计算图片物理大小

//先计算系统dpi，以下是代码
public partial class Form1 : Form
    {
        int dpi_x;
        int dpi_y;
        public Form1()
        {
            InitializeComponent();
         //SetProcessDPIAware(); //重要
            IntPtr screenDC = GetDC(IntPtr.Zero);
            dpi_x = GetDeviceCaps(screenDC, /*DeviceCap.*/LOGPIXELSX);
             dpi_y = GetDeviceCaps(screenDC, /*DeviceCap.*/LOGPIXELSY);
            // dpi_x = 96.0;
            // dpi_y = 96.0;
            ReleaseDC(IntPtr.Zero, screenDC);


        }


        [DllImport("user32.dll")]
        static extern IntPtr GetDC(IntPtr ptr);


        [DllImport("user32.dll", EntryPoint = "ReleaseDC")]
        public static extern IntPtr ReleaseDC(IntPtr hWnd, IntPtr hDc);


        [DllImport("gdi32.dll")]
        public static extern IntPtr CreateDC(
        string lpszDriver, // driver name
        string lpszDevice, // device name
        string lpszOutput, // not used; should be NULL
        Int64 lpInitData // optional printer data
        );


        [DllImport("gdi32.dll")]
        public static extern int GetDeviceCaps(
        IntPtr hdc, // handle to DC
        int nIndex // index of capability
        );


        [DllImport("user32.dll")]
        internal static extern bool SetProcessDPIAware();


        const int DRIVERVERSION = 0;
        const int TECHNOLOGY = 2;
        const int HORZSIZE = 4;
        const int VERTSIZE = 6;
        const int HORZRES = 8;
        const int VERTRES = 10;
        const int BITSPIXEL = 12;
        const int PLANES = 14;
        const int NUMBRUSHES = 16;
        const int NUMPENS = 18;
        const int NUMMARKERS = 20;
        const int NUMFONTS = 22;
        const int NUMCOLORS = 24;
        const int PDEVICESIZE = 26;
        const int CURVECAPS = 28;
        const int LINECAPS = 30;
        const int POLYGONALCAPS = 32;
        const int TEXTCAPS = 34;
        const int CLIPCAPS = 36;
        const int RASTERCAPS = 38;
        const int ASPECTX = 40;
        const int ASPECTY = 42;
        const int ASPECTXY = 44;
        const int SHADEBLENDCAPS = 45;
        const int LOGPIXELSX = 88;
        const int LOGPIXELSY = 90;
        const int SIZEPALETTE = 104;
        const int NUMRESERVED = 106;
        const int COLORRES = 108;
        const int PHYSICALWIDTH = 110;
        const int PHYSICALHEIGHT = 111;
        const int PHYSICALOFFSETX = 112;
        const int PHYSICALOFFSETY = 113;
        const int SCALINGFACTORX = 114;
        const int SCALINGFACTORY = 115;
        const int VREFRESH = 116;
        const int DESKTOPVERTRES = 117;
        const int DESKTOPHORZRES = 118;
        const int BLTALIGNMENT = 119;

        private void button1_Click(object sender, EventArgs e)
        {
            MessageBox.Show(dpi_x.ToString()+"||"+dpi_y.ToString());
        }


    }

 float pw=w/myThumbnail.HorizontalResolution*dpi_x;//物理大小=像素/分辨率*dpi
 float ph=h/2/myThumbnail.VerticalResolution*dpi_y;