WPF Image Zoom with scale shader around specific point

I've an WPF Image control, which use scale shader for zooming around center of the image (ScaleCenter is always 0.5,0.5). But for zooming around specific point on the image (cursor) I guess i should apply some offests to keep the same image pixel under cursor.

I've this code which is works perfect untill Scaleshader < 1.

public void ScalePoint(double zoomFactor, Point point, Image ImageViewElement, bool pushOperation = false)
{
    double w = ImageViewElement.ActualWidth;
    double h = ImageViewElement.ActualHeight;

    double cx = w * 0.5;
    double cy = h * 0.5;
    double dx = point.X - cx;
    double dy = point.Y - cy;

    double contentX = (OffsetX + dx) / ScaleShader;
    double contentY = (OffsetY + dy) / ScaleShader;

    OffsetX = contentX * ScaleShader * zoomFactor - dx;
    OffsetY = contentY * ScaleShader * zoomFactor - dy;

    var m = new Matrix();
    m.Translate(-OffsetX, -OffsetY);
    RenderTransform = new TransformGroup { Children = { new MatrixTransform(m) } };

    ScaleShader *= zoomFactor;
}

When the ScaleShader becomes > 1 - these offsets OffsetX and OffsetY - I guess calculating in a wrong way.

Could you give me some hint on how this parameters should be calculated for any scaleShader values? (I cannot use only matrix transform/scale, because the image sizes are big, so we should use Shader but as I said, scale_center is always 0.5,05 and it cannot be changed because it is used in other parts of the software.

shader code

sampler2D inputSampler : register(S0);
sampler2D inputNNSampler : register(S1);
float scale : register(C0);
float2 scale_center : register(C1); // always (0.5, 0.5)
float scaling_mode : register(C2);

float4 main(float2 uv : TEXCOORD) : COLOR
{
    float2 ray = uv - scale_center;
    uv = scale_center + ray / scale;

    if (uv.x < 0 || uv.y < 0 || uv.x > 1 || uv.y > 1)
        return 0;

    return scaling_mode > 0
        ? tex2D(inputNNSampler, uv)
        : tex2D(inputSampler, uv);
}