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Detect a mouse click on an overlapping grid of boxes - JavaScript Canvas

I am currently trying to detect a mouse click on two grids of boxes simultaneously. One grid is easy, and I've just been using:
var gridPosX = Math.floor(mouseClickX/BoxWidth);
var gridPosY = Math.floor(mouseClickY/BoxHeight);
Now I also want to detect a mouse click on a secondary grid of boxes, located at the corners of the first grid of boxes. This could be achieved in a similar way to the first grid. The problem comes in because I want to detect a click on either the first grid, or the second one, at the same time. What is the best way to differentiate a click on the first grid verses a click on the second grid? I've tried to remove the Math.floor and used the greater than and less than operators (> <) to see if the click was closer to one grid spot than the other, but I've had no luck with that so far.
This is an image example of the grid. The black being the main one, the red being the second one
var WIDTH = 1280, HEIGHT = 1280;
var canvas, context;
var grid = [];
var grid2 = [];
var gridWidth = 10, gridHeight = 10;
var boxWidth = WIDTH/gridWidth, boxHeight = HEIGHT/gridHeight;
function main(){
canvas = document.createElement("canvas");
canvas.width = WIDTH;
canvas.height = HEIGHT;
context = canvas.getContext("2d");
document.body.appendChild(canvas);
canvas.onmousedown = function(e){
if(e.which == 1){
var gridPosX = Math.floor(e.offsetX/boxWidth);
var gridPosY = Math.floor(e.offsetY/boxHeight);
grid[gridPosX][gridPosY] = 0;
}
}
init();
setInterval(draw, 30);
}
function init(){
for(var x = 0; x < gridWidth; x++){
grid[x] = [];
grid2[x] = [];
for(var y = 0; y < gridHeight; y++){
grid[x][y] = 1;
grid2[x][y] = 1;
}
}
}
function draw(){
for(var x = 0; x < gridWidth; x++){
for(var y = 0; y < gridHeight; y++){
if(grid[x][y] == 1){
context.fillStyle = 'gray';
context.fillRect(x*boxWidth, y*boxHeight, boxWidth, boxHeight);
context.strokeRect(x*boxWidth, y*boxHeight, boxWidth, boxHeight);
}
}
}
for(var x = 0; x < gridWidth; x++){
for(var y = 0; y < gridHeight; y++){
if(grid2[x][y] == 1){
context.fillStyle = 'red';
context.fillRect((x*boxWidth)+(boxWidth)-(boxWidth/4), (y*boxHeight)+(boxHeight)-(boxHeight/4), boxWidth/2, boxHeight/2);
context.strokeRect((x*boxWidth)+(boxWidth)-(boxWidth/4), (y*boxHeight)+(boxHeight)-(boxHeight/4), boxWidth/2, boxHeight/2);
}
}
}
}
main();

Since the red grids show on top of the gray ones, I think you can first decide whether a mouse event is on a red grid or not. If not, then it must be on gray grids.
Based on the calculations below to check if a red grid is clicked:
var xRedIndex = Math.floor((e.offsetX - 3 / 4 * boxWidth) / (boxWidth / 2));
var yRedIndex = Math.floor((e.offsetY - 3 / 4 * boxHeight) / (boxHeight / 2));
if (xRedIndex % 2 === 0 && yRedIndex % 2 === 0) {
console.log("red");
console.log("Red grid x: " + (xRedIndex / 2));
console.log("Red grid y: " + (yRedIndex / 2));
} else {
console.log("gray");
var gridPosX = Math.floor(e.offsetX / boxWidth);
var gridPosY = Math.floor(e.offsetY / boxHeight);
grid[gridPosX][gridPosY] = 0;
}
Basically, you first subtract the initial gray area in the first column/row from the offsetX/Y, then see if the rest of the offsetX/Y contains an odd or even number of boxSize/2 (side length of red grid). An even number means the click is on red grids, otherwise it falls on the uncovered gray area.
Working fiddle: https://jsfiddle.net/mwxzgth6/1/

