This question already has answers here:
Real mouse position in canvas
(5 answers)
Closed 4 years ago.
I have a responsive canvas with and it draw a square on mousemove, however I cannot target the mouse position of the canvas, however it works if the canvas is not responsive. how can I target the position of the mouse? Thank you in advance.
https://jsfiddle.net/Paul017/yz028mwe/3/
This is the code.
HTML:
<h4>Drag the mouse to create a rectangle</h4>
<canvas id="canvas" width=300 height=300></canvas>
CSS:
body{ background-color: ivory; }
#canvas{border:1px solid red; width:100%}
JS:
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
ctx.strokeStyle = "rgba(255, 235, 59, 0.5)";
ctx.fillStyle = "red";
ctx.fill();
ctx.lineWidth = 1;
var $canvas = $("#canvas");
var canvasOffset = $canvas.offset();
var offsetX = canvasOffset.left;
var offsetY = canvasOffset.top;
var scrollX = $canvas.scrollLeft();
var scrollY = $canvas.scrollTop();
var isDown = false;
var startX;
var startY;
function handleMouseDown(e) {
e.preventDefault();
e.stopPropagation();
startX = parseInt(e.clientX - offsetX);
startY = parseInt(e.clientY - offsetY);
// set a flag indicating the drag has begun
isDown = true;
}
function handleMouseUp(e) {
e.preventDefault();
e.stopPropagation();
isDown = false;
}
function handleMouseOut(e) {
e.preventDefault();
e.stopPropagation();
isDown = false;
}
function handleMouseMove(e) {
e.preventDefault();
e.stopPropagation();
// if we're not dragging, just return
if (!isDown) {
return;
}
// get the current mouse position
mouseX = parseInt(e.clientX - offsetX);
mouseY = parseInt(e.clientY - offsetY);
ctx.clearRect(0, 0, canvas.width, canvas.height);
var width = mouseX - startX;
var height = mouseY - startY;
ctx.strokeRect(startX, startY, width, height);
}
You can use the selectionStart or selectionEnd property of target method.
You can get target every position with:
e.target.selectionStart
or e.target.selectionEnd
In your code:
function handleMouseMove(e) {
console.log('target event ', e.target);
console.log('selectionStart', e.target.selectionStart);
console.log('selectionEnd', e.target.selectionEnd)
e.preventDefault();
e.stopPropagation();
// if we're not dragging, just return
if (!isDown) {
return;
}
// get the current mouse position
mouseX = parseInt(e.clientX - offsetX);
mouseY = parseInt(e.clientY - offsetY);
ctx.clearRect(0, 0, canvas.width, canvas.height);
var width = mouseX - startX;
var height = mouseY - startY;
ctx.strokeRect(startX, startY, width, height);
}
If the console log show undefined, try to console log you e.target. You will find the selectionStart and selectionEnd method.
Let me know if you need something else.
I have already figured it out by using these mouse positions code to target the x and y location in a responsive canvas
e.offsetX * canvas.width / canvas.clientWidth | 0
e.offsetY * canvas.width / canvas.clientWidth | 0
https://jsfiddle.net/Paul017/yz028mwe/28/
Related
I'm trying to create a mobile app in which user can draw multiple controls (rectangle) on a canvas.
It works fine if canvas fits the screen, but the controls are not drawn well when I increase the canvas height and width out of the screen (a scroll appear when i increase the size of canvas more than the size of mobile screen).
How to fix this in scroll mode?
