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HTML-5 Canvas Magnifying Glass unable to mouse down/drag and scrolling interferes with image

I am creating a magnifying glass for a Canvas application that is supposed to take a canvas image snapshot and draw it onto a smaller canvas at a larger scale where the cursor coordinates are, but I have run into 2 issues.
When I scroll down, the canvas containing the zoomed canvas/image no longer shows what its over but instead the top of the canvas/image.
I am unable to set up a click event where the user can hold down the left mouse and move the zoom area until mouseup. I have tried mouse down and mouse up event listeners to no avail so for reference i have it on mousemove and mouseleave, which also doesnt work well but needs to be replaced with mouseup instead.
var canvas = document.getElementById("canvas1");
var ctx = canvas.getContext("2d");
var ox = canvas.width / 2;
var oy = canvas.height / 2;
ctx.font = "42px serif";
ctx.textAlign = "center";
ctx.textBaseline = "middle";
ctx.fillStyle = "blue";
ctx.fillRect(ox / 2, oy / 2, ox, oy);
function magnify() {
var main = document.getElementById("canvas1");
var ctx = main.getContext('2d')
var base64 = main.toDataURL('image/png', 0);
drawing = new Image();
drawing.onload = () => {
var zoom = document.getElementById("tCanvas");
var zoomCtx = zoom.getContext('2d');
zoomCtx.drawImage(drawing, 0, 0);
}
main.addEventListener("mousemove", function(e) {
var zoom = document.getElementById("tCanvas");
var zoomCtx = zoom.getContext('2d');
zoomCtx.clearRect(0, 0, zoom.width, zoom.height);
zoomCtx.drawImage(main, e.x, e.y, 200, 200, 0, 0, 300, 300);
zoom.style.top = e.pageY - 10 + "px"
zoom.style.left = e.pageX - 10 + "px"
e.pageY = -150
e.pageX = -150
zoom.style.display = "block";
});
main.addEventListener("mouseleave", function() {
var zoom = document.getElementById("tCanvas");
zoom.style.display = "none";
});
drawing.src = base64;
};
<canvas id="tCanvas" class="cgm" height="100" width="100" style="background-color:white; position: absolute; display: none; z-
index:1;border:1px solid red;"> </canvas>
<canvas tabindex=1 class="cgm" id="canvas1" style="position:relative; background:white;
left:0;right:0;margin:auto;z-index:1;margin-top:70px; "></canvas>
<p></p>
<button id="zoom" onclick="magnify();">Zoom</button>
Here's a fiddle for reference (I fixed the height to display the scroll issue).
JSFiddle

I'm going to focus on the part:
make a canvas image snapshot and draw it onto a smaller canvas
at a larger scale where the cursor coordinates are
Your code was doing a lot of stuff with the styles and I have no clue why, your comments do not give me any indication why that was needed, so I removed all that to keep this example as simple as possible
We need to draw so we will use that drawing = new Image() that you have and we need some integration with click I added a new variable drawing = new Image() clicking on the canvas will pause the zoom movement, and clicking again will resume it, that to me was the intuitive way
See my sample code below
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var zoom = document.getElementById("zoom");
var zoomCtx = zoom.getContext('2d');
const z = zoom.width * 5
var pause = false
ctx.fillStyle = "blue";
ctx.fillRect(5, 5, 5, 90);
ctx.fillRect(60, 15, 20, 50);
for (let i = 10; i < 19; i++)
ctx.fillText(i, i * 2, i * 9 - 70);
var drawing = new Image();
drawing.src = canvas.toDataURL('image/png', 0);
canvas.addEventListener("mousemove", (e) => {
if (pause) return
zoomCtx.clearRect(0, 0, zoom.width, zoom.height);
zoomCtx.drawImage(drawing, e.x - 15, e.y - 15, 50, 50, 0, 0, z, z);
});
canvas.addEventListener("mousedown", (e) => {
pause = !pause
});
canvas {
border: solid
}
<canvas id="canvas" height="100" width="100"></canvas>
<canvas id="zoom" height="100" width="100"></canvas>

How to scale an image for a canvas pattern?

