C# How to: Drawing in GDI with Opacity/Alpha components

Article Purpose

In this article we explore GDI+ drawing operations implementing opacity, also known as alpha blending.

Sample source code

This article is accompanied by a sample source code Visual Studio project which is available for download here

C# How to: Drawing in GDI with Opacity/Alpha components

Using the Sample Application

The following is a screenshot of the included sample application:

GDIOpacityDrawing

The screenshot illustrates drawing text and a rectangle onto a Windows Form in Color.SteelBlue with an alpha component of 100.

GDI+ Drawing

In C# it is possible and relatively easy to draw text and two dimensional shapes in GDI+ that support a level of opacity or transparency. The well known abbreviation RGB abbreviates the term Red, Green and Blue. C# supports RGB colours but also what is known as ARGB colours. In the case of ARGB the A abbreviates the word Alpha, in other words an RGB colour with a specified alpha component.

An alpha component specifies a colour’s opacity or transparency. Possible values range from 0 to 255 inclusive, where 0 would represent full transparency and 255 no level of transparency. If a consists of 8 bits and an ARGB colour is composed of 4 components ranging from 0 to 255 each representing a or 8 bits, then an ARGB colour is therefore a 32 bit colour.

The Color structure

The System.Drawing namespace defines the Color structure, which exposes several functions aimed at creating an ARGB Color object instance:

The Paint Event Handler

The bulk of this example’s functionality occurs within the main Form’s handler, as detailed by the following code snippet:

private void MainFormPaintEventHandler(object sender, PaintEventArgs e) 
{ 
    Color alphaForeColor = Color.FromArgb(this.foreColorAlphaValue, this.ForeColor); 
    Pen rectanglePen = new Pen(alphaForeColor, 2.0f); 
    SolidBrush textBrush = new SolidBrush(alphaForeColor); 

float x = this.ClientRectangle.Width / 2.0f; x -= e.Graphics.MeasureString(textToDisplay, this .Font).Width / 2.0f;
float y = this.ClientRectangle.Height / 2.0f; y -= e.Graphics.MeasureString(textToDisplay, this.Font).Height / 2.0f;
e.Graphics.DrawString(textToDisplay, this.Font, textBrush, new PointF(x, y));
e.Graphics.DrawRectangle(rectanglePen, 25, 25, this.ClientRectangle.Width - 50, this.ClientRectangle.Height - 50); }

The main Form defines two member variables used in the handler:

 private byte foreColorAlphaValue = 100;
 textToDisplay = "http://softwarebydefault.com";

In the handler an instance of the structure is created implementing an alpha component defined by the ’s member variable. The object declaration is followed by an ARGB   Pen and SolidBrush declaration.

Next the code determines the X and Y coordinates that would result in drawing the ’s string member variable in the middle of the , after which the actual drawing occurs.

Using the Pen object declared earlier a rectangle is drawn 25 pixels inside the ’s ClientRectangle.

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Dewald Esterhuizen

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