Use Chrome Inspect mobile view (iPhone 6/7/8) to test this code
Use Pointer drag to draw controls
<html>
<body>
<canvas id="canvas_transparent" height="3000" width="1000" style="border: 1px solid black;"></canvas>
</body>
</html>
<script type="text/javascript">
var canvas = document.getElementById("canvas_transparent");
var ctx = canvas.getContext("2d");
var endX;
var startX;
var endY;
var startY;
var positions = [];
//--------------------------------------------------------
canvas.addEventListener("touchstart", function(e) {
mousePos = getTouchPos(canvas, e);
startX = mousePos.x;
startY = mousePos.y;
var touch = e.touches[0];
var mouseEvent = new MouseEvent("mousedown", {
clientX: touch.clientX,
clientY: touch.clientY
});
canvas.dispatchEvent(mouseEvent);
}, false);
//---------------------------------------------------------
canvas.addEventListener("touchend", function(e) {
positions.push({
s_x: startX,
s_y: startY,
e_x: endX,
e_y: endY
});
drawAllControls();
console.log(positions);
var mouseEvent = new MouseEvent("mouseup", {});
canvas.dispatchEvent(mouseEvent);
}, false);
//---------------------------------------------------------
canvas.addEventListener("touchmove", function(e) {
var touch = e.touches[0];
endX = touch.clientX;
endY = touch.clientY;
drawSquare()
var mouseEvent = new MouseEvent("mousemove", {
clientX: touch.clientX,
clientY: touch.clientY
});
canvas.dispatchEvent(mouseEvent);
}, false);
//--------------------------------------------------------
// Get the position of a touch relative to the canvas
function getTouchPos(canvasDom, touchEvent) {
var rect = canvasDom.getBoundingClientRect();
return {
x: touchEvent.touches[0].clientX - rect.left,
y: touchEvent.touches[0].clientY - rect.top
};
}
//--------------------------------------------------------
function drawSquare() {
// creating a square
var w = endX - startX;
var h = endY - startY;
var offsetX = (w < 0) ? w : 0;
var offsetY = (h < 0) ? h : 0;
var width = Math.abs(w);
var height = Math.abs(h);
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.beginPath();
ctx.rect(startX + offsetX, startY + offsetY, width, height);
ctx.fillStyle = "#222222";
ctx.fill();
ctx.lineWidth = 2;
ctx.strokeStyle = 'black';
ctx.stroke();
}
//----------------------------------------------------------
function drawAllControls() {
for (var i = 0; i < positions.length; i++) {
var w = positions[i].e_x - positions[i].s_x;
var h = positions[i].e_y - positions[i].s_y;
var offsetX = (w < 0) ? w : 0;
var offsetY = (h < 0) ? h : 0;
var width = Math.abs(w);
var height = Math.abs(h);
ctx.beginPath();
ctx.rect(positions[i].s_x + offsetX, positions[i].s_y + offsetY, width, height);
ctx.fillStyle = "#222222";
ctx.fill();
ctx.lineWidth = 2;
ctx.strokeStyle = 'black';
ctx.stroke();
}
}
</script>
clientX/clientY do not factor in scroll offset. I think you want pageX/pageY
https://developer.mozilla.org/en-US/docs/Web/API/Touch
// Get the position of a touch relative to the canvas
function getTouchPos(canvasDom, touchEvent) {
var rect = canvasDom.getBoundingClientRect();
return {
x: touchEvent.touches[0].pageX - rect.left,
y: touchEvent.touches[0].pageY - rect.top
};
}
Hi i think this will resolve your problem. Put this in the head tag
<meta name="viewport" content="width=device-width, initial-scale=1.0"/>
working link page
I am trying to create a drag rectangle in Javascript, which allows a user to draw a rectangle by dragging for the purpose of selecting a region on the canvas.
I am trying to use "xor" operation to do the repeated drawing on mouse move handler. But I am unable to get this right.
My code is a single html file, as shown below.
<!DOCTYPE html>
<html>
<head>
<style>
/*body, html {
width: 100%
}*/
#canvas {
background-color: #EFEFEF;
width: 100%;
}
</style>
</head>
<body>
<canvas id="canvas">
canvas
</canvas>
<script src="jquery-1.11.1.js"></script>
<script>
var ctx = document.getElementById("canvas").getContext('2d');
ctx.lineWidth = 1;
var canvasOffset, offsetX, offsetY;
var isDrawing = false;
var prevX, prevY;
canvasOffset = $("#canvas").offset();
offsetX = canvasOffset.left;
offsetY = canvasOffset.top;
$("#canvas").on('mousedown', function(e) {
handleMouseDown(e);
}).on('mouseup', function(e) {
handleMouseUp();
}).on('mousemove', function(e) {
handleMouseMove(e);
});
function handleMouseUp() {
isDrawing = false;
canvas.style.cursor = "default";
}
function handleMouseMove(e) {
if (isDrawing) {
var mouseX = e.clientX - offsetX;
var mouseY = e.clientY - offsetY;
ctx.globalCompositeOperation = "xor";
ctx.strokeStyle = "red";
ctx.beginPath();
console.log("Erasing rect # ", start_X, start_Y, prevX - start_X, prevY - start_Y);
ctx.rect(start_X, start_Y, prevX - start_X, prevY - start_Y);
console.log("Drawing rect # ", start_X, start_Y, mouseX - start_X, mouseY - start_Y);
ctx.rect(start_X, start_Y, mouseX - start_X, mouseY - start_Y);
prevX = mouseX, prevY = mouseY;
ctx.stroke();
}
}
function handleMouseDown(e) {
canvas.style.cursor = "crosshair";
isDrawing = true;
start_X = e.clientX - offsetX;
start_Y = e.clientY - offsetY;
prevX = start_X, prevY = start_Y;
}
</script>
</body>
</html>
Here is the jsfiddle link for my code. http://jsfiddle.net/bjc7bp0q/1/
The problems are:
i) It doesn't start drawing from the point where mouse was clicked.
ii) It doesn't erase previous rectangle strokes.