I want to fill a Canvas with an Image and scale it to a certain width beforehand.
I am trying to achieve an effect where an image in the foreground of the canvas can be erased with the mouse to view an image in the background. This is why I need to use a pattern to fill my canvas instead of just using drawImage(). Everything works apart from the scaling of the foreground image. Here is my code for generating the pattern:
var blueprint_background = new Image();
blueprint_background.src = "myfunurl";
blueprint_background.width = window.innerWidth;
blueprint_background.onload = function(){
var pattern = context.createPattern(this, "no-repeat");
context.fillStyle = pattern;
context.fillRect(0, 0, window.innerWidth, 768);
context.fill();
};
This does exactly what it should do, except that the image keeps its original size.
As you see, I want the image to scale to window.innerWidth (which has the value 1920 when logging it).
If needed, I can provide the rest of the code, but since the error is most likely in this snippet, I decided not to post the rest.
EDIT: Here is my full code with the suggested changes. The front ground image now displays over the full width, however the erasing does not work anymore.
JavaScript (Note that I use jQuery instead of $):
jQuery(document).ready(function() {
var cwidth = window.innerWidth;
var cheight = 768;
function createCanvas(parent, width, height) {
var canvas = {};
canvas.node = document.createElement('canvas');
canvas.context = canvas.node.getContext('2d');
canvas.node.width = width || 100;
canvas.node.height = height || 100;
parent.appendChild(canvas.node);
return canvas;
}
function init(canvas, fillColor) {
var ctx = canvas.context;
canvas.isDrawing = true;
jQuery('#canvas').children().css('position:absolute; top: ' + jQuery('#Top_bar').height() + 'px');
// define a custom fillCircle method
ctx.fillCircle = function(x, y, radius, fillColor) {
this.fillStyle = fillColor;
this.beginPath();
this.moveTo(x, y);
this.arc(x, y, radius, 0, Math.PI * 2, false);
this.fill();
};
// bind mouse events
canvas.onmousemove = function(e) {
if (!canvas.isDrawing) {
return;
}
var x = e.pageX - this.offsetLeft;
var y = e.pageY - jQuery('#Top_bar').outerHeight();
var radius = 30;
var fillColor = '#ff0000';
ctx.globalCompositeOperation = 'destination-out';
ctx.fillCircle(x, y, radius, fillColor);
};
}
var container = document.getElementById('canvas');
jQuery('#canvas').css('position:absolute; top: ' + jQuery('#Top_bar').height() + 'px');
var canvas = createCanvas(container, cwidth, cheight);
init(canvas, '#ddd');
var fgimg = document.getElementById("fgimg");
fgimg.width = cwidth;
var context = canvas.node.getContext("2d");
let canvasP = document.getElementById("pattern");
canvasP.width = window.innerWidth;
canvasP.height = 768;
let ctxP = canvasP.getContext("2d");
ctxP.drawImage( fgimg, 0, 0,window.innerWidth,768 );
context.fillStyle = context.createPattern(canvasP,"no-repeat");
context.fillRect(0,0, canvas.width, canvas.height);
});
CSS:
#canvas {
background:url(http://ulmke-web.de/wp-content/uploads/2019/01/Header-6.jpg);
background-repeat: no-repeat;
background-size: cover;
background-position: center center;
width: 100%;
height: 768px;
}
HTML:
<div id="canvas">
<canvas id="pattern">
</div>
<div style="display:none">
<img id="fgimg" src=" http://ulmke-web.de/wp-content/uploads/2019/01/Header-5.jpg">
</div>

I would use two canvases. On the first one you draw your image and you use this canvas as an image to create the pattern. In order to scale the image you scale the size of the first canvas #pattern in my example.
For example you can do this for a 10/10 image:
canvasP.width = 10;
canvasP.height = 10;
ctxP.drawImage( redpoint, 2.5, 2.5 );
or you can do this for a 20/20 image:
canvasP.width = 20;
canvasP.height = 20;
ctxP.drawImage( redpoint, 5, 5,10,10 );
Furthermore, in my example I'm adding a little margin around the image.
let canvasP = document.getElementById("pattern");
if (canvasP && canvasP.getContext) {
let ctxP = canvasP.getContext("2d");
/*canvasP.width = 10;
canvasP.height = 10;
ctxP.drawImage( redpoint, 2.5, 2.5 ); */
canvasP.width = 20;
canvasP.height = 20;
ctxP.drawImage( redpoint, 5, 5,10,10 );
}
let canvas1 = document.getElementById("canvas");
if (canvas1 && canvas1.getContext) {
let ctx1 = canvas1.getContext("2d");
if (ctx1) {
ctx1.fillStyle = ctx1.createPattern(canvasP,"repeat");
ctx1.fillRect(0,0, canvas1.width, canvas1.height);
}
}
canvas{border:1px solid}
<img id="redpoint" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUA AAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO 9TXL0Y4OHwAAAABJRU5ErkJggg==">
<canvas id="pattern"></canvas>
<canvas id="canvas"></canvas>
I hope it helps.

