I'm pretty new to this and I got the animation working by watching a youtube tutorial.
Here is a canvas animation of a keyboard controlled car.
http://jsfiddle.net/unn9P/
canvas = document.getElementById('canvas');
c = canvas.getContext('2d');
c.clear = function() {
this.clearRect(0,0,1500,1500) };
function wait(fn) {
window.setTimeout(fn, 250) }
function repeat(fn) {
if (requestAnimationFrame) {
var advance = function() {fn(); requestAnimationFrame(advance);};
requestAnimationFrame(advance);
} else window.setInterval(fn, 50);
}
var dx = 0, dy = 0, mousex = 0, mousey=0, mouseclicks = 0;
document.onkeydown = function(e) {
var key = e.keyCode;
if (key == 37) dx=-1;
else if (key == 38) dy=-1;
else if (key == 39) dx=1;
else if (key == 40) dy=1;
else return true;
return false;
};
document.onkeyup = function(e) {
var key = e.keyCode;
if (key == 37 || key == 39) dx=0;
else if (key == 38 || key == 40) dy=0;
else return true;
return false;
};
canvas.onmousemove = function(e) {
var rect = canvas.getBoundingClientRect();
mousex = e.clientX - rect.left;
mousey = e.clientY - rect.top;
};
canvas.onmousedown = function(e) {mouseclicks++;};
a = new Image();
a.src = 'http://o.ooli.ca/car_top.png';
wait(function(){
x = 50;
y = 50;
angle = 0;
repeat(function() {
angle = angle + dx;
x = x - dy * Math.cos(angle * Math.PI / 180);
y = y - dy * Math.sin(angle * Math.PI / 180);
c.clear();
c.translate(x, y);
c.rotate(angle * Math.PI / 180);
c.translate(-37, -19);
c.drawImage(a, 0, 0);
c.setTransform(1, 0, 0, 1, 0, 0); //reset
});
});
Now I would like to add trail behind the car as it moves along and create a reset button inside the canvas which can help me clear the trail, and bring the car to its initial position.
I have read some tutorials but i can't seem to find what I want.
Is there any idea/suggestion on how I should do this?
Simply record your points when moving:
Modified fiddle here
repeat(function () {
angle = angle + dx;
x = x - dy * Math.cos(angle * Math.PI / 180);
y = y - dy * Math.sin(angle * Math.PI / 180);
/// record point
pts.push([x, y]);
c.clear();
/// render points (see below)
renderTrail(pts, c);
c.translate(x, y);
...
Then have a function to render the recorded points:
function renderTrail(pts, c) {
if (pts.length > 1) {
c.beginPath();
c.moveTo(pts[0][0], pts[0][1]);
for(var i = 1, pt; pt = pts[i]; i++) {
c.lineTo(pt[0], pt[1]);
}
c.stroke();
}
}
To reset just clear the point array:
pts = [];
You can do this on a mouse-click event on the canvas where you chose to draw your button or just put a html button on top of canvas (above it, literally on top will reduce the performance of canvas).
Related
So basically i had some code of a car that has speed and everything, now i want another one with the same properties. So i duplicated the whole code and changed the other cars url and all of the variables for it. I get 2 cars on my canvas but they keep blinking. I dont know why that is happening.
You can see it for yourself on JsBin:
http://jsbin.com/fekutakime/3/edit
The car code that i duplicated is:
//Setting the canvas and context
var canvas = document.getElementById('gameCanvas');
var context = canvas.getContext('2d');
//Uploading car
var car = new Image();
car.src = "http://images.clipartpanda.com/car-top-view-clipart-red-racing-car-top-view-fe3a.png";
//Setting properties of car
var x = 450;
var y = 730;
var speed = 10;
var angle = 990;
var mod = 0;
//Event listeners for keys
window.addEventListener("keydown", keypress_handler, false);
window.addEventListener("keyup", keyup_handler, false);
//Interval for animation
var moveInterval = setInterval(function () {
draw();
}, 30);
//Drawing the car turning and changing speed
function draw() {
context.clearRect(0, 0, canvas.width, canvas.height);
x += (speed * mod) * Math.cos(Math.PI / 180 * angle);
y += (speed * mod) * Math.sin(Math.PI / 180 * angle);
context.save();
context.translate(x, y);
context.rotate(Math.PI / 180 * angle);
context.drawImage(car, -(car.width / 2), -(car.height / 2));
context.restore();
}
//Setting the keys
function keyup_handler(event) {
if (event.keyCode == 38 || event.keyCode == 40) {
mod = 0;
}
}
//Setting all of the keys
function keypress_handler(event) {
console.log(x, y);
if (event.keyCode == 38) {
mod = 1;
}
if (event.keyCode == 40) {
mod = -1;
}
if (event.keyCode == 37) {
angle -= 5;
}
if (event.keyCode == 39) {
angle += 5;
}
}
I am building a space shooter game and would like the ship to fire rockets at the direction of the cursor. Therefore, I grab the radian value of the angle it should fire at, multiply it by the ship's speed and set it's x and y velocities respectively.