Any help is highly appreciated.
Update
As suggested by Derek, updated the jsfiddle, with corrections for canvas stretch due to width:100%, and pixel offset by 0.5 for blur compensation.
updated jsfiddle is here (http://jsfiddle.net/bjc7bp0q/8/). This almost works. But I can still see some stray residual line fragments when dragging.
Thanks for any additional help.
Second update:
This seems to work well in Google chrome, but NOT in IE or Firefox. May be its an implementation issue of these browsers.
I'm trying to achieve the same effect I'm posting here using Reactive Extensions for Javascript (RX-JS).
I'm a bit puzzled on how to do it.
Here is the page:
<!DOCTYPE html>
<html>
<head>
<title>drag and drop</title>
</head>
<style type="text/css">
canvas {
border:1px solid steelblue;
background-color: whitesmoke;
}
</style>
<body>
<canvas id="canvas" width=300 height=300></canvas>
<script type="text/javascript" src="http://code.jquery.com/jquery-1.11.0.min.js"></script>
<script type="text/javascript">
$(function() {
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var canvasOffset = $("#canvas").offset();
var offsetX = canvasOffset.left;
var offsetY = canvasOffset.top;
var drawing = false;
var mouseX = 0;
var mouseY = 0;
function handleMouseDown(e) {
mouseX = parseInt(e.clientX - offsetX);
mouseY = parseInt(e.clientY - offsetY);
drawing= true;
}
function handleMouseUp(e) {
drawing = false;
}
function handleMouseMove(e) {
if(drawing){
mouseeX = parseInt(e.clientX - offsetX);
mouseeY = parseInt(e.clientY - offsetY);
$("#movelog").html("Move: " + mouseX + " / " + mouseY);
var ctx = canvas.getContext("2d");
// some cleanup code
ctx.save();
ctx.setTransform(1, 0, 0, 1, 0, 0);
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.restore();
ctx.beginPath();
ctx.moveTo(mouseX,mouseY);
ctx.lineTo(mouseeX,mouseeY);
ctx.stroke();
}
}
$("#canvas").mousedown(function(e) {
handleMouseDown(e);
});
$("#canvas").mousemove(function(e) {
handleMouseMove(e);
});
$("#canvas").mouseup(function(e) {
handleMouseUp(e);
});
});
</script>
</body>
</html>
I think I should create observables for the mouseDown, mouseMove and mouseUp events.
var mouseDown = Rx.Observable.fromEvent(canvas, 'mousedown');
var mouseMove = Rx.Observable.fromEvent(canvas, 'mousemove');
var mouseUp = Rx.Observable.fromEvent(canvas, 'mouseup');
but I don't know how to combine them. I think should start observing the mousedown, then collect all the moves until the mouseup is raised, and in the whil redraw the line from the starting point to the current point where the mouse is during the mousemove.
Do you have any ideas? Thanks a lot.
°°°°°°°°°°°°°°°°°°°°°°EDIT°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°
Here is my code after the answer by Brandon:
$(function() {
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext("2d");
var canvasOffset = $("#canvas").offset();
var offsetX = canvasOffset.left;
var offsetY = canvasOffset.top;
var mouseDown = Rx.Observable.fromEvent($("#canvas"), 'mousedown');
var mouseMove = Rx.Observable.fromEvent($("#canvas"), 'mousemove');
var mouseUp = Rx.Observable.fromEvent($("#canvas"), 'mouseup');
// keep a reference to the pisition when the mouse down was fired
// then flatten the stream with concatAll
var traceLineStream = mouseDown.map(function(md) {
var movesFromMouseDown = mouseMove.takeUntil(mouseUp);
var movesFromMouseDownAndMouseDown = movesFromMouseDown.map(function(mm) {
return {
mouseDownPoint: md,
mouseMovePoint: mm
}
});
return movesFromMouseDownAndMouseDown;
}).concatAll();
var subscription = traceLineStream.subscribe(
function(y) {
var mouseDown = y.mouseDownPoint;
var mouseMove = y.mouseMovePoint;
var mouseDownX = parseInt(mouseDown.clientX - offsetX);
var mouseDownY = parseInt(mouseDown.clientY - offsetY);
var mouseMoveX = parseInt(mouseMove.clientX - offsetX);
var mouseMoveY = parseInt(mouseMove.clientY - offsetY);
ctx.save();
ctx.setTransform(1, 0, 0, 1, 0, 0);
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.restore();
ctx.beginPath();
ctx.moveTo(mouseDownX, mouseDownY);
ctx.lineTo(mouseMoveX, mouseMoveY);
ctx.stroke();
},
function(e) {
console.log('onError: ' + e.message);
},
function() {
console.log('onCompleted');
});
});
First, combined the streams so you have a stream of events that represent a single drag.
