Hi,

I found something like a bug with clipping in a Windows form. While
creating a billiards game program, I found that drawing directly on
the graphics object is much faster than invalidating the area and
forcing the update (3 ms instead of 15 ms). In order to avoid flicker,
I set a clipping region. Everything seemed to be fine except for one
"small" problem...

For some positions of the clipping rectangle, there seems to be a
difference between what is clipped when painting a rectangle and what
is clipped when drawing a line, especially at the right and bottom
edges of the clipping rectangle. The line can be clipped for a pixel
position where the filling is not clipped. Instead of trying to
explain in detail, here is a very simple code that shows the problem.
While running the program, click once in the client area. You will see
that the lines are clipped. If the lines labelled as "Problem" in the
code are commented out and the lines with "No problem" are compiled
instead, there is not clipping problem.

Has this problem been reported? I can probably do something to work
around it but I assume that it is really a bug. And I can say that
when I use the normal Invalidate/Update technique for specific
rectangles, I don't have that problem.

Thanks!

MartinP

--------------------------------

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;

public partial class ClippingForm : Form
{
private Brush m_BackBrush;

public static void Main()
{
Application.Run(new ClippingForm());
}

public ClippingForm()
{
ResizeRedraw = true;
this.Width = 200;
this.Height = 200;
m_BackBrush = Brushes.Yellow;
}

private void DrawForm(Graphics gfx)
{
Pen p = Pens.Teal;
gfx.FillRectangle(m_BackBrush, 0.0F, 0.0F, ClientSize.Width,
ClientSize.Height);

gfx.DrawLine(p, 150.45F, 0.0F, 150.45F, ClientSize.Height); //
Problem
gfx.DrawLine(p, 0.00F, 124.45F, ClientSize.Width, 124.45F); //
Problem

// gfx.DrawLine(p, 150.35F, 0.0F, 150.35F, ClientSize.Height); //
No problem
// gfx.DrawLine(p, 0.00F, 124.35F, ClientSize.Width, 124.35F); //
No problem
}

protected override void OnPaint(System.Windows.Forms.PaintEventArgs
e)
{
DrawForm(e.Graphics);
}

protected override void OnMouseDown(MouseEventArgs e)
{
base.OnMouseDown(e);
m_BackBrush = Brushes.Orange;
Graphics gfx = CreateGraphics();
gfx.SetClip(new RectangleF(51.0F, 75.0F, 100.0F, 50.0F));
DrawForm(gfx);
gfx.Dispose();
}
}