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How to move a canvas element on top of another canvas element

I have 2 canvas elements on top of each other and i want to move the canvas element on top on mouse drag but it produces weird results.
This is my code for the events (the variable cvs is the canvas element which is on top of other canvas element)
var drag = false;
cvs.addEventListener('mousedown', function(event) {
drag = true;
});
cvs.addEventListener('mouseup', function(event) {
drag = false;
});
cvs.addEventListener('mousemove', function(event) {
if (drag) {
const rect = cvs.getBoundingClientRect()
const x = event.clientX - rect.left;
const y = event.clientY - rect.top;
cvs.style.left = x + "px";
cvs.style.top = y + "px";
console.log(x, y);
}
});
When I drag the top canvas it starts to flicker back-and-forth between 2 positions

At a glance, it looks like you are using a relative value to set an absolute position.
So, first iteration, the left position updates to x, then the next iteration you subtract the last value of x from the mouse position. I think this is going to move it on and off screen.
say, clientX is at 100, and left is at 10.
T1 -> x = 100 - 10 = 90,
T2 -> x = 100 - 90 = 10.
Hence the "flickering"
What you want to do, is take the relative movement value of the mouse and move the element by the same amount.
So on mouse down, record the mouse initial position and element initial position.
Subtract the initial mouse position from the mouse position on each mouse move iteration, and assign the initial element position plus the relative change to the element.
var initialPosition = null
var initialMouseCoords = null
cvs.addEventListener('mousedown', function(event) {
initialPosition = cvs.getBoundingClientRect()
initialMouseCoords = {clientX: event.clientX, clientY: event.clientY}
});
cvs.addEventListener('mouseup', function(event) {
initialPosition = null
initialMouseCoords = null
});
cvs.addEventListener('mousemove', function(event) {
if (initialMouseCoords) {
const dx = event.clientX - initialMouseCoords.clientX;
const dy = event.clientY - initialMouseCoords.clientY;
cvs.style.left = initialPosition.left + dx;
cvs.style.top = initialPosition.top + dy;
console.log(dx, dy);
}
});
Bare in mind there are drag events depending on your use case, you might want to explore that as an alternative.

making canvas register mouse move with div over it

I have a <canvas> element that spans the width and height of my webpage, like a background. It has interactive elements that rely on mouse coordinates. When the canvas was in it's own block (i.e. nothing over it) the interactive elements worked fine. But now that it's got divs over it it's not picking up any of the mouse interactions.
Below is my javascript code for the mousemove stuff. Why would items on top affect it picking up mouse xy coordinates, and how do I fix it?
var mouse = {x:-100,y:-100};
var mouseOnScreen = false;
canvas.addEventListener('mousemove', MouseMove, false);
canvas.addEventListener('mouseout', MouseOut, false);
var MouseMove = function(e) {
if (e.layerX || e.layerX == 0) {
//Reset particle positions
mouseOnScreen = true;
mouse.x = e.layerX - canvas.offsetLeft;
mouse.y = e.layerY - canvas.offsetTop;
}
}
var MouseOut = function(e) {
mouseOnScreen = false;
mouse.x = -100;
mouse.y = -100;
}
var update = function(){
var i, dx, dy, sqrDist, scale;
//...... this chunk is the only part of the function that references the mouse
dx = parts[i].x - mouse.x;
dy = parts[i].y - mouse.y;
sqrDist = Math.sqrt(dx*dx + dy*dy);
if (sqrDist < 20){
parts[i].r = true;
}
.....
}

If you don't need the top divs to be mouse-aware then set their CSS pointer-events:none; and the mouse events will filter down to your canvas underneath. Questioner needs responsive buttons placed over the canvas.
If the top divs do need to respond to mouse events, you might have to listen for mouse events on the window and convert those to canvas coordinates that your app can respond to.
You can listen for mousemove events on the window and get all moves -- even when over button elements. Also listen for mouseout events on the window. Since the canvas spans the window, you know mouseout happens when mouseevent.clientX & mouseevent.clientY report the coordinates are outside the window