Trying to add text in canvas

I am working on a PWA, which will be used to conduct surveys, so what I'm doing is,
I'm capturing a snapshot from a video(within the app) and saving it in a canvas, which works fine.
Now I need to add date, time and geo-coordinates on it.
My Javascript code
var video = document.querySelector('video');
var takenPhotosDiv = document.getElementById( "taken-photos" );
var button = document.querySelector('button');
button.onclick = function() {
drawCanvas();
};
var drawCanvas = function(){
var canvas = window.canvas = document.createElement('canvas');
canvas.width = video.videoWidth;
canvas.height = video.videoHeight;
canvas.getContext('2d').drawImage(video, 0, 0, canvas.width, canvas.height);
canvas.getContext('2d').fillText( "30-08-2017" + " " + "15:25" + " " + "(79.85858454, 17.56852655)", 50, 150 );
takenPhotosDiv.appendChild( canvas );
}
The above code works fine and it does get close to what's being expected, here's the sample output of the above code
The final text-format should look like this (the text bar should be at the bottom of the image and not in the middle and with much bigger font)
PS: I don't just have to display this in the above mentioned format, even need to save and push it on Firebase later.
Edit:
var addTextToCanvas = function( canvas ){
canvas.lineWidth = 2;
canvas.fillStyle = "blue";
canvas.font = "bold 20px sans-serif";
canvas.textBaseline = "bottom";
canvas.fillText( "30-08-2017" + " " + "15:25" + " " + "(79.85858454, 17.56852655)", 0, 100 );
return canvas;
};
I tried this, but the font and font size remained the same.
This function is called from drawCanvas(), just before appending it to div, since it didn't work, I simply added called fillText on the canvas there itself
Edit 2:

Make sure that the methods and properties are set on the context not the canvas element itself.
We also need to calculate the actual vertical position of the text. Since it's aligned to the bottom we can use the height of the canvas minus some bottom padding:
var y = canvas.height - 10;
So, for example:
var addTextToCanvas = function( context ) { // pass in 2D context
var y = context.canvas.height - 10;
context.fillStyle = "blue";
context.font = "bold 20px sans-serif";
context.textBaseline = "bottom";
context.fillText( "30-08-2017"+" "+"15:25"+" "+"(79.85858454, 17.56852655)", 10, y );
return context;
};
or if you prefer to pass in the canvas:
var addTextToCanvas = function( canvas ) {
var context = canvas.getContext("2d");
var y = canvas.height - 10;
context.fillStyle = "blue";
context.font = "bold 20px sans-serif";
context.textBaseline = "bottom";
context.fillText( "30-08-2017"+" "+"15:25"+" "+"(79.85858454, 17.56852655)", 10, y );
return context;
};
The lineWidth doesn't do anything here so it can be removed.
I would recommend that you store the context once globally. It's the same context you get each time anyways but there is more overhead requesting it each time it will be used.
Functional example:
var ctx = c.getContext("2d");
var addTextToCanvas = function( context ) {
context.fillStyle = "blue";
context.font = "bold 20px sans-serif";
context.textBaseline = "bottom";
var y = context.canvas.height - 10;
context.fillText( "30-08-2017"+" "+"15:25"+" "+"(79.85858454, 17.56852655)", 10, y );
return context;
};
addTextToCanvas(ctx);
#c {border: 1px solid #999}
<canvas id=c width=600 height=180></canvas>
And finally, to extract as image the call needs to be made on the canvas element not context (can be confusing):
var dataUrl = canvas.toDataURL(); // saves out PNG image
or for JPEG:
var dataUrl = canvas.toDataURL("image/jpeg", 0.75);

Try this
var canvas = window.canvas = document.createElement('canvas');
var ctx = canvas.getContext('2d');
var text = "30-08-2017\n15:25\n(79.85858454, 17.56852655)";
ctx.font = "30px Arial";
ctx.fillStyle = "red";
ctx.textAlign = "center";
ctx.fillText(text, canvas.width/2, canvas.height/2);

