Use Graphics Paths in GDI+


This article has been excerpted from book "Graphics Programming with GDI+".

Graphics paths may be useful when we need to redraw certain graphics items. For example, suppose we have hundreds of graphics items, including lines, rectangles, images, and text associated with a surface but we need to redraw only the rectangles. We can create a graphics path with all rectangles and just redraw that path, instead of the entire surface.

We may also want to use graphics paths when drawing different shapes, depending on the complexity of the application. For example, Listing 13.11 uses draw methods to draw two lines, two rectangles, and an ellipse. We can write code on a button or menu click event handler.

LISTING 13.11: Drawing simple graphics objects

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 WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Paint(object sender, PaintEventArgs e)
        {
            Graphics g = this.CreateGraphics();
            g.Clear(this.BackColor);
            //Create black pen
            Pen blackPen = new Pen(Color.Black, 2);
            // Draw objects
            g.DrawLine(blackPen, 50, 50, 200, 50);
            g.DrawLine(blackPen, 50, 50, 50, 200);
            g.DrawRectangle(blackPen, 60, 60, 150, 150);
            g.DrawRectangle(blackPen, 70, 70, 100, 100);
            g.DrawEllipse(blackPen, 90, 90, 50, 50);
            //Dispose of objects
            blackPen.Dispose();
            g.Dispose();
        }
    }
}


Listing 13.12 draws the same graphics objects. The only difference is that this code uses a graphics path.

Output:

image1.gif

LISTING 13.12: Using a graphics path to draw graphics objects

            Graphics g = this.CreateGraphics();
            g.Clear(this.BackColor);
            //Create a black pen
            Pen blackPen = new Pen(Color.Black, 2);
            //Create a graphics path
            GraphicsPath path = new GraphicsPath();
            path.Addline(50, 50, 200, 50);
            path.Addline(50, 50, 50, 200);
            path.AddRectangle(new Rectangle(60, 60, 150, 150));
            path.AddRectangle(new Rectangle(70, 70, 100, 100));
            path.Ellipse(90, 90, 50, 50);
            g.DrawPath(blackPen, path);
            //Dispose of objects
            blackPen.Dispose();
            g.Dispose();

Both Listing 13.11 and 13.12 generate the output shown in Figure 13.5. There is no straightforward rule for when to use graphics paths. The choice depends on the complexity of your application.

In the preceding example we saw how to replace multiple drawing statements with a single graphics path drawing statement. But graphics paths have some limitations. For example, we can't draw each element (line, rectangle, or an ellipse) of a graphics path with a separate pen or brush. We have to draw or fill them individually.

Conclusion

In this article you learned how to use graphics path in GDI+.

book.gif