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Created Sep 3, 2011
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Control a tank in HTML5 Canvas
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<title>Chapter 4 Example 9: Animating an image with rotation</title>
<script src="http://cdnjs.cloudflare.com/ajax/libs/modernizr/2.0.6/modernizr.min.js"></script>
<script src="http://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.1.7/underscore-min.js"></script>
<script src="ex9.js"></script>
</head>
<body>
<div style="position: absolute; top: 50px; left: 50px;">
<canvas id="canvas" width="500" height="500">
Your browser does not support the HTML 5 Canvas.
</canvas>
</div>
</body>
</html>
window.addEventListener('load', function() { canvasApp(); }, false);
var KEYS = {
UP: 38,
DOWN: 40,
LEFT: 37,
RIGHT: 39
};
var DIRECTIONS = {
38: { dx: 0, dy: -1 }, // UP
40: { dx: 0, dy: 1 }, // DOWN
37: { dx: -1, dy: 0 }, // LEFT
39: { dx: 1, dy: 0 } // RIGHT
};
var keyPressList = {};
document.onkeydown = function(ev) {
ev = ev || window.event;
keyPressList[ev.keyCode] = true;
};
document.onkeyup = function(ev) {
ev = ev || window.event;
keyPressList[ev.keyCode] = false;
};
function getAngle() {
if (keyPressList[KEYS.UP]) {
if (keyPressList[KEYS.LEFT]) { return 315; }
else if (keyPressList[KEYS.RIGHT]) { return 45; }
else { return 0; }
}
else if (keyPressList[KEYS.DOWN]) {
if (keyPressList[KEYS.LEFT]) { return 225; }
else if (keyPressList[KEYS.RIGHT]) { return 135; }
else { return 180; }
}
else if (keyPressList[KEYS.LEFT]) { return 270; }
else if (keyPressList[KEYS.RIGHT]) { return 90; }
else return null;
}
function getDeltas() {
var deltas = { dx: 0, dy: 0 };
var pressedKeys = _.select(_.keys(keyPressList), function(key) { return keyPressList[key]; });
_.each(pressedKeys, function(key) {
var delta = DIRECTIONS[key];
deltas.dx += delta.dx;
deltas.dy += delta.dy;
});
return deltas;
}
function getDirection(currentDirection) {
var deltas = getDeltas();
var angle = getAngle();
return {
dx: deltas.dx,
dy: deltas.dy,
angle: _.isNull(angle) ? currentDirection.angle : angle
};
}
function tankIsStationary(direction) {
return (direction.dx === 0 && direction.dy === 0);
}
function canvasApp() {
if (!Modernizr.canvas) {
return;
} else {
var theCanvas = document.getElementById('canvas');
var context = theCanvas.getContext('2d');
}
var tileSheet = new Image();
tileSheet.addEventListener('load', function() { setInterval(drawScreen, 100); }, false);
tileSheet.src = "tanks_sheet.png";
var animationFrames = [1,2,3,4,5,6,7,8];
var frameIndex = 0;
var x = 50;
var y = 50;
var direction = { dx: 0, dy: 0, angle: 180 };
function drawScreen() {
direction = getDirection(direction);
var speed = 5;
x = x + (direction.dx * speed);
y = y + (direction.dy * speed);
//draw a background so we can wee the Canvas edges
context.fillStyle = "#aaaaaa";
context.fillRect(0,0,500,500);
context.save();
context.setTransform(1, 0, 0, 1, 0, 0);
var rotation = direction.angle;
var angleInRadians = rotation * Math.PI / 180;
context.translate(x+16, y+16)
context.rotate(angleInRadians);
var sourceX = Math.floor(animationFrames[frameIndex] % 8) *32;
var sourceY = Math.floor(animationFrames[frameIndex] / 8) *32;
context.drawImage(tileSheet, sourceX, sourceY, 32, 32, -16, -16, 32, 32);
context.restore();
if (!tankIsStationary(direction)) {
frameIndex++;
if (frameIndex == animationFrames.length) {
frameIndex=0;
}
}
}
}
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