Develop Reference

Getting mouse point coordinates in Javascript HTML5 Canvas

I am having difficulty of getting the mouse coordinates and the hero's coordinates.
When I click on the bottom right corner of the canvas I get a result of x = 641, y = 386 for the mouse click. The enemy coordinate system is 100% accurate.
It seems that the enemy coordinate system is different than the mouse coordinate system. I want them to be on a single coordinate system. Thanks for your help!
This is the initialization of canvas:
<canvas id="canvas" width = "1664" height = "1000" style = "border:1px solid gray; width: 640px; height 480px;"> </canvas>
function getMousePos(canvas, evt) {
var rect = canvas.getBoundingClientRect();
return {
x: evt.clientX - rect.left,
y: evt.clientY - rect.top
};
}
var mousePos = getMousePos(Context.canvas,e);
//Use mousePos.x or mousePos.y to get the coordinates of the mouse click
var mx, my;
if(e.offsetX) {
mx = e.offsetX;
my = e.offsetY;
}
else if(e.layerX) {
mx = e.layerX;
my = e.layerY;
}
Gun.shoot()
for (var i = EnemyManager.enemies.length - 1; i >= 0; i--) {
var enemy = EnemyManager.enemies[i]
console.log("Enemy: " + enemy.x + " " + enemy.y)
console.log("Mouse: " + mousePos.x + " " + mousePos.y)
if ((enemy.x < mx) && (enemy.y < my) && ( (enemy.x + enemy.width) > mx ) && (enemy.y + enemy.height > my)) {
alert("Target HIT")
}
};
})*

I've had the same problem using canvas (especially it needs to work when scrolled down on the page, offset from the top of the page by arbitrary divs, and at different zoom levels). I've found the approach in this answer to be the most robust as a replacement to that getMousePos function.

You are receiving different coordinates probably because you are using two different methods. In the player's position, you are using offsetX instead of mousePos.X (enemy.X in comparison)
Also, since you are defining the return of getMousePos as X and Y, you need to access them as X and Y. So:
var mx, my;
mx = mousePos.X;
my = mousePos.Y;
EDIT: You may also find this link helpful.

You are clicking outside of canvas see my example, http://jsfiddle.net/gn0pkkra/.
document.getElementById('wrapper').addEventListener('click', on_canvas_click, false);
You can compute the exact trajectory even if you are clicking outside of canvas. Then Cartesian coordinates will have the center (0,0) on you game cannon position in canvas. Compute cannon/gun offset over top left corner of canvas (real 0,0) and use those offsets to detect your trajectories.
Clicking on buttom right of the canvas, or even outside of it's impossible to get negative values.

Canvas pixel coordinate offset after scroll bar is used

I'm using a canvas at the top of a page. Im writing out the pixel coordinates from the canvas at a mousemove event. Normally, the most bottom Y-value is equal to the canvas height, i e. 700px. But after scrollbar is used to scroll down a bit on the page, the bottom y-coordinate in the canvas will change accordingly to say 400px instead.
document.getElementById("mapcanvas").addEventListener("mousemove", getPosition, false);
function getPosition(event)
{
var x = event.x;
var y = event.y;
var canvas = document.getElementById("mapcanvas");
x -= canvas.offsetLeft;
y -= canvas.offsetTop;
document.getElementById("canvascoords").innerHTML = "Canvascoords: "+ "x=" + x + ", y=" + y;
}
... Where "mapcanvas" is my div holding the canvas.
Any ideas of making the y-coordinate independent from usage of scroll bar so that the lower y-coordinate always i 700px?

As you've discovered, canvas.offsetLeft & canvas.offsetTop do not account for scrolling.
To account for scrolling, you can use canvas.getBoundingClientRect
var BB=canvas.getBoundingClientRect();
var x=event.clientX-BB.left;
var y=event.clientY-BB.top;
BTW, you might want to fetch a reference to the canvas element just once outside your getPosition() function instead of fetching it repeatedly inside getPosition().
var canvas = document.getElementById("mapcanvas");
function getPosition(event){
...

