Develop Reference

How to detect cursor collision with multiple moving objects in javascript?

I'm making a game where the player will move using their cursor and will have to dodge objects. Each object has a hitbox, so I want to be able to update the number of lives whenever the cursor hits one. I'm just not sure what an efficient way is to keep track of all the hitboxes. I'd also like to record the object that the cursor collided with, so that bonus objects can be counted. The objects constantly respawn and move off the canvas, so I think keeping an array of all the hitboxes would pretty quickly get out of hand. Here's some code with some pseudo code for what I want to do.
canvas.addEventListener("mousemove", e => {
mouseX = e.clientX - rect.left;
mouseY = e.clientY - rect.top;
if (somehitboxX<=mouseX<=somehitboxX + 50 and somehitboxY<=mouseY<=somehitboxY + 50){
doSomething
}
});
EDIT: Instead of having my cursor keep track of all the hitboxes, would it be possible to have the hitboxes keep track of the cursor? Maybe by adding an event listener to each object? If that's possible, how would I go about doing it?
EDIT: Here's some code to get a better idea of what I'm working with. The emoji objects are what need to be dodged by the player. Each one has an emoji, a score, coordinates, speed, a hitbox, and a boolean that says whether it's been hit or not (though I'm not sure how to control it yet since that's what I need help with).
function Emoji(emojicon, score, x, y, speed) {
this.emojicon = emojicon;
this.score = score;
this.x = x;
this.y = y;
this.speed = speed;
this.hitbox = { hbx: x, hby: y - 50, hbWidth: 50, hbHeight: 50 };
this.hit = false;
}
The emoji objects are initialized like this:
var xPos = 0;
for (var i = 0; i < 9; i++) {
var emo = new Emoji("😜", 5, xPos, 520, 1);
emojis.push(emo);
xPos += 55;
}
emojis.forEach(emoji => drawEmoji(emoji));
}
And then Emoji.prototype.update updates both the coordinates of the emoji and its hitbox:
Emoji.prototype.update = function() {
if (this.y < 520) {
this.y -= this.speed;
this.hitbox.hby = this.y - 50;
} else {
var flip = Math.floor(Math.random() * 1000);
if (flip == 1) {
this.y -= this.speed;
this.hitbox.y = this.y - 50;
}
}
};
I hope this is enough code to get an idea of what I have so far.

Fabric.JS and Fabric-Brush - Can't add to lower canvas

I'm trying to use Fabric.js with Fabric Brush This issue that I'm running into is that Fabric Brush only puts the brush strokes onto the Top Canvas and not the lower canvas. (The stock brushes in fabric.js save to the bottom canvas) I think I need to convert "this.canvas.contextTop.canvas" to an object and add that object to the the lower canvas. Any ideas?
I've tried running:
this.canvas.add(this.canvas.contextTop)
in
onMouseUp: function (pointer) {this.canvas.add(this.canvas.contextTop)}
But I'm getting the error
Uncaught TypeError: obj._set is not a function

So the contextTop is CanvasHTMLElement context. You cannot add it.
You can add to the fabricJS canvas just fabric.Object derived classes.
Look like is not possible for now.
They draw as pixel effect and then they allow you to export as an image.
Would be nice to extend fabricJS brush interface to create redrawable objects.
As of now with fabricJS and that particular version of fabric brush, the only thing you can do is:
var canvas = new fabric.Canvas(document.getElementById('c'))
canvas.freeDrawingBrush = new fabric.CrayonBrush(canvas, {
width: 70,
opacity: 0.6,
color: "#ff0000"
});
canvas.isDrawingMode = true
canvas.on('mouse:up', function(opt) {
if (canvas.isDrawingMode) {
var c = fabric.util.copyCanvasElement(canvas.upperCanvasEl);
var img = new fabric.Image(c);
canvas.contextTopDirty = true;
canvas.add(img);
canvas.isDrawingMode = false;
}
})
<script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/2.4.1/fabric.min.js"></script>
<script src="https://tennisonchan.github.io/fabric-brush/bower_components/fabric-brush/dist/fabric-brush.min.js"></script>
<button>Enter free drawing</button>
<canvas id="c" width="500" height="500" ></canvas>
That is just creating an image from the contextTop and add as an object.