I need the ellipse to remain centered and move back and forth along the length of the line

The code is a snippet from the program that I am writing for a project, using the processing.js library. I need the ellipse to travel along the entire length of the diagonal line while remaining centered on the line. When complete, there will be a total of eight lines with the ellipse sliding back and forth. This is the fifth line. You will notice that I had to create a special speed variable in order to get the sphere to move down the diagonal (two other diagonal lines in the code work without this).
It works on all other lines in the graphic. This one travels down perfectly, then goes "off-track" on the way up; when it reaches the top, it just bounces in an odd (random-looking) way. Can I fix it without changing the basic format? Thank you!
var x5 = 113;
var y5 = 27;
var speed5 = 1;
var speed5y = speed5 * 2;
draw = function() {
//lineBallFive
line(100, 1, 300, 400);
fill(0, 225, 255);
ellipse(x5, y5, ballSize, ballSize);
if (x5 > 300) {
speed5 = -3;
}
if (x5 < 100) {
speed5 = 3;
}
if (y5 > 400) {
speed5y = 2 * -3;
}
if (y5 < 1) {
speed5y = 3;
}
}

Instead of moving the ellipse relative to itself, you can use the lerp() function to move the ellipse using linear interpolation. This will make it much easier to sync up your lines.
Here's a little example:
var amount = 0;
var speed = .001;
function setup() {
size(500, 500);
}
void draw() {
var startX = 0;
var startY = 0;
var endX = width;
var endY = height;
var currentX = lerp(startX, endX, amount);
var currentY = lerp(startY, endY, amount);
background(0);
ellipse(currentX, currentY, 20, 20);
amount += speed;
}

Realistic mouse movement coordinates in javascript?

In javascript, is there a way I can create a variable and a function that "simulates" smooth mouse movement? i.e., say the function simulates a user starts from lower left corner of the browser window, and then moves mouse in a random direction slowly...
The function would return x and y value of the next position the mouse would move each time it is called (would probably use something like setInterval to keep calling it to get the next mouse position). Movement should be restricted to the width and height of the screen, assuming the mouse never going off of it.
What I don't want is the mouse to be skipping super fast all over the place. I like smooth movements/positions being returned.