Real mouse position in canvas

I'm trying to draw with the mouse over a HTML5 canvas, but the only way that it seems to work well is if the canvas is in the position 0,0 (upper left corner) if I change the canvas position, for some reason it doesn't draw like it should. Here is my code.
function createImageOnCanvas(imageId){
document.getElementById("imgCanvas").style.display = "block";
document.getElementById("images").style.overflowY= "hidden";
var canvas = document.getElementById("imgCanvas");
var context = canvas.getContext("2d");
var img = new Image(300,300);
img.src = document.getElementById(imageId).src;
context.drawImage(img, (0),(0));
}
function draw(e){
var canvas = document.getElementById("imgCanvas");
var context = canvas.getContext("2d");
posx = e.clientX;
posy = e.clientY;
context.fillStyle = "#000000";
context.fillRect (posx, posy, 4, 4);
}
The HTML part
<body>
<div id="images">
</div>
<canvas onmousemove="draw(event)" style="margin:0;padding:0;" id="imgCanvas"
class="canvasView" width="250" height="250"></canvas>
I have read there's a way of creating a simple function in JavaScript to get the right position, but I have no idea about how to do it.

The Simple 1:1 Scenario
For situations where the canvas element is 1:1 compared to the bitmap size, you can get the mouse positions by using this snippet:
function getMousePos(canvas, evt) {
var rect = canvas.getBoundingClientRect();
return {
x: evt.clientX - rect.left,
y: evt.clientY - rect.top
};
}
Just call it from your event with the event and canvas as arguments. It returns an object with x and y for the mouse positions.
As the mouse position you are getting is relative to the client window you’ll have to subtract the position of the canvas element to convert it relative to the element itself.
Example of integration in your code:
// put this outside the event loop..
var canvas = document.getElementById("imgCanvas");
var context = canvas.getContext("2d");
function draw(evt) {
var pos = getMousePos(canvas, evt);
context.fillStyle = "#000000";
context.fillRect (pos.x, pos.y, 4, 4);
}
Note: borders and padding will affect position if applied directly to the canvas element so these needs to be considered via getComputedStyle() – or apply those styles to a parent div instead.
When Element and Bitmap are of different sizes
When there is the situation of having the element at a different size than the bitmap itself, for example, the element is scaled using CSS or there is pixel-aspect ratio etc. you will have to address this.
Example:
function getMousePos(canvas, evt) {
var rect = canvas.getBoundingClientRect(), // abs. size of element
scaleX = canvas.width / rect.width, // relationship bitmap vs. element for x
scaleY = canvas.height / rect.height; // relationship bitmap vs. element for y
return {
x: (evt.clientX - rect.left) * scaleX, // scale mouse coordinates after they have
y: (evt.clientY - rect.top) * scaleY // been adjusted to be relative to element
}
}
With transformations applied to context (scale, rotation etc.)
Then there is the more complicated case where you have applied transformation to the context such as rotation, skew/shear, scale, translate etc. To deal with this you can calculate the inverse matrix of the current matrix.
Newer browsers let you read the current matrix via the currentTransform property and Firefox (current alpha) even provide an inverted matrix through the mozCurrentTransformInverted. Firefox however, via mozCurrentTransform, will return an Array and not DOMMatrix as it should. Neither Chrome, when enabled via experimental flags, will return a DOMMatrix but a SVGMatrix.
In most cases however you will have to implement a custom matrix solution of your own (such as my own solution here – free/MIT project) until this get full support.
When you eventually have obtained the matrix regardless of path you take to obtain one, you’ll need to invert it and apply it to your mouse coordinates. The coordinates are then passed to the canvas which will use its matrix to convert it to back wherever it is at the moment.
This way the point will be in the correct position relative to the mouse. Also here you need to adjust the coordinates (before applying the inverse matrix to them) to be relative to the element.
An example just showing the matrix steps:
function draw(evt) {
var pos = getMousePos(canvas, evt); // get adjusted coordinates as above
var imatrix = matrix.inverse(); // get inverted matrix somehow
pos = imatrix.applyToPoint(pos.x, pos.y); // apply to adjusted coordinate
context.fillStyle = "#000000";
context.fillRect(pos.x-1, pos.y-1, 2, 2);
}
An example of using currentTransform when implemented would be:
var pos = getMousePos(canvas, e); // get adjusted coordinates as above
var matrix = ctx.currentTransform; // W3C (future)
var imatrix = matrix.invertSelf(); // invert
// apply to point:
var x = pos.x * imatrix.a + pos.y * imatrix.c + imatrix.e;
var y = pos.x * imatrix.b + pos.y * imatrix.d + imatrix.f;
Update: I made a free solution (MIT) to embed all these steps into a single easy-to-use object that can be found here and also takes care of a few other nitty-gritty things most ignore.

