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Objects drawn on my canvas are not updating [duplicate]

I was trying to do an Olympic type flag, purely as a way of learning how to draw in JavaScript. This should draw two circles - one blue, one black...
Here is the code (which I apologise for, been moving things between the two functions - Not sure how to refer to the context non-explicitly):
function drawCircle(ctx,x,y,radius, color){
var startAngle = 0;
var endAngle = (Math.PI*2);
var clockwise = true;
ctx.fillStyle = color;
ctx.arc(x,y,radius,startAngle,endAngle, clockwise);
ctx.fill();
ctx.closePath;
}
function drawCircles(){
var canvas = document.getElementById('myCanvasArea');
var ctx = canvas.getContext('2d');
if (canvas.getContext){
drawCircle(ctx,50,25,25, 'blue');
drawCircle(ctx,100,25,25, 'black');
}
}
I get two black circles. I presume I'm not differentiating between the two shapes, therefore the properties of the 2nd are applied to the 1st.
How do I make that distinction? I was thinking of making clicking each one raise an action. Am I going about this incorrectly from the start?

It's because you are never calling beginPath()!
function drawCircle(ctx,x,y,radius, color){
var startAngle = 0;
var endAngle = (Math.PI*2);
var clockwise = true;
ctx.fillStyle = color;
ctx.beginPath(); // <-- Need me!!
ctx.arc(x,y,radius,startAngle,endAngle, clockwise);
ctx.fill();
ctx.closePath;
}
Since you don't call beginPath, you are drawing one blue circle, then you are continuing a path that now has two circles (the old one and the new one), and drawing that path (and thus both circles) black!
Instead you want to draw one blue circle, fill it blue, begin a new path, and draw that one black.
Live code:
http://jsfiddle.net/5PDUb/1/

html5 canvas drawing position not correct

I am currently writing a simple snake game.
http://jsfiddle.net/jhGq4/3/
I started with drawing the grid for the background
function fill_grid() {
ctx.beginPath();
var row_no = w/cw;
var col_no = h/cw;
for (var i=0;i<row_no;i++)
{
for (var j=0;j<col_no;j++)
{
ctx.rect(i*cw, j*cw, (i+1)*cw, (j+1)*cw);
ctx.fillStyle = 'black';
ctx.fill();
ctx.lineWidth = 1;
ctx.strokeStyle = '#135d80';
ctx.stroke();
}
}
}
It works great but when i paint the snake, the position gets wrong and the length is doubled. I tried to console.log the x position of my snake but they are correct.
function paint_cell(x, y)
{
console.log(x*cw);
console.log((x+1)*cw);
ctx.fillStyle = '#fff799';
ctx.fillRect(x*cw, y*cw, (x+1)*cw, (y+1)*cw);
ctx.lineWidth = 1;
ctx.strokeStyle = '#135d80';
ctx.strokeRect(x*cw, y*cw, (x+1)*cw, (y+1)*cw);
}
***Because someone wants to learn how to make a snake game,
this is my final solution for this game.
http://jsfiddle.net/wd9z9/
You can also visit my site to play:
use WSAD to play.
http://www.catoyeung.com/snake2/single.php
:D

I tested this alternative init function:
function init()
{
fill_grid();
// create_snake1();
// paint_snake1();
paint_cell(5, 5)
}
Which paints a square starting at coords (5,5), but of size 6x6.
I believe you are misusing the fillRect() method. Use
ctx.fillRect(x*cw, y*cw, cw, cw);
Instead of
ctx.fillRect(x*cw, y*cw, (x+1)*cw, (y+1)*cw);
the same probably applies to strokeRect.
Incidentally, I was looking at your grid-fill function. You fill i*j adjacent squares, which works, but you really only need i vertical lines and j horizontal ones - which is a lot less processing. Run a quicker loop with fewer calculations:
function fill_grid() {
ctx.beginPath();
ctx.fillStyle = 'black';
ctx.fillRect(0,0,w,h);
for (var i=0;i<=h;i+=cw)
{
ctx.moveTo(0,i);
ctx.lineTo(w,i);
ctx.moveTo(i,0);
ctx.lineTo(i,h);
}
ctx.stroke();
}
I would also only define line width and stroke colour once, as you initialise the canvas. You may not need to worry about this of course. But this kind of care can make a difference if you refresh the grid often to animate your snakes.

