Generate random images in canvas in javascript
Good afternoon, I am trying to make a javascript game using a canvas. I want to create random enemy objects. So far I've found this as a spawning example: JSFiddle Demo
How can I upload images instead of balls? Any help would be great. Tnx.
// get a refrence to the canvas and its context
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
// newly spawned objects start at Y=25
var spawnLineY = 25;
// spawn a new object every 1500ms
var spawnRate = 1500;
// set how fast the objects will fall
var spawnRateOfDescent = 0.50;
// when was the last object spawned
var lastSpawn = -1;
// this array holds all spawned object
var objects = [];
// save the starting time (used to calc elapsed time)
var startTime = Date.now();
// start animating
animate();
function spawnRandomObject() {
// select a random type for this new object
var t;
// About Math.random()
// Math.random() generates a semi-random number
// between 0-1. So to randomly decide if the next object
// will be A or B, we say if the random# is 0-.49 we
// create A and if the random# is .50-1.00 we create B
if (Math.random() < 0.50) {
t = "red";
} else {
t = "blue";
}
// create the new object
var object = {
// set this objects type
type: t,
// set x randomly but at least 15px off the canvas edges
x: Math.random() * (canvas.width - 30) + 15,
// set y to start on the line where objects are spawned
y: spawnLineY,
}
// add the new object to the objects[] array
objects.push(object);
}
function animate() {
// get the elapsed time
var time = Date.now();
// see if its time to spawn a new object
if (time > (lastSpawn + spawnRate)) {
lastSpawn = time;
spawnRandomObject();
}
// request another animation frame
requestAnimationFrame(animate);
// clear the canvas so all objects can be
// redrawn in new positions
ctx.clearRect(0, 0, canvas.width, canvas.height);
// draw the line where new objects are spawned
ctx.beginPath();
ctx.moveTo(0, spawnLineY);
ctx.lineTo(canvas.width, spawnLineY);
ctx.stroke();
// move each object down the canvas
for (var i = 0; i < objects.length; i++) {
var object = objects[i];
object.y += spawnRateOfDescent;
ctx.beginPath();
ctx.arc(object.x, object.y, 8, 0, Math.PI * 2);
ctx.closePath();
ctx.fillStyle = object.type;
ctx.fill();
}
}
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One way is to have a global array of loaded images:
var images = [img1, img2, ... ];
When an object is created, it is assigned randomly to one of the images in the array images
. We can then render this image using the ctx.drawImage
: method ctx.drawImage(object.image,...))
. The important thing to remember now is that you need to load all of your images first like this:
var img1 = new Image();
img.src = "...";
And to only start the animation on load, we can use onload
:
window.onload = function() {
animate();
}
Here is the obj.image property that we add to our creation object
. This is a selection of a random image from our array images
.
var object = {
...
// give random image
image: images[Math.floor(Math.random()*images.length)]
}
In any case, we can remove the drawing functions arc
and draw, and use object.image
to draw:
for (var i = 0; i < objects.length; i++) {
var object = objects[i];
object.y += spawnRateOfDescent;
ctx.drawImage(object.image, object.x, object.y, 30, 30);
}
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