Gradient Brushes in GDI+

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

Linear gradient brushes allow you to blend two colors together, generating an indefinite range of shades. The Blend class defines a custom falloff for the gradient.

In a gradient, we begin with a starting color and shift to an ending color, with gradual blending in the space between them. In addition to the starting and ending colors, we can specify the direction of the gradient. For example, Figure 4.12 starts with green in the left bottom corner and ends with red in the top right corner. (You may not notice these colors exactly in a black-and-white image.)

You can also specify a range for pattern repetition. For example, you can specify that the gradient will occur from point (0,0) to point (20, 20) and after that will repeat the same pattern, as in Figure 4.13.

FIGURE 4:12: A color gradient

FIGURE 4.13: A gradient pattern with pattern repetition

Linear Gradient Brushes

The LinearGradientBrush class has eight forms of overloaded constructors. Each constructor takes a starting point, an ending point and two gradient colors. The orientation and linear gradient mode are optional.

The following code snippet creates a linear gradient brush using the colors red and green.

Rectangle rect1 = new Rectangle (20, 20, 50, 50);
LinearGradientBrush lgBrush = new LinearGradientBrush (rect1, Color.Red, Color.Green, LinearGradientMode.Horizontal);

Here the mode parameter is represented by the LinearGradientMode enumeration, which specifies the direction of a linear gradient. The members of the LinearGradientMode enumeration are described in Table 4.3.

Now let's look at the properties and methods of the LinearGradientBrush class, which are defined in Table 4.4 and 4.5 respectively.

Linear Gradient Brushes Example

Now let's create an application that uses linear gradient brushes. We create a Window application; add three label controls, a combo box, two text boxes, four buttons, and two check boxes. We also change the Text property and other properties of these controls. The final form looks like Figure 4.14.

The combo box will list the linear gradient modes. The Pick... buttons allow the user to pick starting and ending colors for the gradient process. The Other Rectangle check box uses a rectangle to specify the range of the gradient. We will discuss the Gamma Correction and Properties options later in this section.

TABLE 4.3: LinearGradientMode members

Member	Description
BackwardDiagonal	Specifies a gradient from upper right to lower left.
ForwardDiagonal	Specifies a gradient from upper left to lower right.
Horizontal	Specifies a gradient from left to right.
Vertical	Specifies a gradient from top to bottom.

TABLE 4.4: LinearGradientBrush properties

Property	Description
Blend	Represents the Blend object that specifies gradient position and factors.
GammaCorrection	Represents gamma correction. If it is enabled, the value is true; if not, it is false.
InterPolationColors	Represents a ColorBlend object that defines a multicolor gradient.
LinearColors	Represents the starting and ending colors of a gradient.
Rectangle	Returns a rectangle that defines the starting and ending points of a gradient.
Transform	Represents a Matrix object that defines the transformation.
WrapMode	Represents a WrapMode enumeration that indicates the wrap mode.

TABLE 4.5: LinearGradientBrush methods

Method	Description
MultiplyTransform	Multiplies a Matrix object that represents the transformation.
ResetTransform	Reset the Transform property to identify.
RotateTransform	Rotates the transformation.
ScaleTransform	Scales the transformation.
SetSigmaBellShape	Creates a gradient falloff based on a bell-shaped curve.
TranslateTransform	Translates the transformation by the specified dimensions.

FIGURE 4.14: Our linear gradient brush application

Next we add some class-level variable as follows:

private LinearGradientBrush lgBrush = null;

private LinearGradientMode mode =

new LinearGradientMode();

private Color startColor = Color.Red;

private Color endColor = Color.Green;

After defining the variable, we add the code from Listing 4.11 on the form's load event handler. As the code shows, we add all gradient modes on the AddGradeintMode method. We also set the default background color of text boxes.

