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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.

get mouse position inside resizing div using raycaster and vector

I am able to get the intersects from my click event when I use the window object to acquire height and width, but getting the intersects position on a canvas that's dynamically sized is proving much harder. I'm not certain of the formula I would need to use to calculate the vector.x and vector.y values with a div that isn't always the same size.
The canvas is the size of a div that always has a width: height ratio of 4:3 and resizes to fit in the window and is always positioned in the center of the window.
If I resize the window to be 4:3 then the following code works perfectly:
mouse.x = (ecx/div_width) *2 -1;
mouse.y= -(ecy/div_height) *2 + 1;
when I resize the window, whichever dimension is larger than the size of the canvas has the incorrect value. I've linked an image to roughly describe how the problem presents itself
Image of horizontal dimension issue
I initially thought that the matches would be as simple as dividing the difference between the the sizes of the window and the canvas by
My question is, how would I acquire the correct values to pass to the vector object for it's x and y attributes? (using Vector3 and Raycaster)
here is the function I'm using to try and get the object(s) being clicked:
function getClicked(event){
event.preventDefault();
var ecx = event.clientX;
var ecy = event.clientY;
//elem is the div containing the canvas
//the canvas is not the same size as the window
var elem_w = elem.innerWidth();
var elem_h = elem.innerHeight();
//most examples suggest using the window height and width
//to get the position of the mouse in the scene.
//since the scene isn't the same size as the window, that doesn't work
var ww = window.innerWidth;
var wh = window.innerHeight;
mouse.x = (ecx/ww) *2 -1;
mouse.y= -(ecy/wh) *2 + 1;
var objlist = []
rc.setFromCamera(mouse, camera);
var intersects = rc.intersectObjects(scene.children, true);
for (var i=0;i<names_to_spin.length;i++){
var obj = intersects[i];
objlist.push(obj);
}
//ideally, this should return a list of the objects under the cursor
return objlist;
}

Custom cursor image with javascript

I need to change my mouse cursor to a custom image.
If possible I would like to do it on a spritesheet.
I can't do it from the css because I'm using it in a game. I already know how to decide when etc.
What I need to know is how do I change the cursor to an image, and deciding the image position and size?
Is there any easy solution similar to the drawImage's image position?

You can set the CSS using javascript to hide the cursor:
your_canvas.style.cursor = "none"
You can then get the cursor's position (it's now hidden) with something like this:
your_canvas.addEventListener("mousemove", function (ev) {
var mouseX = ev.pageX - GetTopLeft(your_canvas).Left;
var mouseY = ev.pageX - GetTopLeft(your_canvas).Top;
});
Then you can modify your canvas to show your fancier cursor sprite at that location.
GetTopLeft is defined as follows:
function GetTopLeft(elm){
var x, y = 0;
//set x to elm’s offsetLeft
x = elm.offsetLeft;
//set y to elm’s offsetTop
y = elm.offsetTop;
//set elm to its offsetParent
elm = elm.offsetParent;
//use while loop to check if elm is null
// if not then add current elm’s offsetLeft to x
//offsetTop to y and set elm to its offsetParent
while(elm != null)
{
x = parseInt(x) + parseInt(elm.offsetLeft);
y = parseInt(y) + parseInt(elm.offsetTop);
elm = elm.offsetParent;
}
//here is interesting thing
//it return Object with two properties
//Top and Left
return {Top:y, Left: x};
}
Though I can't remember where I copied the GetTopLeft function from...

If you are using a canvas, just hide the cursor over the canvas and draw your own sprite on the canvas at mouse position.

Get absolute screen bounds of a DOM element from a Firefox extension

I have a firefox extension that needs to get the exact screen co-ordinates of a DOM element and passes it to a native DLL via js/c-types.
Now I have it mostly covered :
var gDomWindowUtils = window.QueryInterface(Components.interfaces.nsIInterfaceRequestor)
.getInterface(nsIDOMWindowUtils);
function getScreenRect(oElem)
{
var rc =
{
x : 0,
y : 0,
w : 0,
h : 0
};
var o = oElement;
while(o != null)
{
rc.y += o.offsetTop;
rc.x += o.offsetLeft;
o = o.offsetParent;
}
var x = {}, y = {};
gDomWindowUtils.getScrollXY(false, x, y);
rc.x -= x.value;
rc.y -= y.value;
var scale = gDomWindowUtils.screenPixelsPerCSSPixel;
rc.x *= scale;
rc.y *= scale;
rc.w *= scale;
rc.h *= scale;
return rc;
};
This handles scrolling and zooming, but the values I get are relative to the browser window, and not the screen.
How do I detect the offset of the client area of the actual rendering area of the browser?
I can even use native code (Win32) via js/ctypes so I tried to see if I could use FindWindow() / GetWindowRect() to get it, but the whole of firefox is a single HWND, the controls are all not native windows.
So one idea I have is, since the UI of firefox is an XUL document, I should be able to get the menubar, tab bar etc etc and find the browser areas absolute offset. But, I have no clue how to access the XUL tree that defines the browser UI.
Can someone give me a pointer?
[Edit]
Ignore rc.w and rc.h being undefined in the above code , it is irrelevant to the question.

You mostly got it already but I would recommend using getBoundingClientRect() instead of offsetLeft/offsetTop:
var rect = oElement.getBoundingClientRect();
var rc = {
x: rect.left,
y: rect.top,
w: rect.width,
h: rect.height
};
getBoundingClientRect() considers scrolling so that you no longer need to add it. You get the coordinates relative to screen using window.mozInnerScreenX and window.mozInnerScreenY:
rc.x += window.mozInnerScreenX;
rc.y += window.mozInnerScreenY;
And after that you multiply the values with screenPixelsPerCSSPixel. That should give you proper screen coordinates.

I will add one more "trick" that helped me with a similar problem.
Whenever my code receives a mousemove event, I capture the mouse-cursor positions in ALL the coordinates that event provides, which includes at least "screen", "client" and "pageXY" coordinates. Then for any OTHER purpose I can compute the difference between these coordinates simply by subtracting the appropriate two of those.
In your case, you'd probably compute an offset like this:
deltaX = event.screenX - event.clientX;
deltaY = event.screenY - event.clientY;
or
offsetX = event.screenX - event.pageX;
offsetY = event.screenY - event.pageY;
Then just add deltaX and deltaY or offsetX and offsetY to those coordinates to get screen coordinates.

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.