I have taken the approach suggested by AndreaBogazzi and modified the Fabric Brush so that it does the transfer from upper to lower canvas (as an image) internal to Fabric Brush. I also used some code I found which crops the image to a smaller bounding box so that is smaller than the full size of the canvas. Each of the brushes in Fabric Brush has an onMouseUp function where the code should be placed. Using the case of the SprayBrush, the original code here was:
onMouseUp: function(pointer) {
},
And it is replaced with this code:
onMouseUp: function(pointer){
function trimbrushandcopytocanvas() {
let ctx = this.canvas.contextTop;
let pixels = ctx.getImageData(0, 0, canvas.upperCanvasEl.width, canvas.upperCanvasEl.height),
l = pixels.data.length,
bound = {
top: null,
left: null,
right: null,
bottom: null
},
x, y;
// Iterate over every pixel to find the highest
// and where it ends on every axis ()
for (let i = 0; i < l; i += 4) {
if (pixels.data[i + 3] !== 0) {
x = (i / 4) % canvas.upperCanvasEl.width;
y = ~~((i / 4) / canvas.upperCanvasEl.width);
if (bound.top === null) {
bound.top = y;
}
if (bound.left === null) {
bound.left = x;
} else if (x < bound.left) {
bound.left = x;
}
if (bound.right === null) {
bound.right = x;
} else if (bound.right < x) {
bound.right = x;
}
if (bound.bottom === null) {
bound.bottom = y;
} else if (bound.bottom < y) {
bound.bottom = y;
}
}
}
// Calculate the height and width of the content
var trimHeight = bound.bottom - bound.top,
trimWidth = bound.right - bound.left,
trimmed = ctx.getImageData(bound.left, bound.top, trimWidth, trimHeight);
// generate a second canvas
var renderer = document.createElement('canvas');
renderer.width = trimWidth;
renderer.height = trimHeight;
// render our ImageData on this canvas
renderer.getContext('2d').putImageData(trimmed, 0, 0);
var img = new fabric.Image(renderer,{
scaleY: 1./fabric.devicePixelRatio,
scaleX: 1./fabric.devicePixelRatio,
left: bound.left/fabric.devicePixelRatio,
top:bound.top/fabric.devicePixelRatio
});
this.canvas.clearContext(ctx);
canvas.add(img);
}
setTimeout(trimbrushandcopytocanvas, this._interval); // added delay because last spray was on delay and may not have finished
},
The setTimeout function was used because Fabric Brush could still be drawing to the upper canvas after the mouseup event occurred, and there were occasions where the brush would continue painting the upper canvas after its context was cleared.

How can I make different shapes of a canvas draggable and particular area of it droppable in the same canvas

I want to create a Canvas in which there will be two areas (Left and right), Left panel will contain some shapes which will be draggable(static as well) and on the right side I would be able to drop them, but I am facing following problem,
I am not able to make the shapes which i draw on the left side, draggable, because there is no id associated with them.
I do not know how to make some particular area droppable.
Here is code to visualize what I am trying to achieve-
<body>
<canvas id="myCanvas" width="800" height="600" style="border:1px solid #000000;">
</canvas>
<script>
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.moveTo(250,0);
ctx.lineTo(250,600);
ctx.stroke();
ctx.fillStyle = "#FF0000";
ctx.fillRect(50,50,160,25);
ctx.fillStyle = "#0000FF";
ctx.font = "15px";
ctx.strokeText("Draggable Elements here",57,67);
ctx.fillStyle = "#FF0000";
ctx.fillRect(500,50,130,25);
ctx.font = "15px";
ctx.strokeText("Droppable area Here",510,67);
</script>
</body>
Here is the JS fiddle for the same -
http://jsfiddle.net/akki166786/4tfyy4o5/
so if anybody can shed some light on how can I achieve this, it will be a great help.
Thanks in Advance