I have this as a Bullet class:
function Bullet(x, y) {
this.x = x;
this.y = y;
this.rotation = 0;
this.width = 6;
this.height = 3;
this.color = utils.getRandomColor();
this.speed = 80;
}
And here is the function which updates the movement of all instances of the bullet class:
function drawBullet(bullet) {
var dx = mouse.x - bullet.x,
dy = mouse.y - bullet.y,
angle = Math.atan2(dy, dx);
bullet.vx = Math.cos(angle) * bullet.speed;
bullet.vy = Math.sin(angle) * bullet.speed;
bullet.x += bullet.vx;
bullet.y += bullet.vy;
bullet.draw(ctx);
}
It starts okay, going in the right direction and velocity and stuff. But as soon as it reaches the mouse, it stops dead there and starts flickering. NOW, I realise that this is because of the way I am getting the angle, using the mouse position as a value - the problem is that I can't figure out a way to use just the angle for the velocity, not the distance to the mouse position. So it doesn't slow down.
All suggestions are welcome, thanks in advance!
If you don't need homing missile type behavior just pass the mouse coordinates when you create the bullet.
Example:
new Bullet(shooterX, shooterY, mouseX, mouseY)
I included an over engineered stack snippet but the relevant part is below.
var Bullet = function(x,y,tx,ty){
this.speed = 15;
this.x = x;
this.y = y;
var radians = Math.atan2(ty-y, tx-x);
// we now have our velX and velY we can just refer to
this.velX = Math.cos(radians) * this.speed;
this.velY = Math.sin(radians) * this.speed;
}
Bullet.prototype.update = function(){
// just update by our previous calculated velX and velY.
this.x += this.velX;
this.y += this.velY;
};
var canvas = document.getElementById("canvas"),
ctx = canvas.getContext("2d"),
width = 250,
height = 250,
output = document.getElementById("radians"),
output2 = document.getElementById("degrees"),
cX = 0,
cY = 0,
mX = 0,
mY = 0,
bullets = [];
canvas.width = width;
canvas.height = height;
canvas.addEventListener("mousemove", function (e) {
mX = e.pageX;
mY = e.pageY;
});
var Ball = function (x, y, radius, color) {
this.x = x || 0;
this.y = y || 0;
this.radius = radius || 10;
// makes our x and y the center of the circle.
this.x = (this.x-this.radius/2);
this.y = (this.y-this.radius/2);
// how far out do we want the point
this.pointLength = 50;
this.px = 0;
this.py = 0;
this.color = color || "rgb(255,0,0)";
}
Ball.prototype.shoot = function(tx, ty){
bullets.push(new Bullet(this.x, this.y, tx, ty));
}
Ball.prototype.update = function (x, y) {
// get the target x and y
this.targetX = x;
this.targetY = y;
var x = this.x - this.targetX,
y = this.y - this.targetY,
radians = Math.atan2(y,x);
this.px = this.x - this.pointLength * Math.cos(radians);
this.py = this.y - this.pointLength * Math.sin(radians);
// -y will make 0 the top, y will 0 us at the bottom.
