如何在移动时给我的javascript球滚动动画？

Question

I have the following HTML code:

<img src="game_files/ball.png" id="ball" /> It's a div containing an image of a ball (top view of a ball)

This ball I can move with my arrow keys and some javacript to go up, left, right, down, etc etc. I do this with this code:

var ball = function  () {
    var inited = false,
        el,
        xcheck = 50,
        x = 50,
        ycheck = 50,
        y = 50,
        xspeed = 0,
        yspeed = 0,
        power = 0.4,
        friction = 0.99,
        xwind = 0,
        ywind = 0,
        dead = true,
        timer = function(keys) {
            if (dead === true) {
                return;
            }
            if (keys[38] === true) {
                yspeed += power;
            }
            if (keys[40] === true) {
                yspeed -= power;
            }
            if (keys[37] === true) {
                xspeed += power;
            }
            if (keys[39] === true) {
                xspeed -= power;
            }
            xspeed = xspeed * friction - xwind;
            yspeed = yspeed * friction - ywind;
            if (Math.abs(xspeed) < 0.004) {
                xspeed = 0;
            }
            if (Math.abs(xspeed) < 0.004) {
                xspeed = 0;
            }
            x -= xspeed;
            y -= yspeed;
            plane.move(x,y);
        };
    return {
        init: function() {
            if (inited != true) {
                inited = true;
                el = document.getElementById('ball');
                roller.register(timer, 'time');
            }
        }
    };
}();

This all works, but the ball has no rolling animation! The image just slides to the left or right How can I add this? I found this tutorial: http://www.emanueleferonato.com/2007/06/10/creation-of-realistic-spheres-in-flash-with-textures-and-masking/ which I thought could help me out.

Unfortunately, this is for a flash ball (I was hoping this would apply in some sort to JS too). This tutorial shows exactly what I want: a ball with rolling animation (see the tutorial page near the end of the tutorial the ball with the leopard skin, just below: Lines 26-37: If the position of the texture relatively to the ball...).

How can I apply this to my JS ball? Any thoughts?

Kind regards and a happy new year

ps I use/load jquery as well

--EDIT--------

Created a fiddle: http://jsfiddle.net/mauricederegt/dhUw5/

1- open the fiddle

2- click the 1

3- use the arrow keys to move the ball around, stay on the green field!

4- see, no rolling effect

Answer 1

To have the same effect as in the Flash example you linked you just have to add a background to the element and move that accordingly.

I replaced your img element with a span and gave it the background-image via CSS and a border-radius to make it round. In your plane.move function I added the following line:

document.getElementById('ball').style.backgroundPosition = x + 'px ' + y +'px';

Voilá, same effect as in the Flash example: http://jsfiddle.net/dhUw5/1/

Answer 2

You would probably use HTML5 canvas. You would clip part of the image used as the ball's surface using drawImage and then mask it to get a circle shape . Then you can animate it by redrawing the canvas, with the clipping position (sx, sy) altered in a similar way to the flash example you linked to. (Following is not tested; make a jsfiddle with your original code if you want to try it.)

// ... when the user moves
this.sy += yspeed;
this.sx += xspeed;
if (this.sx > textureSize) {
    this.sx -= textureSize;
}
else if (this.sx < 0) {
    this.sx += textureSize;
}
if (this.sy > textureSize) {
    this.sy -= textureSize;
}
else if (this.sy < 0) {
    this.sy += textureSize;
}

// ... redraw the ball once every frame
context.clearRect(0, 0, canvasWidth, canvasHeight); // clear the canvas
context.save();
context.beginPath();
context.arc(this.x, this.y, ballRadius, Math.PI * 2, false);
context.clip();    
context.drawImage(ballImage, this.sx, this.sy, 
                  ballDiameter, ballDiameter, 
                  this.x, this.y,
                  ballDiameter, ballDiameter);      // redraw the ball      
context.restore();

Alternatively you could use a div with the texture as a background image, and mask it to make a circle using CSS3 or by overlaying another image (see Best way to mask an image in HTML5 ). Then you would change the coordinates of the background image (texture) using

ballElement.style.backgroundPosition = this.sx + 'px ' + this.sy + 'px';

Answer 3

This is quite easy if you use CSS transforms. In this example the speed of rotation is determined according to the scalar value of the speed the ball is currently traveling at:

    var increment = Math.round(Math.sqrt(Math.pow(xspeed, 2) + Math.pow(yspeed, 2)));
    if (!isNaN(increment)) {
        currangle = currangle + ((xspeed < 0 ? 1 : -1) * increment);
    }

    $('#ball').css({
        '-webkit-transform': 'rotate(' + currangle + 'deg)'
    });

    $('#ball').css({
        '-moz-transform': 'rotate(' + currangle + 'deg)'
    });

This code goes in the move method in plane . Here is a demonstration: http://jsfiddle.net/BahnU/show/