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Visual Basic: ballistics. Review on timers, animation Resolve vectors Homework: Catch up on projects. Read chapter 6. Try cannonball. . 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.
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Visual Basic: ballistics Review on timers, animation Resolve vectors Homework: Catch up on projects. Read chapter 6. Try cannonball.
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
Timer control • Properties: Interval and Enabled. • If Enabled is True, the timer event happens if Interval is > 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.
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.
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.
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
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)
Equations of motion • Constant velocity Distance traveled = velocity * time New_position = velocity*time + old_position • Acceleration (let g=acceleration) New_velocity = g * time + old_velocity Average velocity = .5 * g*time+old_velocity New_position = .5*g*time*time+old_velocity*time+old_position
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
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.
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.
