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;