output.textContent = radians;
output2.textContent = radians/Math.PI * 180
};
Ball.prototype.render = function () {
ctx.fillStyle = this.color;
ctx.beginPath();
ctx.arc(this.x, this.y, this.radius, 0, Math.PI * 2);
ctx.closePath();
ctx.fill();
ctx.strokeStyle = "rgb(0,0,255)";
ctx.beginPath();
ctx.moveTo(this.x, this.y);
ctx.lineTo(this.px, this.py);
ctx.closePath();
ctx.stroke();
};
var Bullet = function(x,y,tx,ty){
this.speed = 15;
this.x = x;
this.y = y;
var radians = Math.atan2(ty-y, tx-x);
this.velX = Math.cos(radians) * this.speed;
this.velY = Math.sin(radians) * this.speed;
}
Bullet.prototype.update = function(){
this.x += this.velX;
this.y += this.velY;
};
Bullet.prototype.render = function(){
ctx.fillStyle = '#000';
ctx.beginPath();
ctx.arc(this.x, this.y, 2, 0, Math.PI * 2);
ctx.closePath();
ctx.fill();
};
var ball1 = new Ball(width/2, height/2, 10);
canvas.addEventListener("click", function (e) {
ball1.shoot(e.pageX, e.pageY);
});
function render() {
ctx.clearRect(0, 0, width, height);
ball1.update(mX, mY);
ball1.render();
bullets.forEach(function(b){
b.update();
b.render();
});
requestAnimationFrame(render);
}
render();
ol{list-style:none;}
<canvas id="canvas"></canvas>
<div>
<ol>
<li>
<span>Radians : </span><span id="radians"></span>
</li>
<li>
<span>Degrees : </span><span id="degrees"></span>
</li>
</ol>
</div>
Add a new property on bullet that stores the angle of motion, initialize it to -1. Then, on the very first drawBullet call, check if it has been initialized first. If not, set the angle...
function Bullet(x, y) {
this.x = x;
this.y = y;
this.rotation = 0;
this.width = 6;
this.height = 3;
this.color = utils.getRandomColor();
this.speed = 80;
this.angle = -1; // New, angle property initialized to -1
}
function drawBullet(bullet) {
if (bullet.angle === -1) { // Only pull the mouse cursor and get an angle
var dx = mouse.x - bullet.x, // If it hasn't already done so.
dy = mouse.y - bullet.y,
angle = Math.atan2(dy, dx);
bullet.angle = angle;
}
bullet.vx = Math.cos(bullet.angle) * bullet.speed; // Re-use the angle value.
bullet.vy = Math.sin(bullet.angle) * bullet.speed;
bullet.x += bullet.vx;
bullet.y += bullet.vy;
bullet.draw(ctx);
}
I am trying to draw on canvas, like drawing using pencil tool in the paint using jquery mobile.
I searched for many links and most of them were for the desktop, i tried to implement the same logic for the mobile app, i am able to obtain only the click events but not able to draw the line on the canvas.
This is what i was trying to implement on the mobile http://jsfiddle.net/loktar/dQppK/23/
This is my code
$(document).on(
'pageshow',
'#canvaspage',
function() {
var painting = false;
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
// ctx.fillStyle="#FF0000";
// ctx.fillRect(0,0,150,75);
// ctx.drawImage(icons-18-black.png)
ctx.canvas.width = window.innerWidth * 0.8;
ctx.canvas.height = window.innerHeight * 0.8;
var imageObj = new Image();
imageObj.onload = function() {
ctx.drawImage(imageObj, 0, 0, ctx.canvas.width * 0.8,
ctx.canvas.height * 0.7);
};
imageObj.src = 'Image.png';
// c.addEventListener('touchstart', function(e) {
$("#myCanvas").on("touchstart",function(e){
painting = true;
e.preventDefault();
ctx.fillStyle = "#FF0000";
lastX = e.pageX - this.offsetLeft;
lastY = e.pageY - this.offsetTop;
});
// c.addEventListener('touchend', function(e) {
$("#myCanvas").on("touchend",function(e){
painting = false;
});
// c.addEventListener('touchmove', function(e) {
$("#myCanvas").on("touchmove",function(e){
if (painting) {
mouseX = e.pageX - this.offsetLeft;
mouseY = e.pageY - this.offsetTop;
// find all points between
var x1 = mouseX,
x2 = lastX,
y1 = mouseY,
y2 = lastY;
var steep = (Math.abs(y2 - y1) > Math.abs(x2 - x1));
if (steep){
var x = x1;
x1 = y1;
y1 = x;
var y = y2;
y2 = x2;
x2 = y;
}
if (x1 > x2) {
var x = x1;
x1 = x2;
x2 = x;
var y = y1;
y1 = y2;
y2 = y;
}
var dx = x2 - x1,
dy = Math.abs(y2 - y1),
error = 0,
de = dy / dx,
yStep = -1,
y = y1;
if (y1 < y2) {
yStep = 1;
}
lineThickness = 5 - Math.sqrt((x2 - x1) *(x2-x1) + (y2 - y1) * (y2-y1))/10;
if(lineThickness < 1){
lineThickness = 1;
}
alert(painting +" " +x1 +" "+x2);
for (var x = x1; x < x2; x++) {
// alert(x +" "+ y +" "+ lineThickness);
if (steep) {
ctx.fillRect(y, x, lineThickness , lineThickness );
} else {
ctx.fillRect(x, y, lineThickness , lineThickness );
}
alert(steep);
error += de;
if (error >= 0.5) {
y += yStep;
error -= 1.0;
}
}
lastX = mouseX;
lastY = mouseY;
}
// ctx.fillRect(0, 0, 150, 75);
e.preventDefault();
}, false);
});
In the above code i am able to obtain all the touch events but the unable to draw the line.