A "realistic mouse movement" doesn't mean anything without context :
Every mouse user have different behaviors with this device, and they won't even do the same gestures given what they have on their screen.
If you take an FPS game, the movements will in majority be in a small vertical range, along the whole horizontal screen.
Here is a "drip painting" I made by recording my mouse movements while playing some FPS game.
If we take the google home page however, I don't even use the mouse. The input is already focused, and I just use my keyboard.
On some infinite scrolling websites, my mouse can stay at the same position for dozens of minutes and just go to a link at some point.
I think that to get the more realistic mouse movements possible, you would have to record all your users' gestures, and repro them.
Also, a good strategy could be to get the coordinates of the elements that will attract user's cursor the more likely (like the "close" link under SO's question) and make movements go to those elements' coordinates.
Anyway, here I made a snippet which uses Math.random() and requestAnimationFrame() in order to make an object move smoothly, with some times of pausing, and variable speeds.
// Canvas is here only to show the output of function
var canvas = document.createElement('canvas');
var ctx = canvas.getContext('2d');
document.body.appendChild(canvas);
var maxX = canvas.width = window.innerWidth;
var maxY = canvas.height = window.innerHeight;
window.onresize = function(){
maxX = canvas.width = window.innerWidth;
maxY = canvas.height = window.innerHeight;
}
gc.onclick = function(){
var coords = mouse.getCoords();
out.innerHTML = 'x : '+coords.x+'<br>y : '+coords.y;
}
var Mouse = function() {
var that = {},
size = 15,
border = size / 2,
maxSpeed = 50, // pixels per frame
maxTimePause = 5000; // ms
that.draw = function() {
if (that.paused)
return;
that.update();
// just for the example
ctx.clearRect(0, 0, canvas.width, canvas.height);
if(show.checked){
ctx.drawImage(that.img, that.x - border, that.y - border, size, size)
}
// use requestAnimationFrame for smooth update
requestAnimationFrame(that.draw);
}
that.update = function() {
// take a random position, in the same direction
that.x += Math.random() * that.speedX;
that.y += Math.random() * that.speedY;
// if we're out of bounds or the interval has passed
if (that.x <= border || that.x >= maxX - border || that.y <= 0 || that.y >= maxY - border || ++that.i > that.interval)
that.reset();
}
that.reset = function() {
that.i = 0; // reset the counter
that.interval = Math.random() * 50; // reset the interval
that.speedX = (Math.random() * (maxSpeed)) - (maxSpeed / 2); // reset the horizontal direction
that.speedY = (Math.random() * (maxSpeed)) - (maxSpeed / 2); // reset the vertical direction
// we're in one of the corner, and random returned farther out of bounds
if (that.x <= border && that.speedX < 0 || that.x >= maxX - border && that.speedX > 0)
// change the direction
that.speedX *= -1;
if (that.y <= border && that.speedY < 0 || that.y >= maxY - border && that.speedY > 0)
that.speedY *= -1;
// check if the interval was complete
if (that.x > border && that.x < maxX - border && that.y > border && that.y < maxY - border) {
if (Math.random() > .5) {
// set a pause and remove it after some time
that.paused = true;
setTimeout(function() {
that.paused = false;
that.draw();
}, (Math.random() * maxTimePause));
}
}
}
that.init = function() {
that.x = 0;
that.y = 0;
that.img = new Image();
that.img.src ="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAB4AAAAeCAYAAAA7MK6iAAABJUlEQVRIic2WXbHEIAyFI6ESKgEJkVIJlYCTSqiESIiESqiEb19gL9Od3f5R5mbmPPHwBTgnIPJfChiAGbCkCQgtG7BpmgAWIALaDDyOI2bGuq40BasqIoKZATgwNAWHEEjHbkBsBhYRVJUYIwBNwVlFaVOwiDDPMylmQ1OwquY7d0CBrglYkuEeidoeOKt61I6Cq0ftKFhqR+0MOKuo2BQsInnndvnOr4JvR+0qWO5G7Q44K0XtOXDf96jqh9z9WXAy1FJ8l0qd+zbtvU7lWs7wIzkuh8SvpqqDi3zGndPQauDkzvdESm8xZvbh4mVZ7k8ud/+aR0C3YPk7mVvgkCZPVrdZV3dHVem6bju1roMPNmbAmq8kG+/ynD7ZwNsAVVz9dL0AhBrZq7F+CSQAAAAASUVORK5CYII=";
that.reset();
}
that.getCoords = function(){
return {x: that.x, y:that.y};
}
that.init()
return that;
}
var mouse = new Mouse()
mouse.draw();
html,body {margin: 0}
canvas {position: absolute; top:0; left:0;z-index:-1}
#out{font-size: 0.8em}
<label for="show">Display cursor</label><input name="show" type="checkbox" id="show" checked="true"/><br>
<button id="gc">get cursor Coords</button>
<p id="out"></p>

Last I heard the browser's mouse position cannot be altered with JavaScript, so the question really has no answer "as is". The mouse position can be locked though. I'm not certain whether it would be possible to implement a custom cursor that allows setting the position. This would include hiding and perhaps locking the stock cursor.