Douglas Crockfords - how to call base method into an inherited class

I'm trying to construct a base class Shape using Crockford's inheritance pattern. Using this base Shape, I'm trying to draw a circle, a rectangle and a triangle. I'm kinda stuck. I didn't know how to call/modify the base method
function points(x,y) {
x = this.x;
y = this.y;
}
function Shape() {
return {
this.points: [ ],
init : function(){
if(typeof this.context === ‘undefined’){
var canvas = document.getElementById(‘canvas’);
var context = canvas.getContext(‘2d’);
}
},
draw: function(){
var context = this.context;
context.beginPath();
context.moveTo(this.points[0].x, this.points[0].y);
for(var i=1; i< this.parameter.length; i++){
context.lineTo(this.parameter[i].x, this.parameter[i].y);
}
context.closePath();
context.stroke();
}
};
}
function Circle(x, y, r){
var points = Shape();
point.x = x;
points.y = y;
points.r = r;
var baseMethod = that.draw;
that.draw = function(){
/*how to modify the base method to draw circle*/
};
}
function Rectangle(a, b, c, d){
var points = Shape();
point.a = a;
points.b = b;
points.c = c;
points.d = d
var baseMethod = that.draw;
that.draw = function(){
/*how to call base method to draw rectangle*/
};
}

You've got quite a few problems going on with your code. Firstly you need to make sure you've got your basic drawing code working before moving on to more complicated shapes such as circles and rectangles. Start with drawing lines. I've tidied up your code and got it working with drawing straight lines:
//returns basic point object which has
//two properties x & y
function point(x, y) {
return {
x: x,
y: y
}
}
//function that returns a shape object with all the
//mechanisms for drawing lines between points
function Shape(canvasID) {
return {
points: [], //not 'this.points' (which would most likely be window.points)
addPoint: function(x, y) {//adding a point to a shape is an operation of shape
this.points.push(point(x, y))
},
init: function() {
if (typeof this.context === 'undefined') {
var canvas = document.getElementById(canvasID);
var ctx = canvas.getContext('2d');
this.context = ctx; //add the context reference to the current shape object
}
},
draw: function() {
this.init();
var context = this.context;
context.beginPath();
var that = this; //create a local reference to the current 'this' object.
//insures us against any possible 'this' scope problems
context.moveTo(that.points[0].x, that.points[0].y);
for (var i = 1; i < that.points.length; i++) {
context.lineTo(that.points[i].x, this.points[i].y);
}
context.closePath();
context.stroke();
}
};
}
//Simple Line object - good for testing your
//basic drawing functionality
function Line(canvasID, x, y, x2, y2) {
var shape = Shape(canvasID);
shape.addPoint(x, y);
shape.addPoint(x2, y2);
shape.draw();
}
//Execute your drawing functionality after the
//window has loaded to make sure all your objects exist before
//trying to use them
window.onload = function() {
Line('canvas', 100, 100, 200, 200);
}
I'm not necessarily sold on whether this is the best way to approach what you are doing - but DC's basic approach is to create objects without having to use the "new" keyword. So he returns an object from a function call using the JavaScript object notation.
Now that you can draw a line, the next step is to draw a series of connected lines one after the other (a path). After that, create your rectangle. You need some code to tell your code where to start drawing the rectangle (the start x/y coordinate) and then you can have parameters denoting the height and width of the rectangle which will be used to calculate the coordinates of the rectangle's corners and passed to the shape object to be drawn in the same way the series of connected lines were drawn. One caveat, though, is to check if there is some sort of 'createRectangle' function on the context object (and same for circle). I don't actually know myself as I've not done this sort of work in HTML5/canvas - although I have in other environments.
Edit
Forgot to mention that you will need to make sure the doctype declaration of your html is html5. A lot of IDE's will automatically declare your html as html4. Html5 just needs: <!DOCTYPE html>
Also, make sure you declare a canvas element in the html body, something like this:
<canvas id="canvas" width="300" height="150">
</canvas>

How i can get hit test with image on canvas?