var drag = mouseDown.first().concat(mouseMove.takeUntil(mouseUp));
Next, project this stream of events into a stream of previous,current tuples.
var moves = drag
.scan({}, function(acc, x) {
return { previous: acc.current, current: x };
})
.skip(1);
Now we have a stream that only works the first time. When it ends, we want to start listening for the next drag:
var allMoves = moves.repeat();
Finally, subscribe:
allMoves.subscribe(function (move) {
var mouseX = move.previous.clientX - offsetX,
mouseY = move.previous.clientY - offsetY,
mouseeX = move.current.clientX - offsetX,
mouseeY = move.current.clientY - offsetY,
...
});
Putting it all together without all of the intermediate variables:
mouseDown
.first()
.concat(mouseMove.takeUntil(mouseUp))
.scan({}, function(acc, x) {
return { previous: acc.current, current: x };
})
.skip(1)
.repeat()
.subscribe(function (move) {
var mouseX = move.previous.clientX - offsetX,
mouseY = move.previous.clientY - offsetY,
mouseeX = move.current.clientX - offsetX,
mouseeY = move.current.clientY - offsetY,
...
});
I am building a paint tool application.I am using two canvases. One for loading a background image and one for do the painting. Painting canvas is placed over the other one. I used two canvases because i don't want the eraser tool to take effect on the image. On my code y position drawn by the application is not always accurate. In most cases the line is drawn higher than the actual drawn path.
var baseCanvas,baseContext,canvasObj,context;
var lastX,lastY,mouseX,mouseY;
var isMouseDown = false;
var mode = "pen";
$(window).load(function()
{
baseCanvas = document.getElementById("imageCanvas");
baseContext = baseCanvas.getContext("2d");
baseContext.strokeStyle = 'Black';
baseContext.fillStyle = "skyBlue";
baseContext.lineWidth = 5;
baseContext.fillRect(0, 0, baseCanvas.width, baseCanvas.height);
canvasObj = document.getElementById("drawingCanvas");
context = canvasObj.getContext("2d");
context.strokeStyle = 'Black';
context.lineCap = "round";
context.lineJoin = "round";
context.lineWidth = 2;
function handleMouseDown(e) {
mouseX = parseInt(e.clientX - $('#drawingCanvas').offset().left);
mouseY = parseInt(e.clientY - $('#drawingCanvas').offset().top);
lastX = mouseX;
lastY = mouseY;
isMouseDown = true;
}
function handleMouseUp(e) {
mouseX = parseInt(e.clientX - $('#drawingCanvas').offset().left);
mouseY = parseInt(e.clientY - $('#drawingCanvas').offset().top);
isMouseDown = false;
}
function handleMouseOut(e) {
mouseX = parseInt(e.clientX - $('#drawingCanvas').offset().left);
mouseY = parseInt(e.clientY - $('#drawingCanvas').offset().top);
isMouseDown = false;
}
function handleMouseMove(e) {
mouseX = parseInt(e.clientX - $('#drawingCanvas').offset().left);
mouseY = parseInt(e.clientY - $('#drawingCanvas').offset().top);
if (isMouseDown) {
context.beginPath();
if (mode == 'pen') {
context.globalCompositeOperation = "source-over";
context.moveTo(lastX, lastY);
context.lineTo(mouseX, mouseY);
context.stroke();
} else {
context.globalCompositeOperation = "destination-out";
context.arc(lastX, lastY, 5, 0, Math.PI * 2, false);
context.fill();
}
lastX = mouseX;
lastY = mouseY;
}
}
$(document).on('mousedown',$("#drawingCanvas"),function (e) {
handleMouseDown(e);
});
$(document).on('mousemove',$("#drawingCanvas"),function (e) {
handleMouseMove(e);
});
$(document).on('mouseup',$("#drawingCanvas"),function (e) {
handleMouseUp(e);
});
$(document).on('mouseout',$("#drawingCanvas"),function (e) {
handleMouseOut(e);
});
});
function setCanvas(imageFile)
{
var base_image = new Image();
base_image.src = window.URL.createObjectURL(imageFile[0]);
baseContext.save();
baseContext.clearRect(0, 0, baseCanvas.width, baseCanvas.height);
baseContext.beginPath();
base_image.onload = function()
{
baseContext.drawImage(base_image,0,0, $('#imageCanvas').width(), $('#imageCanvas').height());
baseContext.restore();
}
}
If you scroll the browser window, you need to adjust for that scrolling.