Having something smoothly follow the cursor is quite straight forward. You may be able to reverse this process to achieve what you need. Here's a code snippet which simply calculates the distance between the cursor and a div every frame and then moves the div 10% of that distance towards the cursor:
http://jsfiddle.net/hpp0qb0d/
var p = document.getElementById('nextmove')
var lastX,lastY,cursorX,cursorY;
window.addEventListener('mousemove', function(e){
cursorX = e.pageX;
cursorY = e.pageY;
})
setInterval(function(){
var newX = p.offsetLeft + (cursorX - lastX)/10
var newY = p.offsetTop + (cursorY - lastY)/10
p.style.left = newX+'px'
p.style.top = newY+'px'
lastX = p.offsetLeft
lastY = p.offsetTop
},20)

identify an object on canvas by a click event

Into this simple code I use an eventListener which doesn't look to work at all. The canvas display an image and the given hitpaint() function is supposed determines whether a click occurs. I cant understand why the eventListener behaves like that. Any insight would be helpful.
mycanv.addEventListener("click", function(e) {
var output = document.getElementByID("output");
ctx.fillStyle = 'blue';
//ctx.clearRect(0,0,100,20);
if (hitpaint) {
//ctx.fillText("hit",100,20);
output.innerHTML = "hit";
} else {
//ctx.fillText("miss",100,20);
output.innerHTML = "miss";
}
}, false);
The hitpaint() function is defined as:
function hitpaint(mouse_event) {
var bounding_box = mycanv.getBoundingClientRect();
var mousex = (mouse_event.clientX - bounding_box.left) *
(mycanv.width / bounding_box.width);
var mousey = (mouse_event.clientY - bounding_box.top) *
(mycanv.height / bounding_box.height);
var pixels = ctx.getImageData(mousex, mousey, 1, 1);
for (var i = 3; i < pixels.data.length; i += 4) {
// If we find a non-zero alpha we can just stop and return
// "true" - the click was on a part of the canvas that's
// got colour on it.
if (pixels.data[i] !== 0) return true;
}
// The function will only get here if none of the pixels matched in
return false;
}
Finally, the main loop which display the picture in random location into the canvas:
function start() {
// main game function, called on page load
setInterval(function() {
ctx.clearRect(cat_x, cat_y, 100, 100);
cat_x = Math.random() * mycanv.width - 20;
cat_y = Math.random() * mycanv.height - 20;
draw_katy(cat_x, cat_y);
}, 1000);
}

There are a some issues here:
As Grundy points out in the comment, the hitpaint is never called; right now it checks for it's existence and will always return true
The mouse coordinates risk ending up as fractional values which is no-go with getImageData
Scaling the mouse coordinates is usually not necessary. Canvas should preferably have a fixed size without an additional CSS size
Add boundary check for x/y to make sure they are inside canvas bitmap
I would suggest this rewrite:
mycanv.addEventListener("click", function(e) {
var output = document.getElementByID("output");
ctx.fillStyle = 'blue';
//ctx.clearRect(0,0,100,20);
if (hitpaint(e)) { // here, call hitpaint()
//ctx.fillText("hit",100,20);
output.innerHTML = "hit";
} else {
//ctx.fillText("miss",100,20);
output.innerHTML = "miss";
}
}, false);
Then in hitpaint:
function hitpaint(mouse_event) {
var bounding_box = mycanv.getBoundingClientRect();
var x = ((mouse_event.clientX - bounding_box.left) *
(mycanv.width / bounding_box.width))|0; // |0 cuts off any fraction
var y = ((mouse_event.clientY - bounding_box.top) *
(mycanv.height / bounding_box.height))|0;
if (x >= 0 && x < mycanv.width && y >= 0 && y < mycanv.height) {
// as we only have one pixel, we can address alpha channel directly
return ctx.getImageData(x, y, 1, 1).data[3] !== 0;
}
else return false; // x/y out of range
}

Zoom Canvas to Mouse Cursor

I'm programming a HTML5 < canvas > project that involves zooming in and out of images using the scroll wheel.
I want to zoom towards the cursor like google maps does but I'm completely lost on how to calculate the movements.
What I have: image x and y (top-left corner); image width and height; cursor x and y relative to the center of the canvas.