Drag and drop in specifik area
UPDATE: Copy of box remains at original position while it's being moved.
First you need to be able to detect your rectangles. You do this by making then into objects in your code:
function box(x,y,w,h,rgb) {
this.x = x,
this.y = y;
this.xS = x; //saving x
this.yS = y; //saving y
this.w = w;
this.h = h;
this.rgb = rgb;
//to determine if the box is being draged
this.draging = false;
}
No you need to add an event listener to determine if someone is clicking, you also need to determine if the person clicked in one of your boxes.
c.addEventListener("mousedown",down);
c.addEventListener("mousemove",move);
c.addEventListener("mouseup",up);
So events have been made to detect when the mouse button is pressed down, released back up and if the mouse moves within the canvas. To these events we have functions, down(), move() and up(), ready to be executed.
All functions will be visible in the example below.
When we're happily draging our boxes and releasing our mouse button, we need to check if the box was dropped in the dropable area. We do this in the up()-function. If the drop was OK, the box can stay, otherwise we send it back to where it came from.
Working example
var c = document.getElementById("canvas");
var ctx = c.getContext("2d");
c.width = 600;
c.height = 300;
//My mouse coordinates
var x,y;
c.addEventListener("mousedown",down);
c.addEventListener("mousemove",move);
c.addEventListener("mouseup",up);
//I'll save my boxes in this array
var myBoxes = new Array();
//This function describes what a box is.
//Each created box gets its own values
function box(x,y,w,h,rgb) {
this.x = x,
this.y = y;
this.xS = x; //saving x
this.yS = y; //saving y
this.w = w;
this.h = h;
this.rgb = rgb;
//to determine if the box is being draged
this.draging = false;
}
//Let's make some boxes!!
myBoxes[0] = new box(10,10,50,100,"green");
myBoxes[1] = new box(80,50,100,75,"blue");
myBoxes[2] = new box(40,150,20,70,"yellow");
//here we draw everything
function draw() {
ctx.clearRect(0,0,c.width,c.height);
//Dropable area
ctx.fillStyle="red";
ctx.fillRect(c.width/2,0,c.width,c.height);
//Boxes!
for (var i = 0; i<myBoxes.length; i++) {
var b = myBoxes[i];
//NEW CODE FOR UPDATE
if (b.draging) { //box on the move
//Also draw it on the original spot
ctx.fillStyle="grey"; //I chose a different color to make it appear more as a shadow of the box that's being moved.
ctx.fillRect(b.xS,b.yS,b.w,b.h);
ctx.strokeRect(b.xS,b.yS,b.w,b.h);
}
//End of new code for update
ctx.fillStyle=b.rgb;
ctx.fillRect(b.x,b.y,b.w,b.h);
ctx.strokeRect(b.x,b.y,b.w,b.h);
}
//Let's keep re-drawing this
requestAnimationFrame(draw);
}
function down(event) {
event = event || window.event;
x = event.pageX - c.offsetLeft,
y = event.pageY - c.offsetTop;
for (var i = 0; i<myBoxes.length; i++) {
var b = myBoxes[i];
if (x>b.x && x<b.x+b.w && y>b.y && y<b.y+b.h) {
b.draging = true;
}
}
}
function move(event) {
event = event || window.event;
x = event.pageX - c.offsetLeft,
y = event.pageY - c.offsetTop;
for (var i = 0; i<myBoxes.length; i++) {
var b = myBoxes[i];
if (b.draging) {
b.x = x;
b.y = y;
}
}
}
function up(event) {
event = event || window.event;
x = event.pageX - c.offsetLeft,
y = event.pageY - c.offsetTop;
for (var i = 0; i<myBoxes.length; i++) {
var b = myBoxes[i];
if (b.draging) {
//Let's see if the rectangle is inside the dropable area
if (b.x>c.width/2) {
//Yes is it!
b.x = x;
b.y = y;
b.draging = false;
}
else {
//No it's not, sending it back to its ordiginal spot
b.x = b.xS;
b.y = b.yS;
b.draging = false;
}
}
}
}
draw();
canvas {
border: 1px solid black;
}
<canvas id="canvas"></canvas>

You're using just one canvas, maybe it would be better if you use two separate canvas, one for each element you want to handle on page. so you'll have one element ID for each one.
plus. if your drawing is simple, consider using a div for it instead a canvas

Once drawn to the canvas, shapes(or lines, images, everything) are no longer accessible.
What you will need to do is store each shape in an object in your code. For example:
var rectangle = {
width: 100,
height: 100,
x: 50,
y: 50
}
Then when you drag rectangle, you will need to update it's x and y properties on mouseup (or while it's being dragged if you want a drag preview).

