Learn how to use canvas and JavaScript to draw lines that display x and y coordinates on top of the mouse pointer at the same time

Question

Learn how to use canvas and JavaScript to draw lines that display x and y coordinates on top of the mouse pointer at the same time

Implement a functionality in which lines are drawn while the mouse button is held down and simultaneously display x & y coordinates on top of the mouse pointer during mouse movement using canvas and JavaScript.

The issue arises when attempting to draw lines while the mouse button is held down due to the use of ctxTemp.clearRect(0,0,canvasTemp.width,canvas.height), which removes the continuously updating x & y coordinates displayed on top of the mouse pointer during mouse movement.

If ctxTemp.clearRect(0,0,canvasTemp.width,canvas.height) is not used, then the x & y coordinates will continue to be drawn on top of the mouse pointer during mouse movement without any interruption.

Your assistance in resolving this dilemma would be greatly appreciated. Thank you in advance.

javascript canvas html5-canvas mouseevent

Answer 1

Answer №1

Utilizing Double Buffer Technique

When working with canvases and needing to render additional guides such as coordinates or widgets, it's essential to separate the content creation from the rendering of these guides.

Using a single canvas for both tasks can create issues as you might end up overwriting the content when clearing or painting the guides on top.

The solution lies in implementing an additional canvas (or more) to keep the content separate from the overlaying guides.

Implementation Example

This example demonstrates how to effectively utilize this strategy:

Create a second canvas named drawing that matches the size of the main canvas on the page.
The strokes made by the mouse are drawn onto this secondary canvas.
The main update function takes care of drawing the second canvas onto the primary one, followed by showcasing the current mouse position in a box above it.
To ensure the mouse position indicator stays within the boundaries of the canvas, extra code is included during its rendering process.

requestAnimationFrame(update);
const ctx = canvas.getContext("2d");
var w = canvas.width, h = canvas.height;
const drawing = createImage(w, h); // create canvas to hold drawing
const pointQueue = [];             // holds points when mouse button down
drawing.ctx.lineWidth = 4;
drawing.ctx.strokeStyle = "#F00";
drawing.ctx.lineJoin = "round";
drawing.ctx.lineCap = "round";
ctx.font = "16px Arial";
ctx.textAlign = "center";
ctx.textBaseline = "middle";
/* add mouse listeners */
const bounds = canvas.getBoundingClientRect();
const mouse = {x: 0, y: 0, button: false}, events = ["down", "up", "move"];
events.forEach(name => document.addEventListener("mouse" + name, mouseEvents));
            
function drawMousePos(ctx) {
    const text = "X: " + mouse.x.toFixed(0) + " Y: " + mouse.y.toFixed(0);
    const width = ctx.measureText(text).width + 8;
    var x = mouse.x, y = mouse.y - 18;
    if (x + width / 2 > w) { x = w - width / 2 }
    if (x - width / 2 < 0) { x = width / 2 }
    if (y - 10 < 0) { y = 10 }
    if (y + 10 > h) { y = h - 10 }
    ctx.fillStyle = "#EEC8";
    ctx.fillRect(x - width / 2, y - 12, width , 20);
    ctx.strokeRect(x - width / 2, y - 12, width, 20);
    ctx.fillStyle = "#000C";
    ctx.fillText(text, x, y);
}
function drawPen(ctx) {
    if (pointQueue.length >= 2) {
        ctx.beginPath();
        ctx.moveTo(...pointQueue.shift());
        while (pointQueue.length > (mouse.button ? 1 : 0)) { ctx.lineTo(...pointQueue.shift()) }
        pointQueue.length && ctx.lineTo(...pointQueue[0]);
        ctx.stroke();
    }
}
function update(){
    if (pointQueue.length) {
        drawPen(drawing.ctx);        
        ctx.clearRect(0, 0, w, h);
        ctx.drawImage(drawing, 0, 0);
        pointQueue.length && drawMousePos(ctx);
        canvas.style.cursor = "none";
    } else { canvas.style.cursor = "crosshair" }
    requestAnimationFrame(update);
}
function createImage(w, h){
    const can = document.createElement("canvas");
    can.width = w;
    can.height = h;
    can.ctx = can.getContext("2d");
    return can;
}
function mouseEvents(e){
    mouse.x = e.pageX - bounds.left - 2;  // offset by 2 pixels for canvas border
    mouse.y = e.pageY - bounds.top - 2;
    if (e.type === "mousedown") { mouse.button = true } 
    if (mouse.button) { pointQueue.push([mouse.x , mouse.y]) }
    if (e.type === "mouseup") { mouse.button = false }
}

canvas { 
    border : 2px solid black; 
    cursor: crosshair;
}

Click drag mouse to draw<br>
<canvas id="canvas" width="512" height="256"></canvas>

Answer 2

Utilizing Double Buffer Technique

When working with canvases and needing to render additional guides such as coordinates or widgets, it's essential to separate the content creation from the rendering of these guides.

Using a single canvas for both tasks can create issues as you might end up overwriting the content when clearing or painting the guides on top.

The solution lies in implementing an additional canvas (or more) to keep the content separate from the overlaying guides.