I create image in this way:
var orc = new Image();
orc.src = "./orc.png";
I use image in objects like this:
function Character(hp, image){
this.hp = hp;
this.image = image;
};
I call it in several times, like:
unit245 = new Character(100, orc);
And I draw it in this way, for example:
ctx.drawImage(unit245.image, 15, 55, 100, 100);
How I can get mouse click or move above my unit245 on canvas?
I need something like this http://easeljs.com/examples/dragAndDrop.html but without any frameworks (except jquery)

There is no built in way. I've written a few tutorials on making movable and selectable shapes on a Canvas to help people get started with this sort of thing though.
In short you need to remember what you have drawn and where, and then check each mouse click to see if you have clicked on something.

HitTesting can be done by checking what is present at the current location over the canvas, which can be called upon mouse click or move event over the canvas (which is the basis of hit testing). This can be done by knowing what has been placed where, like the bounds of an image can be saved, and when user clicks somewhere or moved the mouse over the canvas, you can check whether it is inside the image bounds or outside it. Array or List can be used for this.
Here is how this can be done

You cannot. The canvas has no semblance of what your unit245 or Character object is. You will have to actually manually check the coordinates and see if they fall within the bounds that you have for the character.
For example (assuming your Canvas is a var named canvas):
canvas.onclick = function(e) {
if (e.x >= unit245.x && e.x <= unit245.x + unit245.width && e.y >= unit245.y && e.y <= unit245.y + unit245.height) {
alert("You clicked unit245!");
}
}
In your case:
unit245.x = 15
unit245.y = 55
unit245.width = 100
unit245.height = 100

function Item(img, x, y){
this.image = img;
this.x = x;
this.y = y;
this.canv = document.createElement("canvas");
this.canv.width = this.image.width;
this.canv.height = this.image.height;
this.ctx = this.canv.getContext('2d');
this.ctx.drawImage(this.image, 0, 0, CELL_SIZE, CELL_SIZE);
this.hit = function (mx, my) {
var clr;
clr = this.ctx.getImageData(mx - this.x, my - this.y, 1, 1).data;
if (clr[3] > 250) {
//On object
this.image = gold_glow;
} else {
//Leave object
this.image = gold;
}
};
}