You can get the mouse positions by using this snippet:
function getMousePos(canvas, evt) {
var rect = canvas.getBoundingClientRect();
return {
x: (evt.clientX - rect.left) / (rect.right - rect.left) * canvas.width,
y: (evt.clientY - rect.top) / (rect.bottom - rect.top) * canvas.height
};
}
This code takes into account both changing coordinates to canvas space (evt.clientX - rect.left) and scaling when canvas logical size differs from its style size (/ (rect.right - rect.left) * canvas.width see: Canvas width and height in HTML5).
Example: http://jsfiddle.net/sierawski/4xezb7nL/
Source:
jerryj comment on http://www.html5canvastutorials.com/advanced/html5-canvas-mouse-coordinates/

You need to get the mouse position relative to the canvas
To do that you need to know the X/Y position of the canvas on the page.
This is called the canvas’s “offset”, and here’s how to get the offset. (I’m using jQuery in order to simplify cross-browser compatibility, but if you want to use raw javascript a quick Google will get that too).
var canvasOffset=$("#canvas").offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
Then in your mouse handler, you can get the mouse X/Y like this:
function handleMouseDown(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
}
Here is an illustrating code and fiddle that shows how to successfully track mouse events on the canvas:
http://jsfiddle.net/m1erickson/WB7Zu/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body{ background-color: ivory; }
canvas{border:1px solid red;}
</style>
<script>
$(function(){
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var canvasOffset=$("#canvas").offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
function handleMouseDown(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
$("#downlog").html("Down: "+ mouseX + " / " + mouseY);
// Put your mousedown stuff here
}
function handleMouseUp(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
$("#uplog").html("Up: "+ mouseX + " / " + mouseY);
// Put your mouseup stuff here
}
function handleMouseOut(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
$("#outlog").html("Out: "+ mouseX + " / " + mouseY);
// Put your mouseOut stuff here
}
function handleMouseMove(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
$("#movelog").html("Move: "+ mouseX + " / " + mouseY);
// Put your mousemove stuff here
}
$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});
$("#canvas").mouseup(function(e){handleMouseUp(e);});
$("#canvas").mouseout(function(e){handleMouseOut(e);});
}); // end $(function(){});
</script>
</head>
<body>
<p>Move, press and release the mouse</p>
<p id="downlog">Down</p>
<p id="movelog">Move</p>
<p id="uplog">Up</p>
<p id="outlog">Out</p>
<canvas id="canvas" width=300 height=300></canvas>
</body>
</html>

The easiest way to compute the correct mouse click or mouse move position on a canvas event is to use this little equation:
canvas.addEventListener('click', event =>
{
let bound = canvas.getBoundingClientRect();
let x = event.clientX - bound.left - canvas.clientLeft;
let y = event.clientY - bound.top - canvas.clientTop;
context.fillRect(x, y, 16, 16);
});
If the canvas has padding-left or padding-top, subtract x and y via:
x -= parseFloat(style['padding-left'].replace('px'));
y -= parseFloat(style['padding-top'].replace('px'));

Refer this question: The mouseEvent.offsetX I am getting is much larger than actual canvas size
.I have given a function there which will exactly suit in your situation

How to fix offset issue for mouse pointer on canvas on HTML5 fullscreen mode?

