[转]Regions and Clipping in GDI+

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

The Graphics class provides methods to clip regions. Using these methods, an application can restrict where graphics objects are drawn. One major use of clipping region is to repaint only part of a control. In some cases painting an entire form is costly in terms of time and memory resources. Clipping plays a vital role by painting only the desired area. The Graphics class provides the SetClip, ResetClip, IntersectClip, ExcludeClip and TranslateClip methods to use in clipping operations.

ExcludeClip excludes the area specified by an argument of type Rectangle or a Region and updates the clipping region. Listing 6.11 fills a rectangle, excluding one small rectangle and a region.

LISTING 6.11: Using ExcludeClip to clip regions

            //Create a Graphics object
            Graphics g = this.CreateGraphics();
            g.Clear(this.BackColor);

            //Create rectangles
            Rectangle rect1 = new Rectangle(20, 20, 60, 80);
            Rectangle rect2 = new Rectangle(100, 100, 30, 40);

            //Create a region
            Region rgn1 = new Region(rect2);

            //Exclude clip
            g.ExcludeClip(rect1);
            g.ExcludeClip(rgn1);

            //Fill rectangle
            g.FillRectangle(Brushes.Red, 0, 0, 200, 200);

            //Dispose of object
            g.Dispose();

Figure 6.11 shows output from Listing 6.11. The small rectangle and small region are not updated.

SetClip sets the clipping region of a Graphics object. This method has many overloaded forms and takes parameters of type Rectangle, RectangleF, Region, GraphicsPath, and Graphics with or without the CombineMode enumeration. The CombineMode enumeration defines how different clipping regions can be combined (see Table 6.2).

The ResetClip method resets the clipping region to infinity. Listing 6.12 uses the SetClip, ResetClip, and IntersectClip methods.

Listing 6.12: Using the SetClip, ResetClip, and IntersectClip methods

            //Create a Graphics object
            Graphics g = this.CreateGraphics();
            g.Clear(this.BackColor);

            //Create rectangles and regions
            Rectangle rect1 = new Rectangle(20, 20, 200, 200);
            Rectangle rect2 = new Rectangle(100, 100, 200, 200);
            Region rgn1 = new Region(rect1);
            Region rgn2 = new Region(rect2);

            //Call SetClip
            g.SetClip(rgn1, CombineMode.Exclude);

            //Call IntersectClip
            g.IntersectClip(rgn2);

            //Fill rectangle
            g.FillRectangle(Brushes.Red, 0, 0, 300, 300);

            //Call ResetClip
            g.ResetClip();

            //Draw rectangles
            g.DrawRectangle(Pens.Green, rect1);
            g.DrawRectangle(Pens.Yellow, rect2);
            //Dispose of objects

            g.Dispose();

FIGURE 6.11: ExcludeClip Output

TABLE 6.2: CombineMode members

Note: The CombineMode enumeration is defined in the System.Drawing.Drawing2D namespace.

Figure 6.12 shows the output from Listing 6.12.

TranslateClip translates the clipping region as specified. Listing 6.13 uses the TranslateClip method to translate a region to 20 and 30 points.

LISTING 6.13: Using TranslateClip to translate a region

            //Create a Graphics object
            Graphics g = this.CreateGraphics();
            g.Clear(this.BackColor);

            //Create a Rectangle
            RectangleF rect1 = new RectangleF(20.0f, 20.0f, 200.0f, 200.0f);

            //Create a region
            Region rgn1 = new Region(rect1);

            //Call SetClip
            g.SetClip(rgn1, CombineMode.Exclude);
            float h = 20.0f;
            float w = 30.0f;

            //Call Translate Clip with h and w
            g.TranslateClip(h, w);

            //Fill rectangle
            g.FillRectangle(Brushes.Green, 20, 20, 300, 300);

FIGURE 6.12: Using clip mode

FIGURE 6.13: Using TranslateClip

Figure 6.13 shows the output from Listing 6.13

Clipping Regions Example

Listing 6.14 uses Xor to clip regions

LISTING 6.14: Using the Xor method

            Graphics g = e.Graphics;
            Pen pen = new Pen(Color.Red, 5);
            SolidBrush brush = new SolidBrush(Color.Red);
            Rectangle rect1 = new Rectangle(50, 0, 50, 150);
            Rectangle rect2 = new Rectangle(0, 50, 150, 50);
            Region region = new Region(rect1);
            region.Xor(rect2);
            g.FillRegion(brush, region);

Figure 6.14 shows the output from Listing 6.14.

FIGURE 6.14: Result of the Xor method

Now if we replace Xor with Union:

region.Union(rect2);

the new output looks like Figure 6.15.

Now let's replace Union with Exclude:

region.Exclude(rect2);

FIGURE 6.15: Result of the Union method

FIGURE 6.16: Result of the Exclude method

FIGURE 6.17: Result of the Intersect method

The output looks like Figure 6.16.

If we use the Intersect method:

region.Intersect (rect2);

the output looks like Figure 6.17.

Conclusion

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