Canvas to image problem WPF

public void ExportToPng(Canvas canvas, string path, Size size)
{
    if (path == null) return;
    canvas.Measure(size);
    var rect = new Rect(size);
    canvas.Arrange(rect);
    Render(canvas, size, path);
}


private void Render(Visual visual, Size size, string path)
{    
    double dpi = 128;
    double scale = dpi / 96;
    RenderTargetBitmap renderBitmap =
        new RenderTargetBitmap(
        (int)(size.Width * scale),
        (int)(size.Height * scale),
        dpi,
        dpi,
        PixelFormats.Pbgra32);
    renderBitmap.Render(visual);
    BitmapEncoder encoder = new JpegBitmapEncoder();
    using (FileStream outStream = new FileStream(path, FileMode.Create))
    {
        encoder.Frames.Add(BitmapFrame.Create(renderBitmap));
        encoder.Save(outStream);
    }
}