LISTING 4.11: Adding available linear gradient modes

private void Form1_Load (object sender, System.EventArgs e)

{

AddGradientMode();

textBox1.BackColor = startColor;

textBox2.BackColor = endColor;

}

private void AddGradientMode()

{

//Adds linear gradient mode styles to the combo box

comboBox1.Items.Add(LinearGradientMode.BackwardDiagonal);

comboBox1.Items.Add(LinearGradientMode.ForwardDiagonal);

comboBox1.Items.Add(LinearGradientMode.Horizontal);

comboBox1.Items.Add(LinearGradientMode.Vertical);

comboBox1.Text = LinearGradientMode.BackwardDiagonal.ToString();

}

Next we add code for the Pick... buttons, which allow the user to provide color selections for the starting and ending colors. We also set the color of relative text boxes, as shown in Listing 4.12.

LISTING 4.12: The pick button click event handler

private void StartClrBtn_Click(object sender, System.EventArgs e)

{

//Use ColorDialog to select a color

ColorDialog clrDlg = new ColorDialog();

if (clrDlg.ShowDialog() == DialogResult.OK)

{

//Save color as foregorumd color,

//and fill text box with this color

startColor = clrDlg.Color;

textBox1.BackColor = startColor;

}

private void EndClrBtn_Click(object sender, System.EventArgs e)

{

//Use ColorDialog to select a color

ColorDialog clrDlg = new ColorDialog();

if (clrDlg.ShowDialog() == DialogResult.OK)

{

//Save color as background color,

//and fill text box with this color

endColor = clrDlg.Color;

textBox2.BackColor = endColor;

}

The last step is to write code for the Apply Setting button. This button reads various settings, including the selected gradient mode in the combo box, the starting and ending colors, another rectangle, and gamma correction. As Listing 4.13 shows, the code creates a linear gradient brush using a rectangle, two colors and the gradient mode selection. After creating the brush, it calls the FillRectangle method.

LISTING 4.13: The Apply Settings button click event handler

private void ApplyBtn_Click(object sender, System.EventArgs e)

{

Graphics g = this.CreateGraphics();

g.Clear(this.BackColor);

//Read current style from combo box

string str = comboBox1.Text;

//find out the mode and set is as the current mode

switch (str)

{

case "BackwardDiagonal":

mode = LinearGradientMode.BackwardDiagonal;

break;

case "ForwardDiagonal":

mode = LinearGradientMode.ForwardDiagonal;

break;

case "Horizontal":

mode = LinearGradientMode.Horizontal;

break;

case "Vertical":

mode = LinearGradientMode.Vertical;

break;

default:

break;

}

//Create rectangle

Rectangle rect = new Rectangle(70, 200, 200, 220);

//Create linear gradient brush and mode

if (checkBox1.Checked)

{

Rectangle rect1 = new Rectangle(20, 20, 50, 50);

lgBrush = new LinearGradientBrush

(rect1, startColor, endColor, mode);

}

else

{

lgBrush = new LinearGradientBrush

(rect, startColor, endColor, mode);

}

//Gamma correction check box is checked

if (checkBox1.Checked)

{

lgBrush.GammaCorrection = true;

}

//Fill Rectangle

g.FillRectangle(lgBrush, rect);

//Dispose of objects

if (lgBrush != null)

lgBrush.Dispose();

g.Dispose();

}

When you run the application, the result looks like Figure 4.15.

FIGURE 4.15: The default linear gradient brush output

To generate a different output, let's change the linear gradient mode to Vertical. We'll also change the colors, with the results shown in Figure 4.16.

Let's' change the colors and gradient mode again, this time selecting the Other Rectangle check box. The sets a range of the gradient. If the output is out of range, the gradient repeats itself. The new output looks like Figure 4.17.

Conclusion

Hope the article would have helped you in understanding Gradient Brushes in GDI+. Read other articles on GDI+ on the website.

This book teaches .NET developers how to work with GDI+ as they develop applications that include graphics, or that interact with monitors or printers. It begins by explaining the difference between GDI and GDI+, and covering the basic concepts of graphics programming in Windows.