Random images falling like rain in canvas (Javascript)

问题

So I have been playing around with canvas. Trying to simulate random falling objects, drawing the background image is no problem and not the 2nd img that is supposed to simulate a rain drop.

I can get the drop to come down at a random x but now I am not really sure on how to loop the drop image x times set by the variable noOfDrops.

I left my loop commented out, the working code with just one drop falling randomly is:

<!DOCTYPE html>
<html lang="en">
<head>
<title>Canvas Regn</title>
<script type="text/javascript">
var ctx;
var imgBg;
var imgDrops;
var x = 0;
var y = 0;
var noOfDrops = 50;
var fallingDrops = [];

    function setup() {
        var canvas = document.getElementById('canvasRegn');

        if (canvas.getContext) {
                ctx = canvas.getContext('2d');
        setInterval('draw();', 36);
        imgBg = new Image();
        imgBg.src = 'dimma.jpg';

        imgDrops = new Image();
        imgDrops.src = 'drop.png';

        /*for (var i = 0; i < noOfDrops; i++) {
            var fallingDr = imgDrops[i];
            fallingDr.x = Math.random() * 600;
            fallingDrops.push(fallingDr);
            }*/

        }
    }


    function draw() {
        drawBackground();
        ctx.drawImage (imgDrops, x, y); //The rain drop

        y += 3; //Set the falling speed
        if (y > 450) {  //Repeat the raindrop when it falls out of view
        y = -25 //Account for the image size
        x = Math.random() * 600;    //Make it appear randomly along the width
        }
    }

    function drawBackground(){  
        ctx.drawImage(imgBg, 0, 0); //Background
    }
</script>
</head>
<body onload="setup();">
<canvas id="canvasRegn" width="600" height="450"style="margin:100px;"></canvas>
</body>
</html>

If anyone has some good ideas on how to implement that, I would appreciate it alot.

回答1:

Your loop is actually pretty close. The biggest problem you would have is that you can't just maintain 1 x and 1 y value, you have to maintain that per image. So I modified your loop a bit to push an object on the array that has an x,y, and speed value. The speed value gives you nice randomization of the movement, so everything doesn't come down at the same speed:

var ctx;
var imgBg;
var imgDrops;
var x = 0;
var y = 0;
var noOfDrops = 50;
var fallingDrops = [];


    function drawBackground(){  
        ctx.drawImage(imgBg, 0, 0); //Background
    }

    function draw() {
        drawBackground();

        for (var i=0; i< noOfDrops; i++)
        {
        ctx.drawImage (fallingDrops[i].image, fallingDrops[i].x, fallingDrops[i].y); //The rain drop

        fallingDrops[i].y += fallingDrops[i].speed; //Set the falling speed
        if (fallingDrops[i].y > 450) {  //Repeat the raindrop when it falls out of view
        fallingDrops[i].y = -25 //Account for the image size
        fallingDrops[i].x = Math.random() * 600;    //Make it appear randomly along the width    
        }

        }
    }

    function setup() {
        var canvas = document.getElementById('canvasRegn');

        if (canvas.getContext) {
                ctx = canvas.getContext('2d');

                    imgBg = new Image();
            imgBg.src = "http://lorempixel.com/600/600/sports/";
        setInterval(draw, 36);
        for (var i = 0; i < noOfDrops; i++) {
            var fallingDr = new Object();
            fallingDr["image"] =  new Image();
        fallingDr.image.src = 'http://lorempixel.com/10/10/sports/';

            fallingDr["x"] = Math.random() * 600;
            fallingDr["y"] = Math.random() * 5;
            fallingDr["speed"] = 3 + Math.random() * 5;
            fallingDrops.push(fallingDr);
            }

        }
    }

setup();

Here is a fiddle demo: http://jsfiddle.net/L4Qfb/21/

来源：https://stackoverflow.com/questions/13129479/random-images-falling-like-rain-in-canvas-javascript

标签

javascript

canvas