How can i draw the lines on the canvas??..
Thanks:)
You can use sketch.js (http://intridea.github.io/sketch.js/) with a small modification to make it work on mobile. The modification is given in the comment by leonth here: https://github.com/intridea/sketch.js/issues/1; you basically add 3 lines to the plugin on the mousedown/touchstart event:
switch (e.type) {
case 'mousedown':
case 'touchstart':
if (this.painting) { //add
this.stopPainting(); //add
} //add
this.startPainting();
break;
...
Here is a DEMO FIDDLE, try it from mobile device.
I've created this animation for my project that had to use any form of physics.
I am a total beginner, too :) Anyway, this is my project now :
Bouncing Balls
You can setup gravity and force, then click play, and just drag and drop to shoot the balls. You can change the values and hit update too see an effect.
My question is, how can I create an effect that when I press ratio button (for example) I can see the path that ball makes? Is it complicated? As I was saying I am a beginner, so no complex code for me :)
Also, doyou have any ideas to make the project better? Any additional "physics" effects? Or maybe you know a website that shows tutorials for simile (please) effects made in HTML5/js so I can add additional effects to my project.
One possibility (as you're clearing the canvas each frame) would be to draw ball paths onto a secondary canvas, which would not be cleared each frame. Then, when you come to clear the first frame, render the second frame after clearing, and before rendering the balls.
The second canvas would of course have to be the same dimensions as the first, so that all of the ball points line up correctly. The second canvas should also have a z-index lower than the first, so that it is only shown when you specifically render it to the first canvas (i.e. when the radio button is checked).
To decrease any lag while the radio is not checked, you could skip drawing the ball paths to the second canvas, although I don't think you would see any great increase in performance.
On each frame update, you would mark the position of each ball with a pixel, or line (from the previous position to the current) on the second canvas.
Looking at your code, you seem pretty competent, so I've skipped writing an example as I think this would be good experience for you :)
Modified 'script.js' source demonstrating solution
window.onload = function(){
$("#canvas").hide();
var howManyPaths = 0;
var showPath=false;
// SLIDERS
var gravitySlider = document.getElementById('gravitySlider');
var gravityVal = document.getElementById('gravityValue');
gravitySlider.onchange = function(){
gravityVal.value = gravitySlider.value;
}
gravityVal.onkeyup = function(){
gravitySlider.value = gravityVal.value;
}
var forceSlider = document.getElementById('forceSlider');
var forceValue = document.getElementById('forceValue');
forceSlider.onchange = function(){
forceValue.value = forceSlider.value;
}
forceValue.onkeyup = function(){
forceSlider.value = forceValue.value;
}
// GLOBAL VARIABLES
var test = false;
var gravityCount = $("#gravity").val();
var forceCount = $("#rectangles").val();
// CSS :
var playCSS = document.getElementById("play");
var restartCSS = document.getElementById("restart");
var clickableCSS = document.getElementById("setup");
var clickableBG = document.getElementById("img");
//restartCSS.style.visibility="hidden";
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var canvas2 = document.getElementById("canvas2");
var ctx2 = canvas2.getContext("2d");
//var ctx;
var gravity = 9.86;
var forceFactor = 0.5;
var mouseDown = false;
var balls = new Array();
var mousePos = new Array();
// EVENT HANDLER
function onMouseDown(evt){
mouseDown = true;
mousePos['downX'] = evt.pageX;
mousePos['downY'] = evt.pageY;
}
function onMouseUp(evt){