Javascript on html Canvas element problem

So I'm trying to create a method where the you move an image around a canvas element. This is relevant in that in creating many kinds of games, you'd need a background image to move around properly against the canvas and the player's movement. The problem is that you always draw relative to the canvas's (0,0) point in the top left corner. So what I'm going for in a conceptualization where pressing right (for example) would be conceived as moving the CANVAS right, when really you're moving the image left. It could be argued that this is unnecessary, but honestly thinking about it the other way kind of gives me a headache. I think this way of relating everything to a larger absolute field would be easier to program with a large number of objects.
The problem is, I've messed around with my code in Pycharm but I keep getting canvas not defined and similar errors. Please help me fix this up! So without further ado, here's my code! (and any other ways to clean up my code is appreciated, I'm pretty new to JS!)
//Animates a moving black dot on the canvas.
//Variables for parameters
var gameloopId;
var speed=6;
var canvas;
var background;
var circle;
var ctx;
//Wait for document to be ready then start
$(document).ready(function(){
console.log('document is ready');
init();
});
//Holds the relative coordinates.
function Canvas(){
this.x=0;//relative X
this.y=0;//relative Y
//Calulate screen height and width
this.width = parseInt($("#canvas").attr("width"));
this.height = parseInt($("#canvas").attr("height"));
}
canvas=new Canvas();
//Define an object
function Object(){
this.absX=0;
this.absY=0;
this.x=this.absX-canvas.x;
this.y=this.absY-canvas.y;
}
//Circle Object
function Circle(radius){
this.radius=radius;
}
Circle.prototype= new Object(); //Circle is an Object
function drawCircle(){
// Create the circle
ctx.strokeStyle = "#000000";
ctx.fillStyle = "#000000";
ctx.beginPath();
ctx.arc(circle.x,circle.y,circle.radius,0,Math.PI*2,true);
ctx.closePath();
ctx.stroke();
ctx.fill();
}
Background= Image();
Background.prototype=new Object(); //Background is an Object
background= new Background()
function drawBackground(){
//draw the background
ctx.drawImage(background,background.x,background.y);
}
function init(){
console.log('function init()');
initSettings();
//Insert event handler for keyboard movement of circle (space clearInterval)
$(document).keydown(function(e){
if(e.keyCode=='37'){ //Left key
circle.absX+=speed;
canvas.x+=speed;}
if(e.keyCode=='38'){ //Up key
circle.absY-=speed;
canvas.y-=speed;}
if(e.keyCode=='39'){ //Right key
circle.absX+=speed;
canvas.x+=speed;}
if(e.keyCode=='40'){ //Down key
circle.absX+=speed;
canvas.y+=speed;}
if(e.keyCode=='32'){ //Space Bar
console.log('spacebar');
clearInterval(gameloopId);
initSettings();
gameloopId = setInterval(gameLoop,10);
}
});
$(document).keyup(function(e){
if(e.keyCode=='37'){
console.log('left');}//Left key
if(e.keyCode=='38'){
console.log('up');}//Up key
if(e.keyCode=='39'){
console.log('right');}//Right key
if(e.keyCode=='40'){
console.log('down');}//Down key
});
//Initialize loop of "game"
gameloopId = setInterval(gameLoop,10);
}
function initSettings(){
console.log('initSettings');
//Set up canvas
ctx = document.getElementById('canvas').getContext('2d');
//center circle on the horizontal axis
console.log('setting circle coords');
circle = new Circle(15);
circle.x = parseInt(canvas.width/2);
circle.y = canvas.height - 40;
//Put background at (0,0)
background.x=0;
background.y=0;
background.src="http://127.0.0.1:8000/static/back.jpg";
console.log("background width:"+background.width);
console.log("background height:"+background.height);
}
function gameLoop(){
//console.log('function gameLoop()');
//Has it reached far left side?
if(circle.x<circle.radius)
{
circle.x=circle.radius
}
//Has it reached far right side?
if(circle.x>canvas.width - circle.radius)
{
circle.x=canvas.width - circle.radius
}
//Has it reached top?
if(circle.y<circle.radius)
{
circle.y=circle.radius
}
//has it reached bottom?
if(circle.y>canvas.height - circle.radius)
{
circle.y=canvas.height - circle.radius
}
//has background reached left?
if(background.x < canvas.width-background.width)
{
background.x= canvas.width-background.width;
}
