Wyo VB - Ch. 7 Notes

Objective #1: Understand and use the typesetting and computer display concepts associated with the monitor and hardcopy displays.

• Graphics on a computer screen are formed by arranging pixels (picture elements). A pixel is the smallest graphic element that can be displayed on the screen, however different types of monitors have different capabilities in terms of displaying graphics. Some monitors can display 1024 pixels horizontally and 768 pixels vertically (SVGA resolution). Others can only display a screen that is 640 by 480 pixels (VGA resolution).
  
  
• Since images may look different and distorted on monitors of different resolutions, Visual Basic uses a standard measure called a twip. The twip is the default standard measure of graphic elements. A twip is 1/20 of a point, where a point is 1/72 of an inch. Points were widely used by printer's as a unit of measure long before computer graphics or even computers. Font sizes on the computer screen are measured in points. For example, a 12 font size means 12 points. Since you know that a twip is 1/20 of a point and a point is 1/72 of an inch, it can be calculated that there are 1440 twips in an inch.
  
  
• Because of the nature of the technology that displays graphics on a computer screen, a inch on the screen (called a logical inch) is actually slightly longer than a printed inch depending on the monitor's screen solution. This is partly due to the fact that the resolution of a printed page on any type of printer is MUCH better than the resolution on a screen. Did you ever notice that a picture in a magazine is always more crisp and detailed than that on modern computer screens? You see laser printers can often print at over 1000 dots per inch, dpi whereas a computer can usually only display about 96 pixels per logical inch.
  

• There are seven built-in coordinate systems that can be used when working with graphics on forms and picture boxes. The default system is the twip coordinate system (1).
  
  
• Some coordinate systems do not work consistently on different monitors. However, you can use the two read-only properties called TwipsPerPixelX and TwipsPerPixelY to determine how much of an adjustment is being made by your monitor.

  
  The Coordinate Systems for Visual Basic

  ScaleMode	Coordinate System	Description
  1	Twips	1440 twips per inch
  2	Points	72 per inch
  3	Pixels	depends on monitor settings
  4	Characters	character = 1/6" high by 1/12" wide
  5	Inches	different monitors treat differently
  6	Millimeters	different monitors treat differently
  7	Centimeters	different monitors treat differently

Objective #2: Initialize and use user-defined coordinate systems appropriate to application programs.

• Since a form's default Height and Width properties are probably oddball values such as 3600 twips for height and 4800 for width. It is convenient to relabel the imaginary horizontal and vertical axes with numbers that are easier to work with. You can use the form's ScaleMode, ScaleHeight, and ScaleWidth properties to do this.
  
  
• You may set the ScaleMode property of a form to vbUser (or 0) rather than the default vbTwips (or 1). Then you can set the ScaleHeight and ScaleWidth properties to numbers that are easier to work with such as 100 or 1000.
  
  
• The default window coordinates do not work similarly as the typical mathematics graph. In Visual Basic's coordinate system (and most programming graphics environments) the y-values range from 0 to 100 as you follow down the screen or form from the upper-left corner.

Objective #3: Use the Circle and Line form methods.

• The Circle method is a built-in procedure used to draw circles on certain objects such as Picture Boxes, forms, or the Printer objects.
  
  
• The syntax to be used for this method is
  
  object.Circle (x, y), radius
  
  where object is a form, a shape control, a picture box or other objects that support the Circle method. The center of the circle is the point (x, y) and the radius is the desired radius of the circle.
  
  For example,
  
  frmMain.Circle(300, 400), 200
  
  would draw a circle on form frmMain with the center 300, 400 and radius of 200.
  
  
• The Line method is a built-in procedure that draws a line on certain objects and forms.
  
  The syntax to be used for this method is:
  
  frmMain.Line (x, y) - (x2, y2)    OR
  frmMain.Line - (x, y)
  
  where a line is drawn on the form between the points (x, y) and (x2, y2) in the first example. A line is drawn from the last point drawn to the point (x, y) in the second example.
  