Drawing multiple offscreen canvases into onscreen canvas

I am having an issue when I'm trying to render multiple offscreen canvases into onscreen canvas. I do get one offscreen canvas rendered but the problem is that there should be two other rendered before. In other words, only last canvas is rendered. The expected result would be three overlapping rectangles (or squares :) in red, green and blue. Here's the code:
function rectangle(color) {
var offScreenCanvas = document.createElement('canvas');
var offScreenCtx = offScreenCanvas.getContext('2d');
var width = offScreenCanvas.width = 150;
var height = offScreenCanvas.height = 150;
switch(color) {
case 1:
offScreenCtx.fillStyle='rgb(255,0,0)';
break;
case 2:
offScreenCtx.fillStyle='rgb(0,255,0)';
break;
case 3:
offScreenCtx.fillStyle='rgb(0,0,255)';
break;
}
offScreenCtx.fillRect(0,0,width,height);
return offScreenCanvas;
}
function draw(offScreenCanvas, x , y) {
var canvas = document.getElementById('canvas')
var ctx = canvas.getContext('2d');
var width = canvas.width = window.innerWidth;
var height = canvas.height = window.innerHeight;
ctx.drawImage(offScreenCanvas, x, y);
}
var images = [];
var color = 1;
for (var i=0; i<3; i++) {
var img = new rectangle(color);
images.push(img);
color++;
}
var x = 0;
var y = 0;
for (var i = 0; i < images.length; i++) {
draw(images[i], x, y);
x += 100;
y += 100;
}
I did some searching and it seems that I'm not the first with this issue, but I could not get this working properly.

Setting canvas height or width clears the canvas.
The problem with your code is that you are causing the onscreen canvas to be cleared when you set it size in the function draw
Setting the canvas size, even if that size is the same, will cause the canvas context to reset and clear the canvas. All the other canvases are rendered, but erased when you set the onscreen canvas size.
Your draw function
function draw(offScreenCanvas, x , y) {
var canvas = document.getElementById('canvas')
var ctx = canvas.getContext('2d');
// The cause of the problem ===================================
// Either one of the following lines will clear the canvas
var width = canvas.width = window.innerWidth;
var height = canvas.height = window.innerHeight;
//=============================================================
ctx.drawImage(offScreenCanvas, x, y);
}
To avoid this just set the canvas size once. If you need to resize the canvas and keep its content you first need to create a copy of the canvas, then resize it, then render the copy back to the original.
Demo shows 5 offscreen canvases being rendered onto one onscreen canvas.
const colours = ['#f00', '#ff0', '#0f0', '#0ff', '#00f'];
const ctx = can.getContext('2d');
can.width = innerWidth - 4; // sub 4 px for border
can.height = innerHeight - 4;
function createCanvas(color, i) {
const canvas = document.createElement('canvas');
const ctx = canvas.getContext('2d');
canvas.width = 150;
canvas.height = 150;
ctx.font = "24px arial";
ctx.fillStyle = color;
ctx.fillRect(0, i * 30, canvas.width, 30);
ctx.fillStyle = "black";
ctx.fillText("Canvas "+i,10,(i + 0.75) * 30);
return canvas;
}
colours.forEach((c, i) => {
ctx.drawImage(createCanvas(c, i), 0, 0);
})
canvas {
border: 2px solid black;
position : absolute;
top : 0px;
left : 0px;
}
<canvas id="can"></canvas>

Use Clip Function to with Gradient Effect with JavaScript on Canvas

I'm trying to use the clip() function in canvas to create this effect, as pictured: there is a background image, and when your mouse hover on it, part of the image is shown. I got it to work as a circle, but I want this gradient effect you see the picture. How do I achieve that?
<!DOCTYPE HTML>
<html>
<head>
<link rel="stylesheet" type="text/css" href="./assets/stylesheet/normalize.css">
<link rel="stylesheet" type="text/css" href="./assets/stylesheet/style.css">
</head>
<body>
<canvas id="canvas" width="2000" height="1200"></canvas>
<script>
var can = document.getElementById('canvas');
var ctx = can.getContext('2d');
can.addEventListener('mousemove', function(e) {
var mouse = getMouse(e, can);
redraw(mouse);
}, false);
function redraw(mouse) {
console.log('a');
can.width = can.width;
ctx.canvas.width = window.innerWidth;
ctx.canvas.height = window.innerHeight;
ctx.drawImage(img, 0, 0);
ctx.beginPath();
ctx.rect(0,0,2000,1200);
ctx.arc(mouse.x, mouse.y, 200, 0, Math.PI*2, true)
ctx.clip();
ctx.fillRect(0,0,2000,1200);
}
var img = new Image();
img.onload = function() {
redraw({x: 0, y: 0})
}
img.src = 'http://placekitten.com/2000/1000';
function getMouse(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));
}
mx = e.pageX - offsetX;
my = e.pageY - offsetY;
return {
x: mx,
y: my
};
}
</script>