mouseDown = false;
setup.style.visibility="visible";
if(test == true && !( mousePos['downX'] < 200 && mousePos['downY'] < 150) ){
restartCSS.style.visibility="visible";
forceFactor = forceCount;
balls.push(new ball(mousePos["downX"],
mousePos["downY"],
(evt.pageX - mousePos["downX"]) * forceFactor,
(evt.pageY - mousePos["downY"]) * forceFactor,
10 + (Math.random() * 10),
0.8,
randomColor()
));
}
ctx2.clearRect(0, 0, canvas2.width, canvas2.height);
}
function onMouseMove(evt){
mousePos['currentX'] = evt.pageX;
mousePos['currentY'] = evt.pageY;
}
function resizeWindow(evt){
//canvas.height = 960;
//canvas.width = 720;
canvas.height = $(window).height()-6;
canvas.width = $(window).width();
canvas2.height = $(window).height()-6;
canvas2.width = $(window).width();
}
$(document).mousedown(onMouseDown);
$(document).mouseup(onMouseUp);
$(document).mousemove(onMouseMove);
$(window).bind("resize", resizeWindow);
// GRAPHICS CODE
function circle(x, y, r, col){
ctx.beginPath();
ctx.arc(x, y, r, 0, Math.PI*2, true);
ctx.closePath;
// fill
ctx.fillStyle = col;
ctx.fill();
// stroke
ctx.lineWidth = r * 0.1;
ctx.strokeStyle = "#000000";
ctx.stroke();
}
function circlePath(x, y)
{
ctx2.clearRect(0, 0, canvas2.width, canvas2.height);
ctx2.fillStyle = '#3f4043';
ctx2.fillRect(x, y, 5, 5);
ctx2.strokeStyle = "black";
ctx2.strokeRect(x, y, 5, 5);
}
function randomColor(){
var letter = "0123456789ABCDEF".split("");
var color = "#";
for(var i=0; i<6; i++){
color += letter[Math.round(Math.random()*15)];
}
return color;
}
function arrow(fromX, fromY, toX, toY, color){
// path
ctx.beginPath();
var headLen = 10;
var angle = Math.atan2(toY - fromY, toX - fromX);
ctx.moveTo(fromX, fromY);
ctx.lineTo(toX, toY);
ctx.lineTo(toX - headLen * Math.cos(angle - Math.PI/6), toY - headLen * Math.sin(angle - Math.PI/6));
ctx.moveTo(toX, toY);
ctx.lineTo(toX - headLen * Math.cos(angle + Math.PI/6), toY - headLen * Math.sin(angle + Math.PI/6));
// style
ctx.lineWith = 1;
ctx.strokeStyle = color;
ctx.lineCap = "butt";
ctx.stroke();
}
function drawBall(){
// Gravity
gravity = gravityCount;
this.speedY += gravity * 0.5; // v = a * t
this.x += this.speedX * 0.05; // s = v * t
this.y += this.speedY * 0.05;
// prawa ściana
if(this.x + this.r > canvas.width){
this.x = canvas.width - this.r;
this.speedX *= -1 * this.bounce;
}
// lewa ściana
if(this.x - this.r < 0){
this.x = this.r;
this.speedX *= -1 * this.bounce;
}
// dolna ściana
if(this.y + this.r > canvas.height){
this.y = canvas.height - this.r;
this.speedY *= -1 * this.bounce;
}
// górna ściana
if(this.y - this.r < 0){
this.y = this.r;
this.speedY *= -1 * this.bounce;
}
// zwalnianie na ziemi
if (this.speedX > 0.25){
this.speedX -= 0.25;
if (this.speedY > 0.25)
this.speedY -= 0.25;
}
if (this.speedX < -0.25){
this.speedX += 0.25;
//if (this.speedY < -0.25)
// this.speedY += 0.25;
}
circle(this.x, this.y, this.r, this.col);;
}
// OBJECTS
function ball(positionX, positionY, sX, sY, radius, b, color){
this.x = positionX;
this.y = positionY;
this.speedX = sX;
this.speedY = sY;
this.r = radius;
this.bounce = b;
this.col = color;
this.draw = drawBall;
}
//GAME LOOP
function gameLoop(){
ctx.clearRect(0, 0, canvas.width, canvas.height);
//grab the context from your destination canvas
//if path drawing is enabled, first draw the path canvas to the display canvas
if (showPath) ctx.drawImage(canvas2,0,0);
if(mouseDown == true){
// ctx.clearRect(0, 0, canvas.width, canvas.height); /* !important !!!!!!!!!!!!!!! */
arrow(mousePos['downX'], mousePos['downY'], mousePos['currentX'], mousePos['currentY'], "red");
}
for(var i=0; i<balls.length; i++){
balls[i].draw();
if (i==balls.length-1) {
//draw path
ctx2.fillStyle = '#3f4043';
ctx2.fillRect(balls[i].x, balls[i].y, 5, 5);
ctx2.strokeStyle = "black";
ctx2.strokeRect(balls[i].x, balls[i].y, 5, 5);
}
}