  
• For example,
  
  frmMain.Line(300, 400) - (1000, 1200)
  
  would draw a line on frmMain from the point 300, 400 to the point 1000, 1200.

Objective #4: Use the RGB color function.

•  The RGB function can be used to assign any customized color to certain properties of various objects. This allows you to use a multitude of colors besides the VB color constants such as vbRed, vbBlue, etc.
  
  
• For example, you can set the background of a form to a unique color with the statement
  
  frmMain.BackColor = RGB(50, 23, 99)
  
  
• The RGB function takes 3 arguments. All three arguments must be integers in the range 0 to 255. The first argument refers to the amount of red, the second refers to the amount of green and the fourth refers to the amount of blue.
  
  
• To create the color black use the function call RGB(0, 0, 0). To create the color white, use the function call RGB(255, 255, 255).

Objective #5: Use the Shape control.

• A shape control can be used to place a graphic on a form.
• Use the prefix shp for shape objects.
• Shape controls do not respond to events such as Click.

Objective #6: Use Picture Box and Image controls to add graphics to a VB program.

• Almost any kind of graphic format including .bmp, .gif, .jpg can be displayed within a Picture Box control. After placing the Picture Box control on a form, set its Picture property to the any graphic file on your hard drive. The prefix pic is used for Picture Box objects.
  
  
• An Image control can also display a bitmap graphic through its Picture property. The prefix img is used for Image objects. The purpose of an Image control is only to display an image. It does not have nearly as many useful properties and methods and therefore cannot be used as flexibly as a Picture Box. However, an Image object is more resource-efficient (i.e. memory, etc.) for this reason and should not be overlooked.
  
  
• To load a new image in an Image control at runtime, use the LoadPicture function like this:
  
  imgGraphic.Picture = LoadPicture("Z:\Basic\Ch7\Ch7Proj1\images\spaceinvader.jpg")
  
  where the path of the actual graphic is the parameter of the VB LoadPicture function or simply
  
  imgGraphic.Picture = LoadPicture("spaceinvader.jpg")
  
  if the graphic file is located in the same folder as your project.
  
  To clear an image from an Image control, use the statement
  
  imgGraphic.Picture = ("")
  
  with the null string as the parameter.

Objective #7: Animate graphics on a form and use collision detection.

• You can achieve animation by simplying changing a Shape, Picture Box, or Image control's Top or Left property within a loop.
  
  
• For example, the following loop would animate the Shape object shpObject horizontally across a form (assuming that it is initially positioned along the left edge of the form):
  
  For J = 1 To (frmMain.ScaleWidth - shpObject.Width)
        shpObject.Left = shpObject.Left + 1
  Next J
  
  
• However, Shape objects have a Move method as well that can be used to animate the object across the form.
  
  
• Collision detection is code that responds to graphics when they touch each other or a boundary of the form. For example, the code
  
  Private Sub Form_Activate()
     Form1.ScaleMode = 0
     Form1.ScaleWidth = 100
     Form1.ScaleHeight = 100
     Form1.Line (0, 20)-(100,20)
  End Sub
  
  Private Sub Form_MouseMove(Button As Integer, Shift As Integer, X As Single, Y As Single)
    
   If (shpCircle.Top <= 20) Then
        MsgBox "Collision Occurred"
     End If
  
  End Sub
  
  could be used to detect a collision between the shpCircle object and the line drawn on the form.
  
  
• The following code could also be used to detect a collision by checking to see if the shpCircle object "straddles" the line
  
  Private Sub Form_MouseMove(Button As Integer, Shift As Integer, X As Single, Y As Single)
    
      If (shpCircle.Top > 20 And shpCircle.Top + shpCircle.Height > 20) Then
           MsgBox "Collision Occurred"
      End If
  
  End Sub
  

Objective #8: Create menus with the menu editor.