Solved the issue with this code I found from another SO answer that I modified a bit.
function simulate(e) {
var evt = document.createEvent("MouseEvents");
evt.initMouseEvent("mousemove", true, true, window,
0, e.screenX, e.screenY, e.clientX, e.clientY, false, false, false, false, 0, null);
canvas.dispatchEvent(evt);
console.log("Emulated.");
}
$("body > section, body > header").each(function(){
this.addEventListener("mousemove", simulate);
});
Also, as a side note. Changed original layerX and layerY in mouse code to offsetX and offsetY to work in firefox (in addition to all other browsers)

Skew a line on canvas using JavaScript

I am drawing lines on a canvas. The user can select a particular line and be able to skew that line. By skew, I mean they can drag one end point of the line to a desired point on the same x-axis. How can I do this using JavaScript and HTML5 canvas?

The general way to draw a line is this:
ctx.moveTo(line.startX, line.startY);
ctx.lineTo(line.endX, line.endY);
ctx.stroke();
and then you can add EventListeners and check to see if the mouse is near the line...
window.addEventListener("mousemove", function(e)
{
mouse.x = e.layerX || e.offsetX;
mouse.y = e.layerY || e.offsetY;
// check to see if the mouse is near the line(s) here...
// you can change to x/y and start/end
// example:
if (mouse.x <= line.startX + 5 || mouse.x >= line.startX - 5)
{
// mouse is within 5px of first x
}
});

Controlling window movement by mouse

How do I control a window by mouse movement in javascript?? I mean the page should auto scroll on moving the mouse.

This can be done using parallax like in this http://www.franckmaurin.com/the-parallax-effects-with-jquery/
You are not actually controlling a window but divs.
Here is a sample basic code:
// Variables for current position
var x, y;
function handleMouse(e)
{
if (x && y)
{
window.scrollBy(e.clientX - x, e.clientY - y);
}
x = e.clientX;
y = e.clientY;
}
document.onmousemove = handleMouse;

Getting mouse location in canvas [duplicate]

This question already has answers here:
How do I get the coordinates of a mouse click on a canvas element? [duplicate]
(22 answers)
Closed 3 years ago.
Is there a way to get the location mouse inside a <canvas> tag? I want the location relative to to the upper right corner of the <canvas>, not the entire page.

The accepted answer will not work every time. If you don't use relative position the attributes offsetX and offsetY can be misleading.
You should use the function: canvas.getBoundingClientRect() from the canvas API.
function getMousePos(canvas, evt) {
var rect = canvas.getBoundingClientRect();
return {
x: evt.clientX - rect.left,
y: evt.clientY - rect.top
};
}
canvas.addEventListener('mousemove', function(evt) {
var mousePos = getMousePos(canvas, evt);
console.log('Mouse position: ' + mousePos.x + ',' + mousePos.y);
}, false);

Easiest way is probably to add a onmousemove event listener to the canvas element, and then you can get the coordinates relative to the canvas from the event itself.
This is trivial to accomplish if you only need to support specific browsers, but there are differences between f.ex. Opera and Firefox.
Something like this should work for those two:
function mouseMove(e)
{
var mouseX, mouseY;
if(e.offsetX) {
mouseX = e.offsetX;
mouseY = e.offsetY;
}
else if(e.layerX) {
mouseX = e.layerX;
mouseY = e.layerY;
}
/* do something with mouseX/mouseY */
}

Also note that you'll need CSS:
position: relative;
set to your canvas tag, in order to get the relative mouse position inside the canvas.
And the offset changes if there's a border