//has background reached right?
if(background.x>0)
{
background.x=0;
}
//has background reached top?
if(background.y < canvas.height-background.height)
{
background.y = canvas.height-background.height;
}
//has background reached bottom?
if(background.y>0)
{
background.y=0;
}
//Clear the screen (i.e. a draw a clear rectangle the size of the screen)
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.save();
//draw background
drawBackground();
// draw the circle
drawCircle();
ctx.restore();
}
EDIT:(UPDATED CODE!)
//Animates a moving black dot on the canvas.
//Variables for parameters
var gameloopId;
var speed=6;
var canvas;
var background;
var circle;
var ctx;
//Wait for document to be ready then start
$(document).ready(function(){
console.log('document is ready');
init();
});
//Holds the relative coordinates.
function Canvas(){
this.x=0;//relative X
this.y=0;//relative Y
//Calulate screen height and width
this.width = parseInt($("#canvas").attr("width"));
this.height = parseInt($("#canvas").attr("height"));
}
//Define an object
function MyObject(){
this.absX=0;
this.absY=0;
this.x=this.absX-canvas.x;
this.y=this.absY-canvas.y;
}
//Circle MyObject
function Circle(radius){
this.radius=radius;
}
Circle.prototype= new MyObject(); //Circle is an MyObject
function drawCircle(){
// Create the circle
ctx.strokeStyle = "#000000";
ctx.fillStyle = "#000000";
ctx.beginPath();
ctx.arc(circle.x,circle.y,circle.radius,0,Math.PI*2,true);
ctx.closePath();
ctx.stroke();
ctx.fill();
}
function Background(){
this.img= Image();
}
Background.prototype=new MyObject(); //Background is an MyObject
function drawBackground(){
//draw the background
ctx.drawImage(background,background.x,background.y);
}
function init(){
console.log('function init()');
initSettings();
//Insert event handler for keyboard movement of circle (space clearInterval)
$(document).keydown(function(e){
if(e.keyCode=='37'){ //Left key
circle.absX+=speed;
canvas.x+=speed;}
if(e.keyCode=='38'){ //Up key
circle.absY-=speed;
canvas.y-=speed;}
if(e.keyCode=='39'){ //Right key
circle.absX+=speed;
canvas.x+=speed;}
if(e.keyCode=='40'){ //Down key
circle.absX+=speed;
canvas.y+=speed;}
if(e.keyCode=='32'){ //Space Bar
console.log('spacebar');
clearInterval(gameloopId);
initSettings();
gameloopId = setInterval(gameLoop,10);
}
});
$(document).keyup(function(e){
if(e.keyCode=='37'){
console.log('left');}//Left key
if(e.keyCode=='38'){
console.log('up');}//Up key
if(e.keyCode=='39'){
console.log('right');}//Right key
if(e.keyCode=='40'){
console.log('down');}//Down key
});
//Initialize loop of "game"
gameloopId = setInterval(gameLoop,10);
}
function initSettings(){
console.log('initSettings');
//Set up canvas
canvas=new Canvas();
ctx = document.getElementById('canvas').getContext('2d');
//center circle on the horizontal axis
console.log('setting circle coords');
circle = new Circle(15);
circle.x = parseInt(canvas.width/2);
circle.y = canvas.height - 40;
//Put background at (0,0)
background= new Background();
background.x=0;
background.y=0;
background.img.src="http://127.0.0.1:8000/static/back.jpg";
console.log("background width:"+background.width);
console.log("background height:"+background.height);
}
function gameLoop(){
//console.log('function gameLoop()');
//Has it reached far left side?
if(circle.x<circle.radius)
{
circle.x=circle.radius
}
//Has it reached far right side?
if(circle.x>canvas.width - circle.radius)
{
circle.x=canvas.width - circle.radius
}
//Has it reached top?
if(circle.y<circle.radius)
{
circle.y=circle.radius
}
//has it reached bottom?
if(circle.y>canvas.height - circle.radius)
{
circle.y=canvas.height - circle.radius
}
//has background reached left?
if(background.x < canvas.width-background.width)
{
background.x= canvas.width-background.width;
}
//has background reached right?
if(background.x>0)
{
background.x=0;
}
//has background reached top?
if(background.y < canvas.height-background.height)
{
background.y = canvas.height-background.height;
}
//has background reached bottom?
if(background.y>0)
{
background.y=0;
}
//Clear the screen (i.e. a draw a clear rectangle the size of the screen)
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.save();
//draw background
drawBackground();
// draw the circle
drawCircle();
ctx.restore();
}
enter code here