• Professional-looking menus can be added to a VB project with the menu editor. You can access the menu editor by choosing the Tools/Menu Editor... menu command.
• If a menu command object's Checked property is set to True, then a checkmark will appear to the left of the menu item. If it is set to False (which is the default), then a checkmark does not appear.
• If a menu command object's Enabled property is set to True (the default), then the menu command can be chosen by the user. If it is set to False, then the menu command will be "grayed out" so tha the user knows that he cannot choose that option.
• The Checked and Enabled properties of menu commands can be used to give feedback to the user and enhance the interface of your program. You should always use these properties with menu items when appropriate. Users appreciate it when menu commands are enabled only when they are appropriate choices. If you notice, most commercial Windows applications follow this convention.
• Often, limiting the user's to choosing appropriate commands prevents the programmer from having to test for illegal user inputs. Leaving clues and even error messages for the user explaining why certain commands are indeed inappropriate provides a better experience for the user. Things such as checkmarks placed next to previously selected menu items enhances the user interface of a program as well.
  
  
• A shortcut (popup) menu can be created by simply setting the top-level menu item's Visible property to False. Also, use the following code with the Form_Mouseup event:
  
  Private Sub Form_MouseUp(Button As Integer, Shift As Integer, X As Single, Y As Single)
  
        If (Button = vbRightButton) Then
             frmMain.PopupMenu mnuShortcut
        End If
  
  End Sub
  
  The Form's PopupMenu method is being used to display the menu command object mnuShortcut. The mnuShortcut's Visible property is set to False, however its menu command items are visible. The Button = vbRightButton is being used to tell whether the user's right-mouse button was clicked or not.

Objective #9: Use a scrollbar as a means of input to a program.

• There are two different scroll bar controls, the horizontal scrollbar and the vertical scrollbar. They work similarly except the one is horizontal and the other is vertical. Their prefixes are hsb and vsb.
  
  
• Scrollbars respond to their Change event. As the user moves the scrollbar, the Value property is updated for the scrollbar. By placing code in the scrollbar's Change event, you can make your program respond by the user's movement of the scrollbar.
  
  
• It is sometimes useful to use the value of the scrollbar as the subscript for an array.
  
  
• See Ch. 7 Demo #3 for an example of scrollbars.

Objective #10: Be able to use the three form events MouseDown, MouseUp, and MouseMove as well as a form's KeyPress & KeyDown events.

• A MouseDown event occurs when the user presses the mouse button somewhere on the form.
  
  
• A MouseUp event occurs when the user releases the mouse button. Note that the MouseUp event may occur in the very next instant after its corresponding MouseDown event if the user clicked the mouse quickly. Or, if the user decides to hold down the mouse button for a long time or he/she drags the mouse across the screen, the MouseUp event will occur much later than its corresponding MouseDown event.
  
  
• A MouseMove event occurs when the mouse's position changes.
  
  
• See Ch 7 Demo #5 for an illustration of the MouseDown, MouseUp, and MouseMove events.
  
  
• A form's KeyPress and KeyDown events allow your program to respond to presses of various keys as long as the Form has the focus.
  
  
• See Ch. 7 Demo #6 for an illustration of a program that uses the form'w KeyPress event.
  
  
• See Ch. 7 Demo #7 for an illustration of a program that uses the form's KeyDown event along with the arrow keys on the keyboard.

Objective #11: Use the Rnd function to generate random numbers within desired ranges.

• You can use the built-in VB function Rnd to generate random numbers. The Rnd function returns a decimal number between (but not including) 0 and 1.
  
  
• Using the statement
  
  intRandomNumber = Int (Rnd * (Hi - Lo + 1)) + Lo
  
  you can generate a random whole number between and including the values substituted for Lo and Hi and store that random number in the variable intRandomNumber.
  
  
• Use the Randomize statement first though in the Form_Load event. This "seeds" the VB random number generator so that your program doesn't generate the same random numbers every time it executes.