I am using the HTML5 full_screen API to scale up this letter to full screen mode of the browser.
Follow https://bubbleideas.com/letters/html5-full_screen-api for the demo and steps to reproduce.
There seems a problem/bug with the way browser returns (x,y) value of pointer location of the mouse. In full_screen mode when you scroll down an offset is introduced between the mouse pointer and scribbled path.
Here are the steps to reproduce the issue. (go to above demo link)
Click on button on the right hand top of this page.
Click on "Free hand" drawing tool on the right bottom side. It will open up a stationery panel (Choose pen or pencil tool)
Scribble on the drawing area a couple of times
Now scroll down a bit and try to scribble with the same pen. You ll notice that there is gap/offset between mouse pointer position and scribbled path(this is the issue). Ideally, there should be no gap in the full screen mode either
Has someone been here before? Also note this works perfectly fine for other shapes like the square, circle and triangle without any offset.
UPDATE: (As asked by "Iftah" in the commment below)
As per fabric js I use calcOffset() which recalculates offset on every mouse down. As far as other functions are concerned we do some thing like this. Hopefully this gives some idea
$("#rectangle-function").click(function (evt1) {
doCanvasUp();
initObjectDrawing();
//canvas.isDrawingMode = true;
canvas1 = document.createElement("canvas");
canvas1.height = canvas.height;
canvas1.width = canvas.width;
canvas1.id = "dummy-canvas";
canvas1.style.zIndex = 998;
canvas1.style.position = "absolute";
$(".page-body").prepend(canvas1);
$("#dummy-canvas").mousedown(function (evt) {
var context1 = canvas1.getContext("2d");
var offset = $("#dummy-canvas").offset();
startX = evt.pageX - offset.left;
startY = evt.pageY - offset.top;
context1.beginPath();
$("#dummy-canvas").mousemove(function (event) {
context1.clearRect(0, 0, canvas1.width, canvas1.height);
context1.strokeStyle = "#ff0000";
context1.lineWidth = 1;
context1.moveTo(startX, startY);
var offset1 = $("#dummy-canvas").offset();
var x = event.pageX - offset1.left;
var y = event.pageY - offset1.top;
var diffX = x - startX;
var diffY = y - startY;
context1.strokeRect(startX, startY, diffX, diffY);
context1.closePath();
context1.beginPath();
}).mouseup(function (eventf) {
$("#dummy-canvas").unbind('mousemove');
$("#dummy-canvas").unbind('mouseup');
var offset = $("#dummy-canvas").offset();
//$("#dummy-canvas").remove();
context1.clearRect(0, 0, canvas1.width, canvas1.height);
var endX = eventf.pageX - offset.left;
var endY = eventf.pageY - offset.top;
var diffX = endX - startX;
var diffY = endY - startY;
var rect = new fabric.Rect({
left: startX + diffX * 0.5,
top: startY + diffY * 0.5,
width: diffX,
height: diffY,
opacity: 1,
fill: null,
stroke: color
});
canvas.add(rect);
});
});

Without looking at your code it is impossible to tell exactly where the error is...
One problem could be when reading the mouse coordinates, IE. You could be wrongly using event.pageY instead of event.clientY or a similar confusion (see What is the difference between screenX/Y, clientX/Y and pageX/Y?)
Or you can say the problem is how you use those coordinates,
ie. if you take the mouse event coordinates in screen space then before applying it on the canvas you need to subtract the canvas element screen offset and add the scrolling offset...
Since you have one tool that works and one that doesn't you can compare them and see where they differ.

Html Canvas Resizing And Coordinate Space

Note: I'm using Google Chrome
Currently I have this test page
http://www.evecakes.com/doodles/canvas_size_issue.htm
It should draw a rectangle right below the mouse cursor, but there are some really weird scaling issues which is causing it to draw at a varying offset. I think it's because the canvas coordinate space is not increasing along with its html size. Is there a way to increase that coordinate space size?
Here's the javascript function
$('#mapc').mousemove(function (event) {
var canvas = $('#mapc');
var ctx = canvas[0].getContext('2d');
ctx.fillStyle = "rgb(200,0,0)";
ctx.fillRect(event.clientX, event.clientY, 55, 50);
document.title = event.clientX + " --- " + event.clientY;
});

Set the width and height HTML attributes of the canvas. Right now, it's assuming its default width and the CSS is just stretching it, and it appears as if it is scaling.
A side-note, you can use this in the mousemove event - it is a reference to #mapc element, so you won't have to query the DOM on every mouse move.
var offset = $('#mapc').offset();
$('#mapc').mousemove(function (event) {
var ctx = this.getContext('2d');
ctx.fillStyle = "rgb(200,0,0)";
ctx.fillRect(event.pageX - offset.left, event.pageY - offset.top, 1, 1);
});