In short, you want to translate() the canvas context by your offset, scale() it to zoom in or out, and then translate() back by the opposite of the mouse offset. Note that you need to transform the cursor position from screen space into the transformed canvas context.
ctx.translate(pt.x,pt.y);
ctx.scale(factor,factor);
ctx.translate(-pt.x,-pt.y);
Demo: http://phrogz.net/tmp/canvas_zoom_to_cursor.html
I've put up a full working example on my website for you to examine, supporting dragging, click to zoom in, shift-click to out, or scroll wheel up/down.
The only (current) issue is that Safari zooms too fast compared to Chrome or Firefox.

I hope, these JS libraries will help you:
(HTML5, JS)
Loupe
http://www.netzgesta.de/loupe/
CanvasZoom
https://github.com/akademy/CanvasZoom
Scroller
https://github.com/zynga/scroller
As for me, I'm using loupe. It's awesome!
For you the best case - scroller.

I recently needed to archive same results as Phrogz had already done but instead of using context.scale(), I calculated each object size based on ratio.
This is what I came up with. Logic behind it is very simple. Before scaling, I calculate point distance from edge in percentages and later adjust viewport to correct place.
It took me quite a while to come up with it, hope it saves someones time.
$(function () {
var canvas = $('canvas.main').get(0)
var canvasContext = canvas.getContext('2d')
var ratio = 1
var vpx = 0
var vpy = 0
var vpw = window.innerWidth
var vph = window.innerHeight
var orig_width = 4000
var orig_height = 4000
var width = 4000
var height = 4000
$(window).on('resize', function () {
$(canvas).prop({
width: window.innerWidth,
height: window.innerHeight,
})
}).trigger('resize')
$(canvas).on('wheel', function (ev) {
ev.preventDefault() // for stackoverflow
var step
if (ev.originalEvent.wheelDelta) {
step = (ev.originalEvent.wheelDelta > 0) ? 0.05 : -0.05
}
if (ev.originalEvent.deltaY) {
step = (ev.originalEvent.deltaY > 0) ? 0.05 : -0.05
}
if (!step) return false // yea..
var new_ratio = ratio + step
var min_ratio = Math.max(vpw / orig_width, vph / orig_height)
var max_ratio = 3.0
if (new_ratio < min_ratio) {
new_ratio = min_ratio
}
if (new_ratio > max_ratio) {
new_ratio = max_ratio
}
// zoom center point
var targetX = ev.originalEvent.clientX || (vpw / 2)
var targetY = ev.originalEvent.clientY || (vph / 2)
// percentages from side
var pX = ((vpx * -1) + targetX) * 100 / width
var pY = ((vpy * -1) + targetY) * 100 / height
// update ratio and dimentsions
ratio = new_ratio
width = orig_width * new_ratio
height = orig_height * new_ratio
// translate view back to center point
var x = ((width * pX / 100) - targetX)
var y = ((height * pY / 100) - targetY)
// don't let viewport go over edges
if (x < 0) {
x = 0
}
if (x + vpw > width) {
x = width - vpw
}
if (y < 0) {
y = 0
}
if (y + vph > height) {
y = height - vph
}
vpx = x * -1
vpy = y * -1
})
var is_down, is_drag, last_drag
$(canvas).on('mousedown', function (ev) {
is_down = true
is_drag = false
last_drag = { x: ev.clientX, y: ev.clientY }
})
$(canvas).on('mousemove', function (ev) {
is_drag = true
if (is_down) {
var x = vpx - (last_drag.x - ev.clientX)
var y = vpy - (last_drag.y - ev.clientY)
if (x <= 0 && vpw < x + width) {
vpx = x
}
if (y <= 0 && vph < y + height) {
vpy = y
}
last_drag = { x: ev.clientX, y: ev.clientY }
}
})
$(canvas).on('mouseup', function (ev) {
is_down = false
last_drag = null
var was_click = !is_drag
is_drag = false
if (was_click) {
}
})
$(canvas).css({ position: 'absolute', top: 0, left: 0 }).appendTo(document.body)
function animate () {
window.requestAnimationFrame(animate)
canvasContext.clearRect(0, 0, canvas.width, canvas.height)
canvasContext.lineWidth = 1
canvasContext.strokeStyle = '#ccc'
var step = 100 * ratio
for (var x = vpx; x < width + vpx; x += step) {
canvasContext.beginPath()
canvasContext.moveTo(x, vpy)
canvasContext.lineTo(x, vpy + height)
canvasContext.stroke()
}
for (var y = vpy; y < height + vpy; y += step) {
canvasContext.beginPath()
canvasContext.moveTo(vpx, y)
canvasContext.lineTo(vpx + width, y)
canvasContext.stroke()
}
canvasContext.strokeRect(vpx, vpy, width, height)
canvasContext.beginPath()
canvasContext.moveTo(vpx, vpy)
canvasContext.lineTo(vpx + width, vpy + height)
canvasContext.stroke()
canvasContext.beginPath()
canvasContext.moveTo(vpx + width, vpy)
canvasContext.lineTo(vpx, vpy + height)
canvasContext.stroke()
canvasContext.restore()
}
animate()
})
<!DOCTYPE html>
<html>
<head>
<title></title>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
</head>
<body>
<canvas class="main"></canvas>
</body>
</html>