ctx.fillStyle = "#000000";
ctx.font = "15px Arial";
ctx.fillText("Balls: " + balls.length + " " + gravityCount + " " + forceCount + " " + howManyPaths, 10, canvas.height -10);
}
// START THE GAME
function init(){
//$("#setup").hide();
$("#canvas").show();
$("#canvas2").hide();
ctx = $('canvas')[0].getContext("2d");
canvas.height = $(window).height()-6;
canvas.width = $(window).width();
//canvas.width = 960;
//canvas.height = 720;
canvas2.height = $(window).height()-6;
canvas2.width = $(window).width();
return setInterval(gameLoop, 10);
}
$("#play").click(function() {
test = true;
playCSS.style.visibility="hidden";
gravityCount = $("#gravitySlider").val();
forceCount = $("#forceSlider").val();
init();
});
$("#restart").click(function() {
window.location.href="index.html";
});
$("#refresh").click(function() {
gravityCount = $("#gravitySlider").val();
forceCount = $("#forceSlider").val();
});
$("#showPath").click(function() {
showPath=true;
});
$("#hidePath").click(function() {
showPath=false;
});
}
I am using very similar code to create a pie chart using canvas as per this article:
http://wickedlysmart.com/how-to-make-a-pie-chart-with-html5s-canvas/
As you can see from this image, there are cases where the labels are upside down:
Here is the code that writes the labels to the graph:
var drawSegmentLabel = function(canvas, context, i) {
context.save();
var x = Math.floor(canvas.width / 2);
var y = Math.floor(canvas.height / 2);
var degrees = sumTo(data, i);
var angle = degreesToRadians(degrees);
context.translate(x, y);
context.rotate(angle);
context.textAlign = 'right';
var fontSize = Math.floor(canvas.height / 32);
context.font = fontSize + 'pt Helvetica';
var dx = Math.floor(canvas.width * 0.3) - 20;
var dy = Math.floor(canvas.height * 0.05);
context.fillText(labels[i], dx, dy);
context.restore();
};
I am trying to rectify this so the text is always readable and not upside down but cant work out how to do it!
Here's my solution! (A little kludgey but seems to work on the basic example, I haven't tested in on edge cases...)
var drawSegmentLabel = function(canvas, context, i) {
context.save();
var x = Math.floor(canvas.width / 2);
var y = Math.floor(canvas.height / 2);
var angle;
var angleD = sumTo(data, i);
var flip = (angleD < 90 || angleD > 270) ? false : true;
context.translate(x, y);
if (flip) {
angleD = angleD-180;
context.textAlign = "left";
angle = degreesToRadians(angleD);
context.rotate(angle);
context.translate(-(x + (canvas.width * 0.5))+15, -(canvas.height * 0.05)-10);
}
else {
context.textAlign = "right";
angle = degreesToRadians(angleD);
context.rotate(angle);
}
var fontSize = Math.floor(canvas.height / 25);
context.font = fontSize + "pt Helvetica";
var dx = Math.floor(canvas.width * 0.5) - 10;
var dy = Math.floor(canvas.height * 0.05);
context.fillText(labels[i], dx, dy);
context.restore();
};
To display the text in the correct way you have to check if the rotation angle is between 90 and 270 degree. If it is then you know the text will be display upside down.
To switch it correctly you then have to rotate you canvas of planed rotation - 180 degree and then to align it in left not right :
var drawSegmentLabel = function(canvas, context, i) {
context.save();
var x = Math.floor(canvas.width / 2);
var y = Math.floor(canvas.height / 2);
var degrees = sumTo(data, i);
var angle = 0;
if (degree > 90 && degree < 270)
angle = degreesToRadians(degrees - 180);
else
angle = degreesToRadians(degrees);
context.translate(x, y);
context.rotate(angle);
context.textAlign = 'right';
var fontSize = Math.floor(canvas.height / 32);
context.font = fontSize + 'pt Helvetica';
var dx = Math.floor(canvas.width * 0.3) - 20;
if (degree > 90 && degree < 270)
dx = 20;
var dy = Math.floor(canvas.height * 0.05);
context.fillText(labels[i], dx, dy);
context.restore();
};