I don't think you can write your own Object You definitely can't use Object, it's a reserved keyword. Object is the built in javascript object that all objects inherit from. You have basically overwritten it. That could be your problem.
Try calling it myObject to check if this is the problem.
//Define an myObject
function myObject(){
this.absX=0;
this.absY=0;
this.x=this.absX-canvas.x;
this.y=this.absY-canvas.y;
}
Circle.prototype= new myObject(); //Circle is a myObject

Background= Image();
Background.prototype=new Object(); //Background is an Object
background= new Background()
seems suspicious.
edit : Background is an Element. You add a prototype even though it is not a function.
Then you call Background as a constructor but it is not.
So background is likely to be undefined. I am surprised background.x gives you 0.
By the way, you should parseInt(arg, 10) to get your result in decimal and not octal.

I figured it out! I had a lot of stupid stuff in my ode and lots of bugs - for example background.img is an Image, but all over the place I was trying to get background.width instead of background.img.width. I also refactored several functions to make things prettier (to em at least). Thanks to the above for your help! Here's my "final" code, at least as of right now:
//Animates a moving black dot on the canvas.
//Variables for parameters
var gameloopId;
var speed=6;
//var canvas;
var background;
var circle;
var ctx;
//Wait for document to be ready then start
$(document).ready(function(){
console.log('document is ready');
init();
});
//Holds the relative coordinates.
var canvas = new function Canvas(){
this.x=0;//relative X
this.y=0;//relative Y
//Calulate screen height and width
this.width = parseInt($("#canvas").attr("width"));
this.height = parseInt($("#canvas").attr("height"));
};
//Define an object
function MyObject(){
this.absX=0;
this.absY=0;
this.x=this.absX-canvas.x;
this.y=this.absY-canvas.y;
this.updateplace = function (){
this.x=this.absX-canvas.x;
this.y=this.absY-canvas.y;
};
}
//Circle MyObject
function Circle(radius){
this.radius=radius;
this.draw=function(){
// Create the circle
ctx.strokeStyle = "#000000";
ctx.fillStyle = "#000000";
ctx.beginPath();
ctx.arc(circle.x,circle.y,circle.radius,0,Math.PI*2,true);
ctx.closePath();
ctx.stroke();
ctx.fill();
}
}
Circle.prototype= new MyObject(); //Circle is an MyObject
function Background(){
this.img= Image();
this.draw=function(){
ctx.drawImage(background.img,background.x,background.y);
}
}
Background.prototype=new MyObject(); //Background is an MyObject
function init(){
console.log('function init()');
initSettings();
//Insert event handler for keyboard movement of circle (space clearInterval)
$(document).keydown(function(e){
if(e.keyCode=='37'){ //Left key
circle.absX-=speed;
canvas.x-=speed;}
if(e.keyCode=='38'){ //Up key
circle.absY-=speed;
canvas.y-=speed;}
if(e.keyCode=='39'){ //Right key
circle.absX+=speed;
canvas.x+=speed;}
if(e.keyCode=='40'){ //Down key
circle.absY+=speed;
canvas.y+=speed;}
if(e.keyCode=='32'){ //Space Bar
console.log('spacebar');
clearInterval(gameloopId);
initSettings();
gameloopId = setInterval(gameLoop,10);
}
});
$(document).keyup(function(e){
if(e.keyCode=='37'){
console.log('left');}//Left key
if(e.keyCode=='38'){
console.log('up');}//Up key
if(e.keyCode=='39'){
console.log('right');}//Right key
if(e.keyCode=='40'){
console.log('down');}//Down key
});
//Initialize loop of "game"
gameloopId = setInterval(gameLoop,10);
}
function initSettings(){
console.log('initSettings');
//Set up canvas
ctx = document.getElementById('canvas').getContext('2d');
canvas.width = parseInt($("#canvas").attr("width"));
canvas.height = parseInt($("#canvas").attr("height"));
//center circle on the horizontal axis
console.log('setting circle coords');
circle = new Circle(15);
circle.absX = parseInt(canvas.width/2);
circle.absY = canvas.height - 40;
//Put background at (0,0)
background= new Background();
background.x=0;
background.y=0;
background.img.src="http://127.0.0.1:8000/static/back.jpg";
console.log("background width:"+background.img.width);
console.log("background height:"+background.img.height);
console.log("Right Bound:"+(background.img.width- canvas.width))
}
function gameLoop(){
//console.log('function gameLoop()');
//Has it reached far left side?
if(circle.absX<circle.radius)
{
circle.absX=circle.radius
}
//Has it reached far right side?
if(circle.absX>background.img.width - circle.radius)
{
circle.absX=background.img.width - circle.radius
}
//Has it reached top?
if(circle.absY<circle.radius)
{
circle.absY=circle.radius
}
//has it reached bottom?
if(circle.absY>background.img.height - circle.radius)
{
circle.absY=background.img.height - circle.radius
}
//has canvas reached right bound?
if(canvas.x > background.img.width- canvas.width)
{
canvas.x= background.img.width- canvas.width;
}
//has canvas reached left bound?
if(canvas.x<0)
{
canvas.x=0;
}
//has background reached bottom bound?
if(canvas.y > background.img.height - canvas.height)
{
canvas.y = background.img.height - canvas.height;
}
//has background reached top bound?
if(canvas.y<0)
{
canvas.y=0;
}
//Clear the screen (i.e. a draw a clear rectangle the size of the screen)
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.save();
//draw background
background.updateplace();
background.draw();
// draw the circle
circle.updateplace();
circle.draw();
ctx.restore();
}