Screen capture without active window

Question

I'm trying to write a magnifier application using a winform or wpf windows. The idea is to drag the window over a spot on the screen and magnify it. I know it exists commercially but need to build a customized version. The challenge I'm having is to capture the screen image behind the active application. I have found code to capture a screen image below. But it includes the active window

    {
        Rectangle bounds = new Rectangle(this.Location, this.Size);
        Bitmap bitmap = new Bitmap(bounds.Width, bounds.Height);
        Graphics g = Graphics.FromImage(bitmap);
        g.CopyFromScreen(this.Location, Point.Empty, bounds.Size);
        Bitmap bp2 = new Bitmap(bitmap); //  local copy of image...
        pictureBox1.Image = bp2;
    }

Adding statements to hide the active application correct the image capture, but introduce a screen flicker which I'd like to avoid. (modified code below)

    {
        this.StartPosition = FormStartPosition.Manual;  // get current window location
        Point cur = this.Location;
        this.Location = new Point(-500, -500);  //  hide the active app off screen.

        Rectangle bounds = new Rectangle(cur, this.Size);
        Bitmap bitmap = new Bitmap(bounds.Width, bounds.Height);
        Graphics g = Graphics.FromImage(bitmap);
        g.CopyFromScreen(cur, Point.Empty, bounds.Size);
        Bitmap bp2 = new Bitmap(bitmap); //  local copy of image...
        pictureBox1.Image = bp2;

        this.Location = cur;  // restore application location
    }

Can someone suggest an alternative to capture a screen region, behind an active windows?

Thx.

Answer 1

Wrapping the Magnification API is pretty useful. I created a Winforms control that does this. Add a new class to your project and paste the code shown below. Compile. Drop the new control from the top of the toolbox onto a form.

The TrackMouse property controls which part of the screen is shown magnified. Set to False, it magnifies the area covered by the control, making it act like a magnifying glass. Set to True, it will operate like Windows' Magnifier, following the mouse.

The Magnification property controls the amount of magnification. You can already adjust it by using the mouse wheel.

The form you drop it should have its TopMost property set to True. You might want to tinker with its Region property to make it resemble a spyglass.

using System;
using System.ComponentModel;
using System.Drawing;
using System.Windows.Forms;
using System.Runtime.InteropServices;

class Magnifier : Control {
    public Magnifier() {
        if (!MagInitialize()) throw new NotSupportedException();
        timer = new Timer { Interval = 45 };
        timer.Tick += (o, ea) => { if (trackMouse) setSource(false); else Invalidate(); };
    }
    [DefaultValue(false)]
    public bool TrackMouse {
        get { return trackMouse; }
        set { trackMouse = value; setSource(false); }
    }
    [DefaultValue(2.0f)]
    public float Magnification {
        get { return magnification; }
        set { magnification = Math.Max(1, value); setSource(true); }
    }

    protected override CreateParams CreateParams {
        get {
            var cp = base.CreateParams;
            if (!this.DesignMode) {
                cp.ClassName = "Magnifier";
                //cp.Style |= MS_SHOWMAGNIFIEDCURSOR;
                this.SetStyle(ControlStyles.UserPaint, true);
            }
            return cp;
        }
    }

    protected override void OnHandleCreated(EventArgs e) {
        base.OnHandleCreated(e);
        if (!this.DesignMode) {
            setSource(true);
            this.FindForm().LocationChanged += ParentLocationChanged;
            timer.Start();
        }
    }
    protected override void Dispose(bool disposing) {
        if (disposing) {
            var frm = this.FindForm();
            if (frm != null) frm.LocationChanged -= ParentLocationChanged;
            timer.Dispose();
            MagUninitialize();
        }
        base.Dispose(disposing);
    }

    private void ParentLocationChanged(object sender, EventArgs e) {
        if (!trackMouse) setSource(false);
    }
    protected override void OnSizeChanged(EventArgs e) {
        setSource(false);
        base.OnSizeChanged(e);
    }
    protected override void OnMouseWheel(MouseEventArgs e) {
        this.Magnification += e.Delta / 100f;
        ((HandledMouseEventArgs)e).Handled = true;
    }

    private void setSource(bool newmag) {
        if (!this.IsHandleCreated || this.DesignMode) return;
        if (newmag) {
            var xform = new MAGTRANSFORM();
            xform.v11 = xform.v22 = magnification;
            xform.v33 = 1.0f;
            MagSetWindowTransform(this.Handle, ref xform);
        }
        Point center;
        if (trackMouse) center = Cursor.Position;
        else {
            var rc = this.RectangleToScreen(this.Bounds);
            center = new Point(rc.Left + rc.Width / 2, rc.Top + rc.Height / 2);
        }
        var scr = Screen.FromPoint(center);
        var rect = new RECT();
        rect.left = Math.Max(scr.Bounds.Left, center.X - (int)(this.Width / magnification / 2));
        rect.top = Math.Max(scr.Bounds.Top, center.Y - (int)(this.Height / magnification / 2));
        rect.right = rect.left + (int)(this.Width / magnification);
        if (rect.right > scr.Bounds.Right) {
            rect.right = scr.Bounds.Right;
            rect.left = rect.right - (int)(this.Width / magnification);
        }
        rect.bottom = center.Y + (int)(this.Height / magnification);
        if (rect.bottom > scr.Bounds.Bottom) {
            rect.bottom = scr.Bounds.Bottom;
            rect.top = rect.bottom - (int)(this.Height / magnification);
        }
        MagSetWindowSource(this.Handle, ref rect);
        this.Invalidate();
    }

    private Timer timer;
    private bool trackMouse;
    private float magnification = 2.0f;

    private struct RECT {
        public int left, top, right, bottom;
    }
    private struct MAGTRANSFORM {
        public float v11, v12, v13;
        public float v21, v22, v23;
        public float v31, v32, v33;
    }
    [DllImport("magnification.dll")]
    private static extern bool MagInitialize();
    [DllImport("magnification.dll")]
    private static extern bool MagUninitialize();
    [DllImport("magnification.dll")]
    private static extern bool MagSetWindowSource(IntPtr hWnd, ref RECT rc);
    [DllImport("magnification.dll")]
    private static extern bool MagSetWindowTransform(IntPtr hWnd, ref MAGTRANSFORM xform);
    private const int MS_SHOWMAGNIFIEDCURSOR = 1;
    private const int MS_CLIPAROUNDCURSOR = 2;
    private const int MS_INVERTCOLORS = 4;
}