I'll share the most bulletproof mouse code that I have created thus far. It works on all browsers will all manner of padding, margin, border, and add-ons (like the stumbleupon top bar)
// Creates an object with x and y defined,
// set to the mouse position relative to the state's canvas
// If you wanna be super-correct this can be tricky,
// we have to worry about padding and borders
// takes an event and a reference to the canvas
function getMouse = function(e, canvas) {
var element = canvas, offsetX = 0, offsetY = 0, mx, my;
// Compute the total offset. It's possible to cache this if you want
if (element.offsetParent !== undefined) {
do {
offsetX += element.offsetLeft;
offsetY += element.offsetTop;
} while ((element = element.offsetParent));
}
// Add padding and border style widths to offset
// Also add the <html> offsets in case there's a position:fixed bar (like the stumbleupon bar)
// This part is not strictly necessary, it depends on your styling
offsetX += stylePaddingLeft + styleBorderLeft + htmlLeft;
offsetY += stylePaddingTop + styleBorderTop + htmlTop;
mx = e.pageX - offsetX;
my = e.pageY - offsetY;
// We return a simple javascript object with x and y defined
return {x: mx, y: my};
}
You'll notice that I use some (optional) variables that are undefined in the function. They are:
stylePaddingLeft = parseInt(document.defaultView.getComputedStyle(canvas, null)['paddingLeft'], 10) || 0;
stylePaddingTop = parseInt(document.defaultView.getComputedStyle(canvas, null)['paddingTop'], 10) || 0;
styleBorderLeft = parseInt(document.defaultView.getComputedStyle(canvas, null)['borderLeftWidth'], 10) || 0;
styleBorderTop = parseInt(document.defaultView.getComputedStyle(canvas, null)['borderTopWidth'], 10) || 0;
// Some pages have fixed-position bars (like the stumbleupon bar) at the top or left of the page
// They will mess up mouse coordinates and this fixes that
var html = document.body.parentNode;
htmlTop = html.offsetTop;
htmlLeft = html.offsetLeft;
I'd recommend only computing those once, which is why they are not in the getMouse function.

For mouse position, I usually use jQuery since it normalizes some of the event attributes.
function getPosition(e) {
//this section is from http://www.quirksmode.org/js/events_properties.html
var targ;
if (!e)
e = window.event;
if (e.target)
targ = e.target;
else if (e.srcElement)
targ = e.srcElement;
if (targ.nodeType == 3) // defeat Safari bug
targ = targ.parentNode;
// jQuery normalizes the pageX and pageY
// pageX,Y are the mouse positions relative to the document
// offset() returns the position of the element relative to the document
var x = e.pageX - $(targ).offset().left;
var y = e.pageY - $(targ).offset().top;
return {"x": x, "y": y};
};
// now just make sure you use this with jQuery
// obviously you can use other events other than click
$(elm).click(function(event) {
// jQuery would normalize the event
position = getPosition(event);
//now you can use the x and y positions
alert("X: " + position.x + " Y: " + position.y);
});
This works for me in all the browsers.
EDIT:
I copied the code from one of my classes I was using, so the jQuery call to this.canvas was wrong. The updated function figures out which DOM element (targ) caused the event and then uses that element's offset to figure out the correct position.

GEE is an endlessly helpful library for smoothing out troubles with canvas, including mouse location.

Simple approach using mouse event and canvas properties:
JSFiddle demo of functionality http://jsfiddle.net/Dwqy7/5/
(Note: borders are not accounted for, resulting in off-by-one):
Add a mouse event to your canvas
canvas.addEventListener("mousemove", mouseMoved);
Adjust event.clientX and event.clientY based on:
canvas.offsetLeft
window.pageXOffset
window.pageYOffset
canvas.offsetTop
Thus:
canvasMouseX = event.clientX - (canvas.offsetLeft - window.pageXOffset);
canvasMouseY = event.clientY - (canvas.offsetTop - window.pageYOffset);
The original question asked for coordinates from the upper right (second function).
These functions will need to be within a scope where they can access the canvas element.
0,0 at upper left:
function mouseMoved(event){
var canvasMouseX = event.clientX - (canvas.offsetLeft - window.pageXOffset);
var canvasMouseY = event.clientY - (canvas.offsetTop - window.pageYOffset);
}
0,0 at upper right:
function mouseMoved(event){
var canvasMouseX = canvas.width - (event.clientX - canvas.offsetLeft)- window.pageXOffset;
var canvasMouseY = event.clientY - (canvas.offsetTop - window.pageYOffset);
}

I'd use jQuery.
$(document).ready(function() {
$("#canvas_id").bind( "mousedown", function(e){ canvasClick(e); } );
}
function canvasClick( e ){
var x = e.offsetX;
var y = e.offsetY;
}
This way your canvas can be anywhere on your page, relative or absolute.

Subtract the X and Y offsets of the canvas DOM element from the mouse position to get the local position inside the canvas.