Here's how:
var scrollAdjustment=$("html,body").scrollTop();
mouseY+=scrollAdjustment;
And if you scroll horizontally, you would need to adjust for horizontal scrolling too.
BTW, since the canvas offset doesn't change, you could pre-calculate the canvas offsets one time instead of calculating it each time in your event handlers.
In your setup up top:
var canvasOffset=$("#drawingCanvas").offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
And in your event handlers:
var scrollAdjustment=$("html,body").scrollTop();
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY+scrollAdjustment);
Provided you are using the clientX, clientY properties of your mouse events, and that you hooked the events on the window, you should getBoundingClientRect on your canvas to know its top and left position, then substract them.
https://developer.mozilla.org/en-US/docs/Web/API/element.getBoundingClientRect
Rq : You might think also about hooking the events on the canvas.
I am trying to create a simple canvas program where the user can consistently create new shapes. This one is just a basic rectangle creator (I am hoping to expand it more to circles, lines, and maybe even other stuff). Right now though I have created something that is working in a really weird way.
<html>
<head>
<meta chartset="utf-8">
<title>Dragging a square</title>
<script type="text/javascript">
var canvas, context, startX, endX, startY, endY;
var mouseIsDown = 0;
function init() {
canvas = document.getElementById("canvas");
context = canvas.getContext("2d");
canvas.addEventListener("mousedown", mouseDown, false);
canvas.addEventListener("mousemove", mouseXY, false);
document.body.addEventListener("mouseup", mouseUp, false);
}
function mouseUp() {
mouseIsDown = 0;
//mouseXY();
}
function mouseDown() {
mouseIsDown = 1;
startX = event.clientX;
startY = event.clientY;
mouseXY();
}
function mouseXY(eve) {
if (!eve) {
var eve = event;
}
endX = event.pageX - canvas.offsetLeft;
endY = event.pageY - canvas.offsetTop;
drawSquare();
}
function drawSquare() {
// creating a square
var width = Math.abs(startX - endX);
var height = Math.abs(startY - endY);
context.beginPath();
context.rect(startX, startY, width, height);
context.fillStyle = "yellow";
context.fill();
context.lineWidth = 7;
context.strokeStyle = 'black';
context.stroke();
}
</script>
</head>
<body onload="init()">
<canvas id="canvas" width="400" height="400" style="border: 1px solid black; cursor: pointer;"></canvas>
</body>
</html>
Sorry about the slightly weird formatting when I copy and pasted my code. I think the problem is my mouseXY function. What I want is the user to click somewhere on the canvas a drag the mouse to create a rectangle, when the user lets go that is the end of that operation and they can create a whole new rectangle right after. At this point the program kind of just lets me click and create a new rectangle but if I let go of the mouse button it doesn't stop, in fact I have to click again to make it stop which then creates a new rectangle. I am still very new to this and I am having a lot of trouble with this, I will continue to work on this and if I figure it out I will let the site know. Thank you and have a great day!
