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    <p>Instead of using <code>Invalidate()</code> which will invalidate the entire painting area of your control each time you do a mousemove....and thus cause more painting work to be done...you can be more efficient by only invalidating the area that you need to change by using <code>Invalidate()</code> with a specified <code>Rectangle</code>.</p> <p>When <code>OnPaint</code> is called...the <code>e.ClipRectangle</code>...equates to the "dirty" area that needs updating.....you can use that to optimize the painting work that you do in <code>OnPaint</code>.</p> <p>You could also cache the creation of your Red <code>Pen</code> and White <code>SolidBrush</code> instead of creating them each time the <code>OnPaint</code> happened...you may or may not see any difference doing that.</p> <p>Here's an update of your example:</p> <pre><code>using System; using System.Collections.Generic; using System.ComponentModel; using System.Data; using System.Drawing; using System.Linq; using System.Text; using System.Windows.Forms; namespace WindowsFormsApplication3 { public partial class CaptureScreen : Form { public Image Image { get; set; } private Rectangle selection; private Rectangle previousselection; private Point startPoint; public static Image Snip() { using (var bmp = new Bitmap(SystemInformation.VirtualScreen.Width, SystemInformation.VirtualScreen.Height, System.Drawing.Imaging.PixelFormat.Format32bppPArgb)) { using (var graphics = Graphics.FromImage(bmp)) graphics.CopyFromScreen(SystemInformation.VirtualScreen.Left, SystemInformation.VirtualScreen.Top, 0, 0, bmp.Size); using (var snipper = new CaptureScreen(bmp, new Point(SystemInformation.VirtualScreen.Left, SystemInformation.VirtualScreen.Top))) { if (snipper.ShowDialog() == DialogResult.OK) return snipper.Image; } return null; } } public CaptureScreen(Image screenShot, Point startPos) { InitializeComponent(); Cursor = Cursors.Cross; BackgroundImage = screenShot; ShowInTaskbar = false; FormBorderStyle = FormBorderStyle.None; StartPosition = FormStartPosition.Manual; Size = screenShot.Size; Location = startPos; SetStyle(ControlStyles.AllPaintingInWmPaint | ControlStyles.UserPaint | ControlStyles.DoubleBuffer, true); } protected override void OnMouseDown(MouseEventArgs e) { if (e.Button != MouseButtons.Left) return; startPoint = e.Location; selection = new Rectangle(e.Location, new Size(0, 0)); previousselection = selection; Invalidate(); } protected override void OnMouseMove(MouseEventArgs e) { if (e.Button != MouseButtons.Left) return; var x1 = Math.Min(e.X, startPoint.X); var y1 = Math.Min(e.Y, startPoint.Y); var x2 = Math.Max(e.X, startPoint.X); var y2 = Math.Max(e.Y, startPoint.Y); Invalidate(previousselection); // invalidate old rect area so it gets blanked out previousselection = selection; selection = new Rectangle(x1, y1, x2 - x1, y2 - y1); Invalidate(selection); // invalidate new rect area so it gets drawn } protected override void OnMouseUp(MouseEventArgs e) { if (selection.Width &lt;= 0 || selection.Height &lt;= 0) return; Image = new Bitmap(selection.Width, selection.Height); using (var gr = Graphics.FromImage(Image)) { gr.DrawImage(BackgroundImage, new Rectangle(0, 0, Image.Width, Image.Height), selection, GraphicsUnit.Pixel); } DialogResult = DialogResult.OK; } protected override void OnPaint(PaintEventArgs e) { using (var br = new SolidBrush(Color.FromArgb(127, Color.White))) using (var pen = new Pen(Color.Red, 1)) using (Region region = new Region(new Rectangle(0,0,Width,Height))) { region.Exclude(selection); region.Intersect(e.ClipRectangle); e.Graphics.FillRegion(br, region); e.Graphics.DrawRectangle(pen, selection.X, selection.Y, selection.Width - 1, selection.Height - 1); } } } } </code></pre>
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    1. COThis definitely increases performance tenfold. The only thing that I notice now is that the selection starts to lag again if it becomes too large. I'm using a dual monitor, and the effects vaguely start repeating themselves (not as bad) when I drag from both screens. It's not as noticeable as it previously was but it's definitely much better. Thanks!
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    2. COActually, scratch that, I just compared it with Windows' Snipping tool, it's practically the same! Once again, thanks!
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    3. COSee the revision where the Region is disposed via the Using.
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