HTML5 canvas zoom my drawings

Hello i create program like a paint on HTML5 canvas. I have problem i need create few tools drawing and zoom. I don't have idea how to create zoom without delay. Drawing example: http://jsfiddle.net/x5rrvcr0/
How i can zooming my drawings?
drawing code:
<style>
canvas {
background-color: #CECECE;
}
html, body {
background-color: #FFFFFF;
}
</style>
<script>
$(document).ready(function () {
var paintCanvas = document.getElementById("paintCanvas");
var paintCtx = paintCanvas.getContext("2d");
var size = 500;
paintCanvas.width = size;
paintCanvas.height = size;
var draw = false;
var prevMouseX = 0;
var prevMouseY = 0;
function getMousePos(canvas, evt) {
evt = evt.originalEvent || window.event || evt;
var rect = canvas.getBoundingClientRect();
if (evt.clientX !== undefined && evt.clientY !== undefined) {
return {
x: evt.clientX - rect.left,
y: evt.clientY - rect.top
};
}
}
$("#paintCanvas").on("mousedown", function(e) {
draw = true;
var coords = getMousePos(paintCanvas);
prevMouseX = coords.x;
prevMouseY = coords.y;
});
$("#paintCanvas").on("mousemove", function(e) {
if(draw) {
var coords = getMousePos(paintCanvas, e);
paintCtx.beginPath();
paintCtx.lineWidth = 10;
paintCtx.strokeStyle = "#000000";
paintCtx.moveTo(prevMouseX, prevMouseY);
paintCtx.lineTo(coords.x, coords.y);
paintCtx.stroke();
prevMouseX = coords.x;
prevMouseY = coords.y;
}
});
$("#paintCanvas").on("mouseup", function(e) {
draw = false;
});
});
</script>
<body>
<canvas id="paintCanvas"></canvas>
</body>

If you want to keep the pixelated effect in the zoom, you need to draw on a temp canvas, then only after copy that temp canvas to the main screen.
You no longer need to zoom in the temp canvas, just draw on 1:1 scale always. When copying to the view canvas, then you apply the zoom (and maybe translate) that you want.
Keep in mind that drawings are anti-aliased, so you when zooming you will see some shades of grey when drawing in black, for instance.
I kept the recording code of #FurqanZafar since it is a good idea to record things in case you want to perform undo : in that case just delete the last record entry and redraw everything.
http://jsfiddle.net/gamealchemist/x5rrvcr0/4/
function updatePaintCanvas() {
paintContext.clearRect(0, 0, paintContext.canvas.width, paintContext.canvas.height);
paintContext.save();
paintContext.translate(cvSize * 0.5, cvSize * 0.5);
paintContext.scale(scale, scale);
paintContext.drawImage(drawCanvas, -cvSize * 0.5, -cvSize * 0.5);
paintContext.restore();
}

Heres the updated fiddle: http://jsfiddle.net/x5rrvcr0/2/ with basic zooming functionality
If you draw multiple paths on mouse move then your sketch will appear broken or disconnected, instead you should only stroke a single path until "mouseup" event.
You can then store these paths in an array and later redraw them at different zoom levels:
function zoom(context, paths, styles, scale) {
context.clearRect(0, 0, context.canvas.width, context.canvas.height);
context.save();
applyStyles(context, styles);
scaleFromCenter(context, scale);
for (var i = 0; i < paths.length; i++) {
context.beginPath();
context.moveTo(paths[i][0].x, paths[i][0].y);
for (var j = 1; j < paths[i].length; j++)
context.lineTo(paths[i][j].x, paths[i][j].y);
context.stroke();
}
context.restore();
};

Canvas variable not updating when link is clicked

See link: http://jndgn.com/colortheory/bezold.html
When clicking the 'Bigger Stripes' link in the nav I'm attempting to simply update the number of colored stripes in the canvas below it. The variable 'stripeNum' is set at 100 and when clicked should change to 20 via:
$(function(){
$("#stripes").on("click",function(e){
e.preventDefault();
if(stripeNum == 100) {
stripeNum = 20;
} else {
stripeNum = 100;
}
});
})
...but when clicked, no action occurs. Do I somehow need to get the canvas to update itself? A bit of an HTML5 rookie here. Thanks in advance for any thoughts.
JSFiddle attached for easier code transparency.

You need to re-draw the canvas, as well as clearing it when you do this. You could simply make this adjustment and you'd be good:
First, wrap the canvas drawing you have in a function:
var contextFill = function() {
context.clearRect(0, 0, canvas.width, canvas.height); // Clear on draw
for (var i = 0; i < stripeNum + 1; i++) {
context.beginPath();
context.rect(0, i * canvas.height / stripeNum, canvas.width, canvas.height / (2 * stripeNum));
context.fillStyle = randomColor;
context.fill();
}
}
Then, call contextFill() on document ready. The only addition I made to the above is context.clearRect, to clear the canvas.
Finally, also call it when your bigger stripes is called, e.g.:
...
if (stripeNum == 100) {
stripeNum = 20;
} else {
stripeNum = 100;
}
contextFill();
If you want to update canvas on window resize, you can watch the resize event on the window. The downside is it can occur many times depending on how the user is resizing. You can use a library like lodash to ensure that the function only is called every N milliseconds. For example:
window.addEventListener('resize', _.debounce(function() {
contextFill();
}, 500));