HTML and CSS Reference
In-Depth Information
You have seen the code to create an effect like this already. In the canvasApp() function,
we would create the starting variables like this:
var speed = 4;
var angle = 305;
var radians = angle * Math.PI/ 180;
var radius = 15;
var vx = Math.cos(radians) * speed;
var vy = Math.sin(radians) * speed;
Next, we create the starting point for the ball as p1 , and then create a dynamic object
that holds all the values we created for the ball object:
var p1 = {x:20,y:theCanvas.width-radius};
var ball = {x:p1.x, y:p1.y, velocityx: vx, velocityy:vy, radius:radius};
If we want to add gravity to the application, we would first create a new variable named
gravity and set it to a constant value of .1 :
var gravity = .1;
Next, in the drawScreen() function, we apply this gravity value to the ball object when
it is drawn to the canvas ( ball.velocityy += gravity ). We want the ball to stop moving
when it hits the “ground” (the bottom of the canvas), so we test to see whether the y
position of the ball plus the radius of the ball (the outer edge) has passed the bottom
of the canvas ( ball.y + ball.radius <= theCanvas.height ). If so, we stop the ball's
if (ball.y + ball.radius <= theCanvas.height) {
ball.velocityy += gravity;
} else {
ball.velocityx = 0;
ball.velocityy = 0;
ball.y = theCanvas.height - ball.radius;
Next, we apply the constant x velocity and the new y velocity to ball , and draw it to
the canvas:
ball.y += ball.velocityy;
ball.x += ball.velocityx;
context.fillStyle = "#000000";
Figure 5-17 shows what the path looks like when simple gravity is applied to a ball
moving on a vector. We have added the points to illustrate the path.
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