Scale）应用以下代码片段：

mCanvasScale = new ScaleTransform();
mCanvas.LayoutTransform = mCanvasScale;

如何获得mTranslation的正确偏移量？

最好的问候 丹尼斯

Answer 1

解决方案应该是

class CanvasManager
{
    Point mTranslation; // holds the translation
    Canvas mCanvas;
    ScaleTransform mCanvasScale; // linked to mCanvas

    // Constructor omitted

    public void Scale(double factor, Point pt)
    {
        // retrieve the previous scaling factor
        double oldfactor = mCanvasScale.ScaleX;
        //apply the new factor
        mCanvasScale.ScaleX = factor;
        mCanvasScale.ScaleY = factor;
        // now offset so Point pt gets back at its original position
        mTranslation.X -= ( (pt.X * factor) - (pt.X * oldfactor) ) / factor;
        mTranslation.Y -= ( (pt.Y * factor) - (pt.Y * oldfactor) ) / factor;
    }
}