Implementation Example

This example demonstrates how to effectively utilize this strategy:

Create a second canvas named drawing that matches the size of the main canvas on the page.
The strokes made by the mouse are drawn onto this secondary canvas.
The main update function takes care of drawing the second canvas onto the primary one, followed by showcasing the current mouse position in a box above it.
To ensure the mouse position indicator stays within the boundaries of the canvas, extra code is included during its rendering process.

requestAnimationFrame(update);
const ctx = canvas.getContext("2d");
var w = canvas.width, h = canvas.height;
const drawing = createImage(w, h); // create canvas to hold drawing
const pointQueue = [];             // holds points when mouse button down
drawing.ctx.lineWidth = 4;
drawing.ctx.strokeStyle = "#F00";
drawing.ctx.lineJoin = "round";
drawing.ctx.lineCap = "round";
ctx.font = "16px Arial";
ctx.textAlign = "center";
ctx.textBaseline = "middle";
/* add mouse listeners */
const bounds = canvas.getBoundingClientRect();
const mouse = {x: 0, y: 0, button: false}, events = ["down", "up", "move"];
events.forEach(name => document.addEventListener("mouse" + name, mouseEvents));
            
function drawMousePos(ctx) {
    const text = "X: " + mouse.x.toFixed(0) + " Y: " + mouse.y.toFixed(0);
    const width = ctx.measureText(text).width + 8;
    var x = mouse.x, y = mouse.y - 18;
    if (x + width / 2 > w) { x = w - width / 2 }
    if (x - width / 2 < 0) { x = width / 2 }
    if (y - 10 < 0) { y = 10 }
    if (y + 10 > h) { y = h - 10 }
    ctx.fillStyle = "#EEC8";
    ctx.fillRect(x - width / 2, y - 12, width , 20);
    ctx.strokeRect(x - width / 2, y - 12, width, 20);
    ctx.fillStyle = "#000C";
    ctx.fillText(text, x, y);
}
function drawPen(ctx) {
    if (pointQueue.length >= 2) {
        ctx.beginPath();
        ctx.moveTo(...pointQueue.shift());
        while (pointQueue.length > (mouse.button ? 1 : 0)) { ctx.lineTo(...pointQueue.shift()) }
        pointQueue.length && ctx.lineTo(...pointQueue[0]);
        ctx.stroke();
    }
}
function update(){
    if (pointQueue.length) {
        drawPen(drawing.ctx);        
        ctx.clearRect(0, 0, w, h);
        ctx.drawImage(drawing, 0, 0);
        pointQueue.length && drawMousePos(ctx);
        canvas.style.cursor = "none";
    } else { canvas.style.cursor = "crosshair" }
    requestAnimationFrame(update);
}
function createImage(w, h){
    const can = document.createElement("canvas");
    can.width = w;
    can.height = h;
    can.ctx = can.getContext("2d");
    return can;
}
function mouseEvents(e){
    mouse.x = e.pageX - bounds.left - 2;  // offset by 2 pixels for canvas border
    mouse.y = e.pageY - bounds.top - 2;
    if (e.type === "mousedown") { mouse.button = true } 
    if (mouse.button) { pointQueue.push([mouse.x , mouse.y]) }
    if (e.type === "mouseup") { mouse.button = false }
}

canvas { 
    border : 2px solid black; 
    cursor: crosshair;
}

Click drag mouse to draw<br>
<canvas id="canvas" width="512" height="256"></canvas>

Answer 3

Answer №2

If you're looking to track your mouse movements, one approach is to bind a div or span to your mouse and assign it an event listener for 'mousemove'. Within this element, you can include two spans:

<div id='mouseTracker' style='position: absolute; top: 0;left: 0'>
   <span id='mouseX'></span>
   <span id='mouseY'></span>
</div>

The div with the id #mouseTracker would be positioned absolutely, and as the mouse moves, you can update its position using the coordinates.

const mouseDiv = document.getElementById('mouseTracker');
const mouseX = document.getElementById('mouseX');
const mouseY = document.getElementById('mouseY');
mouseDiv.addEventListener('mousemove', e => {
  mouseDiv.setAttribute('style', `left: ${e.offsetX}; top: ${e.offsetY}`);
  mouseX.innerText = `x: ${e.offsetX}`;
  mouseY.innerText = `y: ${e.offsetY}`;
});

This setup allows you to implement additional mouse events to customize when the coordinates are displayed, such as only showing them while the mouse is in motion.

I hope this solution proves helpful!

Answer 4

If you're looking to track your mouse movements, one approach is to bind a div or span to your mouse and assign it an event listener for 'mousemove'. Within this element, you can include two spans:

<div id='mouseTracker' style='position: absolute; top: 0;left: 0'>
   <span id='mouseX'></span>
   <span id='mouseY'></span>
</div>

The div with the id #mouseTracker would be positioned absolutely, and as the mouse moves, you can update its position using the coordinates.

const mouseDiv = document.getElementById('mouseTracker');
const mouseX = document.getElementById('mouseX');
const mouseY = document.getElementById('mouseY');
mouseDiv.addEventListener('mousemove', e => {
  mouseDiv.setAttribute('style', `left: ${e.offsetX}; top: ${e.offsetY}`);
  mouseX.innerText = `x: ${e.offsetX}`;
  mouseY.innerText = `y: ${e.offsetY}`;
});

This setup allows you to implement additional mouse events to customize when the coordinates are displayed, such as only showing them while the mouse is in motion.

I hope this solution proves helpful!

Learn how to use canvas and JavaScript to draw lines that display x and y coordinates on top of the mouse pointer at the same time

Answer №1

Utilizing Double Buffer Technique

Implementation Example

Answer №2

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