I took #Phrogz's answer as a basis and made a small library that enables canvas with dragging, zooming and rotating.
Here is the example.
var canvas = document.getElementById('canvas')
//assuming that #param draw is a function where you do your main drawing.
var control = new CanvasManipulation(canvas, draw)
control.init()
control.layout()
//now you can drag, zoom and rotate in canvas
You can find more detailed examples and documentation on the project's page

Faster
Using ctx.setTransform gives you more performance than multiple matrix calls ctx.translate, ctx.scale, ctx.translate.
No need for complex transformation inversions as and expensive DOM matrix calls tp converts point between zoomed and screen coordinate systems.
Flexible
Flexibility as you don't need to use ctx.save and ctx.restore if you are rendering content at using different transforms. Returning to the transform with ctx.setTransform rather than the potentially frame rate wreaking ctx.restorecall
Easy to invert the transform and get the world coordinates of a (screen) pixel position and the other way round.
Examples
Using mouse and mouse wheel to zoom in and out at mouse position
An example using this method to scale page content at a point (mouse) via CSS transform CSS Demo at bottom of answer also has a copy of the demo from the next example.
And an example of this method used to scale canvas content at a point using setTransform
How
Given a scale and pixel position you can get the new scale as follow...
const origin = {x:0, y:0}; // canvas origin
var scale = 1; // current scale
function scaleAt(x, y, scaleBy) { // at pixel coords x, y scale by scaleBy
scale *= scaleBy;
origin.x = x - (x - origin.x) * scaleBy;
origin.y = y - (y - origin.y) * scaleBy;
}
To position the canvas and draw content
ctx.setTransform(scale, 0, 0, scale, origin.x, origin.y);
ctx.drawImage(img, 0, 0);
To use if you have the mouse coordinates
const zoomBy = 1.1; // zoom in amount
scaleAt(mouse.x, mouse.y, zoomBy); // will zoom in at mouse x, y
scaleAt(mouse.x, mouse.y, 1 / zoomBy); // will zoom out by same amount at mouse x,y
To restore the default transform
ctx.setTransform(1,0,0,1,0,0);
The inversions
To get the coordinates of a point in the zoomed coordinate system and the screen position of a point in the zoomed coordinate system
Screen to world
function toWorld(x, y) { // convert to world coordinates
x = (x - origin.x) / scale;
y = (y - origin.y) / scale;
return {x, y};
}
World to screen
function toScreen(x, y) {
x = x * scale + origin.x;
y = y * scale + origin.y;
return {x, y};
}