Visual Basic: ballistics

1
Visual Basic ballistics

• Review on timers, animation
• Resolve vectors
• Homework Catch up on projects. Read chapter 6.
  Try cannonball.

2
Coordinate system

• Most Visual Basic controls are positioned in
  terms of Top and Left.
• Lines are positioned in terms of x1, y1, x2, y2
  values.
• As in most computer systems,
• the Left/X values increase moving to the right
• the Top/Y values increase moving DOWN the screen

3
Timer control

• Properties Interval and Enabled.
• If Enabled is True, the timer event happens if
  Interval is gt 0
• Interval is set in milliseconds
• If a timer named timFalling has Enabled set to
  True and Interval set to 500 then
  timFalling_Timer will happen every ½ second.

4
Sub timFalling_Timer

• shpBall.Top shpBall.Top shpBall.Height
• This causes the ball to drop its whole height
  every interval.
• shpBall.Top shpBall.Top delta v_speed
• Assume delta represents the elapsed time and
  v_speed represents a speed, this causes the ball
  to drop an amount equal to time speed.
• shpBall.Top originalTop T v_speed
• This positions the ball each time based on a
  calculation from originalTop (presumably the
  starting position), T is the elapsed time.

5
Bouncing ball

• Recall that in the timer event, the object was
  moved horizontally (Left) and vertically (Top)
• Also, check was made if object struck sides of
  the form.

6
Ballistics

• Simulation of motion must take gravity into
  account.
• Horizontal motion continues with no acceleration
  (no change)
• Vertical motion is changed by gravity.
• Physics computer graphics requires when
  cannonball is shot out of cannon at an angle,
  your code must resolve the initial velocity
  vector into horizontal and vertical components
• Velocity is defined as speed and direction

7
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8
Calculating the vectors

• Angle
• vx v Cos (Theta)
• vy v Sin (Theta)
• Where v is velocity coming out of cannon, theta
  is the angle.

Angle (traditional name is theta)
9
Equations of motion

• Constant velocity
• Distance traveled velocity time
• New_position velocitytime old_position
• Acceleration (let gacceleration)
• New_velocity g time old_velocity
• Average velocity .5 gtimeold_velocity
• New_position .5gtimetimeold_velocitytimeol
  d_position

10
Horizontal and vertical velocities

• (switching to VB names for variables)
• X2,Y2 is endpoint of cannon
• Initial horizontal velocity remains the same
• sngXX sngVx sngTT X2
• Vertical velocity changes
• sngYY .5 g (sngTT sngTT) - sngVy sngTT
  Y2

11
Overview of cannonball

• Note read chapter 6!
• Command button with caption FIRE! will calculate
  initial horizontal and vertical velocity from the
  scroll bar for the speed and the angle of the
  line representing the cannon
• Timer event will increment variable for time and
  apply the equations of motion to the cannonball
  (a shape control object)
• Timer event also does calculation to determine if
  cannonball has hit the ground or hit the target.
• Mouse events used to drag cannon tip and target.
  Horizontal scroll bar used for speed.

12
Staged implementation

• Cannonball moves through the air. No checks to
  stop it! Stop execution by clicking on stop
  button on toolbar.
• Check for hitting ground or hitting target.
• Implement event handlers for changing speed,
  moving tip of cannon.
• Staged implementation is highly recommended for
  all but the smallest of projects.