Question

美好的一天，我正在尝试使用画布制作一个javascript游戏。我想产生随机的敌人物体。到目前为止，我发现这是一个产卵示例：JSFiddle Demo

如何加载图像而不是球？任何帮助都会很棒。 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();
}

}

Answer 1

Fiddle Here

一种方法是拥有一个全局加载图像数组：

var images = [img1, img2, ... ];

创建对象时，它会随机分配给images数组中的一个图像。然后，我们可以使用ctx.drawImage方法绘制图像：ctx.drawImage(object.image,...))。现在记住你需要首先加载所有图像是非常重要的：

var img1 = new Image();
img.src = "...";

要仅在加载所有内容时运行动画，我们可以使用onload：

window.onload = function() {
    animate();
}

这是我们添加到object创建的obj.image属性。这是从images数组中选择一个随机图像。

var object = {
    ...
    // give random image
    image: images[Math.floor(Math.random()*images.length)]
}

最后我们可以删除arc绘制函数并绘制，并使用object.image绘制：

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);
}

在javascript中在画布中生成随机图像

1 个答案: