Develop Reference

Cursor wont align with HTML Canvas on longer and thinner images

I have a canvas that I am trying to draw on whenever I use square images with dimensions like such:
Width:2925
Height:2354
My cursor works correctly. When I use images with dimensions like Width: 585 Height: 2354 My cursor refuses to align correctly. Here is my JS I am using:
var canvas;
var ctx;
var canvasOffset;
var offsetX;
var offsetY;
var isDrawing = false;
var drag = false;
var rect = {};
var background = new Image();
//canvas.height = background.height;
//canvas.width = background.width;
var aspectRatio;
var parentOffsetWidth;
var parentOffsetHeight;
// Make sure the image is loaded first otherwise nothing will draw.
$(background).on("load", function () {
//ctx.drawImage(background,0,0);
//ctx.width = background.width;
//drawImageProp(ctx, background, 0, 0, background.width, background.height);.
//coverImg(background, 'cover');
//coverImg(background, 'contain');
//ctx.drawImage(background, 0, 0, background.width,background.height,0, 0, ctx.width, ctx.height);
console.log("--background img load--");
parentOffsetWidth = document.getElementById("imgGrid").offsetWidth;
parentOffsetHeight = document.getElementById("imgGrid").offsetHeight;
console.log("w:" + parentOffsetWidth);
console.log("h:" + parentOffsetHeight);
if (parentOffsetWidth < 500) {
//if img is to small to render properly
//parentOffsetWidth = 500;
}
if (background != null) {
canvas.height = (background.height / background.width) * parentOffsetWidth;
canvas.width = parentOffsetWidth;
}
//coverImg(background, 'cover');
drawImageProp(ctx, background, 0, 0, canvas.width, canvas.height, offsetX, offsetY);
})
$(document).ready(function () {
//ctx.drawImage(background,0,0);
})
function Load(backgroundImgSrc) {
console.log("--load--");
console.log(backgroundImgSrc);
background.src = backgroundImgSrc;
aspectRatio = background.height / background.width;
canvas = document.getElementById("canvas");
canvasOffset = $("#canvas").offset();
offsetX = canvasOffset.left;
offsetY = canvasOffset.top;
ctx = canvas.getContext("2d");
init();
}
var startX;
var startY;
const coverImg = (img, type) => {
const imgRatio = img.height / img.width
const winRatio = window.innerHeight / window.innerWidth
if ((imgRatio < winRatio && type === 'contain') || (imgRatio > winRatio && type === 'cover')) {
const h = window.innerWidth * imgRatio
ctx.drawImage(img, 0, (window.innerHeight - h) / 2, window.innerWidth, h)
}
if ((imgRatio > winRatio && type === 'contain') || (imgRatio < winRatio && type === 'cover')) {
const w = window.innerWidth * winRatio / imgRatio
ctx.drawImage(img, (window.w - w) / 2, 0, w, window.innerHeight)
}
}
function drawImageProp(ctx, img, x, y, w, h, offsetX, offsetY) {
if (arguments.length === 2) {
x = y = 0;
w = ctx.canvas.width;
h = ctx.canvas.height;
}
// default offset is center
offsetX = typeof offsetX === "number" ? offsetX : 0.5;
offsetY = typeof offsetY === "number" ? offsetY : 0.5;
// keep bounds [0.0, 1.0]
if (offsetX < 0) offsetX = 0;
if (offsetY < 0) offsetY = 0;
if (offsetX > 1) offsetX = 1;
if (offsetY > 1) offsetY = 1;
var iw = img.width,
ih = img.height,
r = Math.min(w / iw, h / ih),
nw = iw * r, // new prop. width
nh = ih * r, // new prop. height
cx, cy, cw, ch, ar = 1;
// decide which gap to fill
if (nw < w) ar = w / nw;
if (Math.abs(ar - 1) < 1e-14 && nh < h) ar = h / nh; // updated
nw *= ar;
nh *= ar;
// calc source rectangle
cw = iw / (nw / w);
ch = ih / (nh / h);
cx = (iw - cw) * offsetX;
cy = (ih - ch) * offsetY;
// make sure source rectangle is valid
if (cx < 0) cx = 0;
if (cy < 0) cy = 0;
if (cw > iw) cw = iw;
if (ch > ih) ch = ih;
// fill image in dest. rectangle
ctx.drawImage(img, cx, cy, cw, ch, x, y, w, h);
}
////////////////////////////////////
function init() {
canvas.addEventListener('mousedown', mouseDown, false);
canvas.addEventListener('mouseup', mouseUp, false);
canvas.addEventListener('mousemove', mouseMove, false);
}
function mouseDown(e) {
console.log(e);
rect.Left = e.pageX - offsetX;
rect.Top = e.pageY - offsetY;
rect.Left_Percentage = (rect.Left / canvas.width) * 100;
rect.Top_Percentage = (rect.Top / canvas.height) * 100;
drag = true;
}
function mouseUp() {
drag = false;
//ctx.clearRect(0,0,canvas.width,canvas.height);
canvas.style.cursor = "default";
window.dotnetInstance.invokeMethodAsync("ShowPopup", rect);
}
function mouseMove(e) {
if (drag) {
rect.Width = (e.pageX - offsetX) - rect.Left;
rect.Height = (e.pageY - offsetY) - rect.Top;
rect.Height_Percentage = (rect.Height/canvas.height) * 100;
rect.Width_Percentage = (rect.Width / canvas.width) * 100;
ctx.clearRect(0, 0, canvas.width, canvas.height);
draw();
}
}
function draw() {
ctx.setLineDash([10]);
ctx.strokeRect(rect.Left, rect.Top, rect.Width, rect.Height);
}
window.SetDotNetInstance = (dotnetHelper) => {
console.log(dotnetHelper);
if (window.dotnetInstance == null) {
window.dotnetInstance = dotnetHelper;
}
}
function toggleDisplay(current) {
if ($(current).is(":checked")){
$(".clickRegionBlock").hide();
$(".blockNumberInfo").hide();
$("#imgGrid>.border").removeClass("border")
}
else {
$(".clickRegionBlock").show();
$(".blockNumberInfo").show();
$("#imgGrid>.box-border").addClass("border");
}
}
function Zoom() {
var gridSizes = $("#imgGrid").css("grid-template-columns");
var splitArrPx = gridSizes.split(" ");
var firstpx = parseInt(splitArrPx[0].replace("px", ""));
firstpx += 100;
var zoomedGridSizes = "";
splitArrPx.forEach(function (item, index, arr) {
var replacedItem = item.replace(/\d+/, firstpx);
zoomedGridSizes += replacedItem + " ";
})
console.log(zoomedGridSizes);
$("#imgGrid").css("grid-template-columns", zoomedGridSizes);
Load(background.src);
}
And here is my HTML code I am using .Net Blazor as well to achieve this that is why there are variables in my HTML it should not effect my javascript:
<canvas id="canvas" style="background-image:url('api/contentpreview/blocks/#b.PageId/#b.LocalIndex/#Resolution/#zone'); width:100%; height:auto; background-size:cover;" #onload='() => LoadImg("api/contentpreview/blocks/" + b.PageId + "/" + b.LocalIndex + "/" + Resolution + "/" + zone)' />
initCanvas = true;
canvasImg = "api/contentpreview/blocks/" + b.PageId + "/" + b.LocalIndex + "/" + Resolution + "/" + zone;

Rectangle vertices coordinates using canvas rect

I'm trying to draw rectangle on canvas in this way:
<canvas id="canvas" height="893" width="1262"></canvas>
<div id="output"></div>
<script>
var canvasSizeX = 1262;
var canvasSizeY = 893;
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
ctx.globalAlpha = 0.5;
//Variables
var canvasx = $(canvas).offset().left;
var canvasy = $(canvas).offset().top;
var canvasybottom = $(canvas).offset().bottom;
var last_mousex = last_mousey = 0;
var mousex = mousey = 0;
var mousedown = false;
// points
var A;
var B;
var C;
var D;
//Mousedown
$(canvas).on('mousedown', function (e) {
last_mousex = parseInt(e.clientX - canvasx);
last_mousey = parseInt(e.clientY - canvasy);
mousedown = true;
});
//Mouseup
$(canvas).on('mouseup', function (e) {
mousedown = false;
});
//Mousemove
$(canvas).on('mousemove', function (e) {
mousex = parseInt(e.clientX - canvasx);
mousey = parseInt(e.clientY - canvasy);
if (mousedown) {
ctx.clearRect(0, 0, canvas.width, canvas.height); //clear canvas
ctx.beginPath();
var width = mousex - last_mousex;
var height = mousey - last_mousey;
ctx.rect(last_mousex, last_mousey, width, height);
ctx.strokeStyle = 'red';
ctx.lineWidth = 7;
ctx.stroke();
//
// Here I need the A, B, C and D values
//
}
//Output
$('#output').html('current: ' + mousex + ', ' + mousey + '<br/>last: ' + last_mousex + ', ' + last_mousey + '<br/>mousedown: ' + mousedown + '<br/>A: ' + A + '<br/>B: ' + B + '<br/>C: ' + C + '<br/>D: ' + D);
});
</script>
When the rect is drawn, I'd like to know the A, B, C and D (vertices) coordinates. On mousemove event actually I can get only the width the mouse X and mouse Y coordinates.
Is there a way to do it or I'm missing something?

Why is camera's position not correctly calculated (canvas)

I am making a clone of agar.io and I am stuck in my code. I can't understand why my camera's position is not correctly calculated. I want my camera's position to half the vector between the farthest blob and the closest blob.
Below is a picture and my code:
<html>
<head>
<title>Play Agario Clone</title>
<style>
body {
margin: 0;
padding: 0;
}
</style>
</head>
<body>
<canvas id="game">
kindly update your browser.
</canvas>
<script>
var
canvas,
ctx,
width = innerWidth,
height = innerHeight,
mouseX = 0,
mouseY = 0;
var
camera = {
x: 0,
y: 0,
// camera
update: function(obj) {
var farthestBlobX = Math.max.apply(0, obj.blobs.map(function(cell) { return cell.x }));
var farthestBlobY = Math.max.apply(0, obj.blobs.map(function(cell) { return cell.y }));
var closestBlobX = Math.min.apply(0, obj.blobs.map(function(cell) { return cell.x }));
var closestBlobY = Math.min.apply(0, obj.blobs.map(function(cell) { return cell.y }));
var x = farthestBlobX - closestBlobX;
var y = farthestBlobY - closestBlobY;
var length = Math.sqrt(x * x + y * y);
this.x = length/2 - width/2;
this.y = length/2 - height/2;
}
},
player = {
defaultMass: 54,
x: 0,
y: 0,
blobs: [],
update: function () {
for (var i = 0; i < this.blobs.length; i ++) {
var x = mouseX + camera.x - this.blobs[i].x;
var y = mouseY + camera.y - this.blobs[i].y;
var length = Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2));
var speed = 54/this.blobs[i].mass;
this.blobs[i].velX = x/length * speed * Math.min(1, Math.pow(x / this.blobs[i].mass, 2));
this.blobs[i].velY = y/length * speed * Math.min(1, Math.pow(x / this.blobs[i].mass, 2));
this.blobs[i].x += this.blobs[i].velX;
this.blobs[i].y += this.blobs[i].velY;
for (var j = 0; j < this.blobs.length; j ++) {
if (j != i && this.blobs[i] !== undefined) {
var blob1 = this.blobs[i];
var blob2 = this.blobs[j];
var x = blob2.x - blob1.x;
var y = blob2.y - blob1.y;
var dist = Math.sqrt(x * x + y * y);
if (dist < blob1.mass + blob2.mass) {
x /= dist;
y /= dist;
blob1.x = blob2.x - x * (blob1.mass + blob2.mass);
blob1.y = blob2.y - y * (blob1.mass + blob2.mass);
}
}
}
}
this.x += (mouseX - width/2)/(width/2) * 1;
this.y += (mouseY - height/2)/(height/2) * 1
},
split: function (cell) {
cell.mass /= 2;
this.blobs.push({
x: cell.x,
y: cell.y,
mass: cell.mass
});
},
draw: function () {
for (var i = 0; i < this.blobs.length; i ++) {
ctx.fillStyle = "red";
ctx.beginPath();
ctx.arc(-camera.x + this.blobs[i].x, -camera.y + this.blobs[i].y, this.blobs[i].mass, 0, Math.PI*2);
ctx.fill();
ctx.closePath();
}
}
};
function handleMouseMove (e) {
mouseX = e.clientX;
mouseY = e.clientY;
}
function setup () {
canvas = document.getElementById("game");
ctx = canvas.getContext("2d");
canvas.width = width;
canvas.height = height;
addEventListener("mousemove", handleMouseMove);
player.blobs.push({
x: 0,
y: 0,
mass: player.defaultMass
});
player.blobs.push({
x: 100,
y: 100,
mass: player.defaultMass/2
});
player.blobs.push({
x: 100,
y: 100,
mass: player.defaultMass*2
});
var loop = function () {
update();
draw();
requestAnimationFrame(loop);
}
requestAnimationFrame(loop);
}
function update () {
camera.update(player);
player.update();
}
function draw () {
ctx.fillStyle = "#fff";
ctx.fillRect(0, 0, width, height);
player.draw();
}
setup();
</script>
</body>
</html>

Instead of computing everything relative to your camera, use your camera to set the global transformation matrix of your canvas, and only for this.
This way, your blobs' updates will be cleaner, and your camera easier to manage.
Now to get the middle position between two points, do (pt1 + pt2) / 2.
You were not clear in your question, if fartherstX and fartherstY should represent the same blob. In your code it wasn't, so I didn't change it.
Also, I didn't gone into all your logics, but beware NaN values, I got some while doing the edit.
function draw() {
var cw = ctx.canvas.width / 2;
var ch = ctx.canvas.height / 2;
// reset transform to clear the canvas
ctx.setTransform(1, 0, 0, 1, 0, 0);
ctx.fillStyle = "#fff";
ctx.fillRect(0, 0, width, height);
// here we really set the camera position
ctx.setTransform(1, 0, 0, 1, -camera.x + cw, -camera.y + ch);
ctx.strokeRect(0, 0, width, height); // just to show the original area
player.draw();
}
var
canvas,
ctx,
width = innerWidth,
height = innerHeight,
mouseX = 0,
mouseY = 0;
var camera = {
x: 0,
y: 0,
// camera
update: function(obj) {
var farthestBlobX = Math.max.apply(0, obj.blobs.map(function(cell) {
return cell.x
}));
var farthestBlobY = Math.max.apply(0, obj.blobs.map(function(cell) {
return cell.y
}));
var closestBlobX = Math.min.apply(0, obj.blobs.map(function(cell) {
return cell.x
}));
var closestBlobY = Math.min.apply(0, obj.blobs.map(function(cell) {
return cell.y
}));
this.x = (closestBlobX + farthestBlobX) / 2 || 0;
this.y = (closestBlobY + farthestBlobY) / 2 || 0;
}
},
player = {
defaultMass: 54,
x: 0,
y: 0,
blobs: [],
update: function() {
for (var i = 0; i < this.blobs.length; i++) {
var x = mouseX - this.blobs[i].x || 0;
var y = mouseY - this.blobs[i].y || 0;
var length = Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2));
var speed = 54 / this.blobs[i].mass;
this.blobs[i].velX = x / length * speed * Math.min(1, Math.pow(x / this.blobs[i].mass, 2));
this.blobs[i].velY = y / length * speed * Math.min(1, Math.pow(x / this.blobs[i].mass, 2));
this.blobs[i].x += this.blobs[i].velX;
this.blobs[i].y += this.blobs[i].velY;
for (var j = 0; j < this.blobs.length; j++) {
if (j != i && this.blobs[i] !== undefined) {
var blob1 = this.blobs[i];
var blob2 = this.blobs[j];
var x = blob2.x - blob1.x;
var y = blob2.y - blob1.y;
var dist = Math.sqrt(x * x + y * y);
if (dist < blob1.mass + blob2.mass) {
x /= dist;
y /= dist;
blob1.x = blob2.x - x * (blob1.mass + blob2.mass);
blob1.y = blob2.y - y * (blob1.mass + blob2.mass);
}
}
}
}
this.x += (mouseX - width / 2) / (width / 2) * 1;
this.y += (mouseY - height / 2) / (height / 2) * 1;
},
split: function(cell) {
cell.mass /= 2;
this.blobs.push({
x: cell.x,
y: cell.y,
mass: cell.mass
});
},
draw: function() {
for (var i = 0; i < this.blobs.length; i++) {
ctx.fillStyle = "red";
ctx.beginPath();
ctx.arc(this.blobs[i].x, this.blobs[i].y, this.blobs[i].mass, 0, Math.PI * 2);
ctx.fill();
ctx.closePath();
}
}
};
function handleMouseMove(e) {
mouseX = e.clientX;
mouseY = e.clientY;
}
function setup() {
canvas = document.getElementById("game");
ctx = canvas.getContext("2d");
canvas.width = width;
canvas.height = height;
addEventListener("mousemove", handleMouseMove);
player.blobs.push({
x: 10,
y: 10,
mass: player.defaultMass
});
player.blobs.push({
x: 100,
y: 100,
mass: player.defaultMass / 2
});
player.blobs.push({
x: 100,
y: 100,
mass: player.defaultMass * 2
});
var loop = function() {
update();
draw();
requestAnimationFrame(loop);
}
requestAnimationFrame(loop);
}
function update() {
camera.update(player);
player.update();
}
setup();
body {
margin: 0;
padding: 0;
}
<canvas id="game">kindly update your browser.</canvas>

I see there is an answer already..
var canvas;
var ctx;
var width = innerWidth;
var height = innerHeight;
var mouseX = 0;
var mouseY = 0;
const camera = {
x : 0,
y : 0,
update(obj) { // camera
this.x = (obj.blobsExtent.minx + obj.blobsExtent.maxx) / 2;
this.y = (obj.blobsExtent.miny + obj.blobsExtent.maxy) / 2;
this.x -= width / 2;
this.y -= height / 2;
}
};
const player = {
defaultMass : 54,
blobs : [],
blobsExtent : { // get the extent while updating the blobs save you having to iterate all the objects a second time to get extent
minx :0,
miny : 0,
maxx : 0,
maxy : 0,
},
update () {
var be = this.blobsExtent; // be for Blob Extent alias to save typing and make code easier to read
for (var i = 0; i < this.blobs.length; i++) {
var blob1 = this.blobs[i];
var x = mouseX - blob1.x;
var y = mouseY - blob1.y;
// to stop the divide by zero propigating NaN set length to 1 if less than 1
var length = Math.max(1,Math.sqrt(x * x + y * y)); // x * x is quicker than Math.pow(x,2)
var speed = 54 / blob1.mass;
blob1.velX = x / length * speed * Math.min(1, Math.pow(x / blob1.mass, 2));
blob1.velY = y / length * speed * Math.min(1, Math.pow(x / blob1.mass, 2));
blob1.x += blob1.velX;
blob1.y += blob1.velY;
for (var j = 0; j < this.blobs.length; j++) {
if (j != i) {
var blob2 = this.blobs[j];
var x = blob2.x - blob1.x;
var y = blob2.y - blob1.y;
var dist = Math.sqrt(x * x + y * y);
var radTotal = blob1.mass + blob2.mass;
if (dist < radTotal) {
x /= dist;
y /= dist;
blob1.x = blob2.x - x * radTotal;
blob1.y = blob2.y - y * radTotal;
}
}
}
if(i === 0){ // use first blob to setup min max
be.maxx = be.minx = blob1.x;
be.maxy = be.miny = blob1.y;
}else{
be.maxx = Math.max(be.maxx, blob1.x);
be.maxy = Math.max(be.maxy, blob1.y);
be.minx = Math.min(be.minx, blob1.x);
be.miny = Math.min(be.miny, blob1.y);
}
}
},
split (cell) {
cell.mass /= 2;
this.blobs.push(createBlob(cell.x, cell.y, cell.mass));
},
draw () {
var b; // alias for blob
ctx.fillStyle = "red"; // all the same colour then can render as one path
ctx.setTransform(1,0,0,1,-camera.x,-camera.y);
ctx.beginPath();
for (var i = 0; i < this.blobs.length; i++) {
b = this.blobs[i];
ctx.arc( b.x, b.y, b.mass, 0, Math.PI * 2);
ctx.closePath();
}
ctx.fill();
ctx.setTransform(1,0,0,1,0,0); // restore default transform
}
};
function handleMouseMove(e) {
mouseX = e.clientX + camera.x;
mouseY = e.clientY + camera.y;
}
function createBlob(x,y,mass){ return {x,y,mass} }
function setup() {
canvas = document.getElementById("game");
ctx = canvas.getContext("2d");
canvas.width = width;
canvas.height = height;
addEventListener("mousemove", handleMouseMove);
player.blobs.push(createBlob(0,0,player.defaultMass));
player.blobs.push(createBlob(100,100,player.defaultMass / 2));
player.blobs.push(createBlob(100,100,player.defaultMass * 2));
}
function update() {
camera.update(player);
player.update();
}
function draw() {
ctx.fillStyle = "#fff";
ctx.fillRect(0, 0, width, height);
player.draw();
}
function loop() {
update();
draw();
requestAnimationFrame(loop);
}
setup();
requestAnimationFrame(loop);
body {
margin: 0;
padding: 0;
}
<canvas id="game"></canvas>

Capture user input on HTML canvas/Javascript and vice versa

I am trying to draw a rectangle in HTML5 canvas based on user input. I am also trying to do the opposite. If a user draws a rectangle in the canvas, the width and height values are dropped into the form. Please see my jsfiddle.
http://jsfiddle.net/hbrennan72/FyTx5/
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");
var rect = {};
//var drag = false;
function getarea()
{
var wid = document.getElementById("wid").value;
var hgt = document.getElementById("hgt").value;
var area = wid * hgt;
var perim = wid * 2 + hgt * 2;
if (wid =="") {
document.getElementById("errors").innerHTML = "The value for the width is blank.";
document.getElementById('wid').focus();
}
else if (hgt == "") {
document.getElementById("errors").innerHTML = "The value for the height is blank.";
document.getElementById("hgt").focus();
}
else if (isNaN (wid)) {
document.getElementById("errors").innerHTML = "Please enter a numeric value for the width.";
document.getElementById('wid').focus();
}
else if (isNaN (hgt)) {
document.getElementById("errors").innerHTML = "Please enter a numeric value for the height.";
document.getElementById("hgt").focus();
}
else if (wid <= 0) {
document.getElementById("errors").innerHTML = "The width is less than or equal to zero.";
document.getElementById('wid').focus();
}
else if (hgt <= 0) {
document.getElementById("errors").innerHTML = "The height is less than or equal to zero.";
document.getElementById("hgt").focus();
}
else if (wid > 500) {
document.getElementById("errors").innerHTML = "The width is greater than 500.";
document.getElementById('wid').focus();
}
else if (hgt > 500) {
document.getElementById("errors").innerHTML = "The height is greater than 500.";
document.getElementById("hgt").focus();
}
else {
window.document.getElementById("area").innerHTML = area;
window.document.getElementById("perim").innerHTML = perim;
document.getElementById("errors").innerHTML = "Please see results listed above.";
}
}
function updateform (){
"use strict"
var wid = document.getElementById("wid").value;
var hgt = document.getElementById("hgt").value;
var area = wid * hgt;
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2D");
context.clearRect(0, 0, canvas.width, canvas.height);
context.fillRect(0,0, rect.wid, rect.hgt);
context.fillStyle = "#FF0000"; //red color
draw();
}
function mouseDown(e)
{
rect.startX = e.pageX - this.offsetLeft;
rect.startY = e.pageY - this.offsetTop;
canvas.style.cursor="crosshair";
drag = true;
}
function mouseUp () {
drag = false;
}
function mouseMove(e) {
if (drag) {
rect.wid = (e.pageX - this.offsetLeft) - rect.startX;
rect.hgt = (e.pageY - this.offsetTop) - rect.startY ;
context.clearRect(0,0,canvas.width,canvas.height);
draw();
}
}
function init() {
canvas.addEventListener("mousedown", mouseDown, false);
canvas.addEventListener("mouseup", mouseUp, false);
canvas.addEventListener("mousemove", mouseMove, false);
}
init();
function drawform() {
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2D");
context.clearRect(0, 0, canvas.width, canvas.height);
context.fillRect(0,0, rect.wid, rect.hgt);
context.fillStyle = "#FF0000"; //red color
}
function draw (){
context.clearRect(0, 0, canvas.width, canvas.height);
context.fillRect(rect.startX, rect.startY, rect.wid, rect.hgt);
context.fillStyle = "#378E37"; //green color
}
draw();

I have tried to do it in this way.It is working fine.The code is
$("#smt").click(function(){
var canvasel=' <canvas id="myCanvas" width="400" height="400"></canvas>'
$("#canvas").append(canvasel)
var w=$("#wid").val();
var h=$("#hgt").val();
var c=document.getElementById("myCanvas");
var ctx=c.getContext("2d");
ctx.rect(20,20,w,h);
ctx.stroke();
})
$("#clr").click(function(){
$("#canvas").empty();
$("#wid").val('');
$("#hgt").val('');
})
initDraw(document.getElementById('canvas'));
function initDraw(canvas) {
function setMousePosition(e) {
var ev = e || window.event; //Moz || IE
if (ev.pageX) { //Moz
mouse.x = ev.pageX + window.pageXOffset;
mouse.y = ev.pageY + window.pageYOffset;
} else if (ev.clientX) { //IE
mouse.x = ev.clientX + document.body.scrollLeft;
mouse.y = ev.clientY + document.body.scrollTop;
}
};
var mouse = {
x: 0,
y: 0,
startX: 0,
startY: 0
};
var element = null;
canvas.onmousemove = function (e) {
setMousePosition();
if (element !== null) {
element.style.width = Math.abs(mouse.x - mouse.startX) + 'px';
element.style.height = Math.abs(mouse.y - mouse.startY) + 'px';
element.style.left = (mouse.x - mouse.startX < 0) ? mouse.x + 'px' : mouse.startX + 'px';
element.style.top = (mouse.y - mouse.startY < 0) ? mouse.y + 'px' : mouse.startY + 'px';
}
}
canvas.onclick = function (e) {
if (element !== null) {
element = null;
canvas.style.cursor = "default";
console.log("finsihed.");
var width=$(".rectangle").width();
var height=$(".rectangle").height();
$("#wid").val(width);
$("#hgt").val(height)
} else {
console.log("begun.");
mouse.startX = mouse.x;
mouse.startY = mouse.y;
element = document.createElement('div');
element.className = 'rectangle'
element.style.left = mouse.x + 'px';
element.style.top = mouse.y + 'px';
canvas.appendChild(element)
canvas.style.cursor = "crosshair";
}
}
}
SEE DEMO HERE
NOTE:Its not working in Mozilla. I am trying to fix it.Check in Chrome

Html5 canvas image on click

I'm having a problem whit my code.
I draw some circles in a circular path and I expect when to click on them to return something other than 0 in firebug console but that's not happening;
I don't know what is wrong with my code and i hope someone will tell me.
Here's my code:
var canvas, ctx;
var circle_data = [];
function circles(x, y, radius) {
this.x = x;
this.y = y;
this.radius = radius;
circle_data.push(this);
}
circles.prototype = {
draw: function (context) {
context.beginPath();
context.arc(this.x, this.y, this.radius / 5, 0, 2 * Math.PI, false);
context.fillStyle = "red";
context.fill();
}
}
function draw() {
ctx.translate(250, 250);
for (var n = 0; n < 10; n++) {
var radi = (Math.PI / 180);
var x = Math.sin(radi * n * 36) * 70;
var y = Math.cos(radi * n * 36) * 70;
var radius = 50;
var thiscircle = new circles(x, y, radius);
thiscircle.draw(ctx);
}
}
function mouseDown(e) {
var img_data = ctx.getImageData(e.pageX, e.pageY, 1, 1);
console.log(img_data.data[3]);
}
function init() {
canvas = document.getElementById('canvas'),
ctx = canvas.getContext('2d');
canvas.addEventListener('mousedown', mouseDown, false);
}
init();
It dosen't matter is i use data[3];
I tried whit console.log(img_data.data[0]+" "+img_data.data[1]+" "+img_data.data[2]);
Still getting 0 0 0

Your detecting the mouse position relative to the page and not the canvas, you need to get the position of the canvas on the page and subtract that from the X and Y of the mouse to find you position relative to the canvas. I use functions similar to the ones below when working with canvas.
getOffsetPosition = function(obj){
/*obj is the Canvas element*/
var offsetX = offsetY = 0;
if (obj.offsetParent) {
do {
offsetX += obj.offsetLeft;
offsetY += obj.offsetTop;
}while(obj = obj.offsetParent);
}
return [offsetX,offsetY];
}
getMouse = function(e,canvasElement){
OFFSET = getOffsetPosition(canvasElement);
mouse_x = (e.pageX || (e.clientX + document.body.scrollLeft + document.documentElement.scrollLeft)) - OFFSET[0];
mouse_y = (e.pageY || (e.clientY + document.body.scrollTop + document.documentElement.scrollTop)) - OFFSET[1];
return [mouse_x,mouse_y];
}
The following code works.
<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>Untitled Document</title>
</head>
<body>
<canvas id="canvas" width="500" height="500" style="background-color:#999999;"></canvas>
</body>
<script>
var canvas,ctx;
var circle_data = [];
function circles(x,y,radius)
{
this.x = x;
this.y = y;
this.radius = radius;
circle_data.push(this);
}
circles.prototype = {
draw: function(context){
context.beginPath();
context.arc(this.x, this.y, this.radius / 5, 0, 2* Math.PI, false);
context.fillStyle = "red";
context.fill();
}
}
getOffsetPosition = function(obj){
/*obj is the Canvas element*/
var offsetX = offsetY = 0;
if (obj.offsetParent) {
do {
offsetX += obj.offsetLeft;
offsetY += obj.offsetTop;
}while(obj = obj.offsetParent);
}
return [offsetX,offsetY];
}
getMouse = function(e,canvasElement){
OFFSET = getOffsetPosition(canvasElement);
mouse_x = (e.pageX || (e.clientX + document.body.scrollLeft + document.documentElement.scrollLeft)) - OFFSET[0];
mouse_y = (e.pageY || (e.clientY + document.body.scrollTop + document.documentElement.scrollTop)) - OFFSET[1];
return [mouse_x,mouse_y];
}
function draw(){
ctx.translate(250, 250);
for (var n = 0; n < 10; n++) {
var radi = (Math.PI/180);
var x = Math.sin(radi*n*36)*70;
var y = Math.cos(radi*n*36)*70;
var radius = 50;
var thiscircle = new circles(x,y,radius);
thiscircle.draw(ctx);
}
}
function mouseDown(e)
{
var pos = getMouse(e,ctx.canvas);
var img_data = ctx.getImageData(pos[0],pos[1],1,1);
console.log(img_data.data[3]);
}
function init() {
canvas = document.getElementById('canvas'),
ctx = canvas.getContext('2d');
draw();
canvas.addEventListener('mousedown', mouseDown, false);
}
init();
</script>
</html>