Computer Science 196 Introductory Programming with Visual Basic 'NET

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Computer Science 196Introductory
Programmingwith Visual Basic .NET

• Drawing

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System.Drawing Namespace

• Rectangle structure defines size and position for
  rectangle on form.
• SolidBrush class used to paint interior of shape
  with single color.
• Pen class used to draw lines and curves.
• Font class defines typeface, size, etc. for text.
• Color structure for predefined or programmer
  designed colors.
• Graphics class has methods for drawing shapes,
  lines, etc.

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Rough idea of how things work

• To paint a rectangle we need
• A rectangle
• A color to paint
• A brush dipped into the paint
• A painter (graphics object) to use the brush to
  paint the rectangle

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The graphics coordinate system

• A graphics coordinate system is a system for
  identifying points on a form (or other object)
• The points are referred to as picture elements or
  pixels
• The points within the form all fall within one
  quadrant of the coordinate system - other points
  are not visible.
• The origin is in the upper left corner
• The x-coordinate increases to the right.
• The y-coordinate increases downward.

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Coordinate system
(0,0)
(-50,0)
(0,0)
(50,0)
(50,50)
(0,50)
Form 150 x 100
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Constructors revisited

• Recall that a constructor is a method called New
  in the given class.
• It is used to create a new instance of the class
• Typically a class will have more than one
  constructor (different argument lists).

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Defining a Rectangle

• One constructor for the Rectangle class has four
  Integer parameters.
• The first two give the x,y coordinates for the
  upper left corner of the rectangle.
• The last two give the width and height of the
  rectangle (in pixels).
• Dim recBigRect As New Rectangle(10,20,100,75)
• Dim recLittleRect As Rectangle
  recLittleRect New Rectangle (50,200,15,10)

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Color structure

• Colors use the RGB system
• Red component has value of 0-255
• Same for Green and Blue
• There is also an alpha value for the brightness,
  also 0-255
• To create a color
• Declare a Color variable
• Use Colors FromArgb method to create the color
• Dim colMyColor As Color colMyColor
  Color.FromArgb(125,222,105,200)
• The parameters are Alpha, Red, Green, Blue

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Color structure

• The Color structure has many built in colors
• Dim colMyColor as Color Color.Red

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SolidBrush

• The SolidBrush class has a constructor with a
  single parameter, a color.
• This creates a SolidBrush with the specified
  color (dipped in the color)
• Dim sbMyBrush As New SolidBrush(Color.Red)

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Pen

• The Pen class has a constructor with a Color
  parameter and a Single parameter for the width
• Dim penMyPen As New Pen(Color.Blue,3)

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The Paint event

• Occurs
• Right after form is loaded
• Form resized
• Form moved, etc
• Refresh method called
• The event handler for this event is one place
  where we can do drawing

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The Paint event - event handler

• The event handler has a parameter that is of type
  System.Windows.Forms.PaintEvenArgs
• This parameter has a Graphics property that can
  be used to draw instances of shapes, etc.
• Typical start

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Graphics methods

• FillRectangle with parameters a brush and a
  rectangle
• graCurrent.FillRectangle(sbMyBrush,recMyRect)
• DrawRectangle with parameters a pen and a
  rectangle
• graCurrent.DrawRectangle(penMyPen,recMyRect)
• DrawLine with parameters a pen and four integers
  for x,y coordinates of two points
• graCurrent.DrawLine(penMyPen, 30,40,6,80)