USING a RADIAL gradient
There are many ways to do that but the simplest is a gradient with an alpha.
First you need to define the size of the circle you wish to show.
var cirRadius = 300;
Then the location (canvas coordinates) where this circle will be centered
var posX = 100;
var posY = 100;
Now define the rgb colour
var RGB = [0,0,0] ; // black
Then an array of alpha values to define what is transparent
var alphas = [0,0,0.2,0.5,1]; // zero is transparent;
Now all you do is render the background image
// assume ctx is context and image is loaded
ctx.drawImage(image, 0, 0, ctx.canvas.width, ctx.canvas.height); // fill the canvas
Then create the gradient with it centered at the position you want and the second circle at the radius you want. The first 3 numbers define the center and radius of the start of the gradient, the last 3 define the center and radius of the end
var grad = ctx.createRadialGradient(posX,posY,0,posX,posY,cirRadius);
Now add the colour stops using the CSS color string rgba(255,255,255,1) where the last is the alpha value from 0 to 1.
var len = alphas.length-1;
alphas.forEach((a,i) => {
grad.addColorStop(i/len,`rgba(${RGB[0]},${RGB[1]},${RGB[2]},${a})`);
});
or for legacy browsers that do not support arrow functions or template strings
var i,len = alphas.length;
for(i = 0; i < len; i++){
grad.addColorStop(i / (len - 1), "rgba(" + RGB[0] + "," + RGB[1] + "," + RGB[2] + "," + alphas[i] + ")");
}
Then set the fill style to the gradient
ctx.fillStyle = grad;
then just fill a rectangle covering the image
ctx.fillRect(0,0,ctx.canvas.width,ctx.canvas.height);
And you are done.
By setting the position with via a mouse event and then doing the above steps 60times a second using window.requestAnimationFrame you can get the effect you are looking for in real time.
Here is an example
// create a full screen canvas
var canvas = document.createElement("canvas");
canvas.style.position = "absolute";
canvas.style.left = "0px";
canvas.style.top = "0px";
canvas.style.zIndex = 10;
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
document.body.appendChild(canvas);
// var to hold context
var ctx;
// load an image
var image = new Image();
image.src = "https://i.stack.imgur.com/C7qq2.png?s=328&g=1";
// add resize event
var resize = function(){
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
ctx = canvas.getContext("2d");
}
// add mouse event. Because it is full screen no need to bother with offsets
var mouse = function(event){
posX = event.clientX;
posY = event.clientY;
}
// incase the canvas size is changed
window.addEventListener("resize",resize);
// listen to the mouse move
canvas.addEventListener("mousemove",mouse)
// Call resize as that gets our context
resize();
// define the gradient
var cirRadius = 300;
var posX = 100; // this will be set by the mouse
var posY = 100;
var RGB = [0,0,0] ; // black any values from 0 to 255
var alphas = [0,0,0.2,0.5,0.9,0.95,1]; // zero is transparent one is not
// the update function
var update = function(){
if(ctx){ // make sure all is in order..
if(image.complete){ // draw the image when it is ready
ctx.drawImage(image,0,0,canvas.width,canvas.height)
}else{ // while waiting for image clear the canvas
ctx.clearRect(0,0,canvas.width,canvas.height);
}
// create gradient
var grad = ctx.createRadialGradient(posX,posY,0,posX,posY,cirRadius);
// add colour stops
var len = alphas.length-1;
alphas.forEach((a,i) => {
grad.addColorStop(i/len,`rgba(${RGB[0]},${RGB[1]},${RGB[2]},${a})`);
});
// set fill style to gradient
ctx.fillStyle = grad;
// render that gradient
ctx.fillRect(0,0,canvas.width,canvas.height);
}
requestAnimationFrame(update); // keep doing it till cows come home.
}
// start it all happening;
requestAnimationFrame(update);