Well I got this to work (thanks to #Ken) but now I am trying to solve a new problem. I want to be able to put multiple rectangles on the canvas. I created a function that represents the Rectangle and then created a draw function within the rectangle function to draw out a rectangle. I created a new function called addShape() that ideally creates the rectangle object and pushes into an array called square and drawShapes() that is supposed to erase everything on the canvas and redraws everything. Here is what I have so far:
<html>
<head>
<meta chartset="utf-8">
<title>Dragging a square</title>
<script type="text/javascript">
function Rectangle(canvas, x, y, width, height,color) {
//this.context = canvas.getContext("2d");
this.x = x;
this.y = y;
this.width = width;
this.height = height;
this.color = color;
this.draw = function() {
this.context.globalAlpha = 0.85;
this.context.beginPath();
this.context.rect(this.x, this.y, this.width, this.height);
this.context.fillStyle = this.color;
this.context.strokeStyle = "black";
this.context.lineWidth = 1;
this.context.fill();
this.context.stroke();
};
};
// hold the canvas and context variable, as well as the
// starting point of X and Y and the end ones
var canvas, context, startX, endX, startY, endY;
var mouseIsDown = 0;
// An array that holds all the squares
var squares = [];
window.onload = function() {
canvas = document.getElementById("canvas");
context = canvas.getContext("2d");
canvas.addEventListener("mousedown", mouseDown, false);
canvas.addEventListener("mousemove", mouseXY, false);
canvas.addEventListener("mouseup", mouseUp, false);
}
function mouseUp(eve) {
if (mouseIsDown !== 0) {
mouseIsDown = 0;
var pos = getMousePos(canvas, eve);
endX = pos.x;
endY = pos.y;
//Square(); //update on mouse-up
addShape(); // Update on mouse-up
}
}
function mouseDown(eve) {
mouseIsDown = 1;
var pos = getMousePos(canvas, eve);
startX = endX = pos.x;
startY = endY = pos.y;
// Square(); //update
addShape();
}
function mouseXY(eve) {
if (mouseIsDown !== 0) {
var pos = getMousePos(canvas, eve);
endX = pos.x;
endY = pos.y;
//Square();
addShape();
}
}
function getMousePos(canvas, evt) {
var rect = canvas.getBoundingClientRect();
return {
x: evt.clientX - rect.left,
y: evt.clientY - rect.top
};
}
function addShape() {
var w = endX - startX;
var h = endY - startY;
var offsetX = (w < 0) ? w : 0;
var offsetY = (h < 0) ? h : 0;
var width = Math.abs(w);
var height = Math.abs(h);
var s = new Rectangle(startX + offsetX, startY + offsetY, width, height, "yellow");
squares.push(s);
// Update the display
drawShapes();
}
function drawShapes() {
context.clearRect(0,0,canvas.width,canvas.height);
for (var i = 0; i < squares.length; i++) {
var shape = squares[i];
shape.draw();
};
}
function clearCanvas() {
squares = [];
drawShapes();
}
</script>
</head>
<body onload="addShape()">
<canvas id="canvas" width="400" height="400" style="border: 1px solid black; cursor: pointer;"></canvas><br>
<button onclick="clearCanvas()">Clear Canvas</button>
</body>
</html>
I am pretty sure I broke the original code... thank you for any help!
You need to modify a couple of things in the code: (edit: there are many issues with this code. I went through some of them inline here, but haven't tested. If you put it in a fiddle it's easier for us to check)..
Fiddle
When mouse down occur initialize both start and end points. Call a common draw function that is not dependent on the event itself:
function mouseDown(eve) {
mouseIsDown = 1;
var pos = getMousePos(canvas, eve);
startX = endX = pos.x;
startY = endY = pos.y;
drawSquare(); //update
}
At mouse up, only register if isMouseDown is true, else this function will handle all incoming up-events (as you have attatched it to document, which is correct - window could have been used too):
function mouseUp(eve) {
if (mouseIsDown !== 0) {
mouseIsDown = 0;
var pos = getMousePos(canvas, eve);
endX = pos.x;
endY = pos.y;
drawSquare(); //update on mouse-up
}
}
Only draw if mouseisdown is true:
function mouseXY(eve) {
if (mouseIsDown !== 0) {
var pos = getMousePos(canvas, eve);
endX = pos.x;
endY = pos.y;
drawSquare();
}
}
In addition you will need to clear the previous area of the rectangle before drawing a new or else it won't show when you draw a bigger rectangle and then move the mouse back to draw a smaller one.
For simplicity you can do:
function drawSquare() {
// creating a square
var width = Math.abs(startX - endX);
var height = Math.abs(startY - endY);
context.clearRect(0, 0, context.width, context.height);
//or use fillRect if you use a bg color
context.beginPath();
context.rect(startX, startY, width, height);
context.fillStyle = "yellow";
context.fill();
context.lineWidth = 7;
context.strokeStyle = 'black';
context.stroke();
}
Use this for mouse position:
function getMousePos(canvas, evt) {
var rect = canvas.getBoundingClientRect();
return {
x: evt.clientX - rect.left,
y: evt.clientY - rect.top
};
}