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I am making a clicker game where you click on a potato for stonks. I have this code:
var canvas, canvasContext, imgaeAssets;
window.onload = function() {
canvas = document.getElementById('gaming');
canvasContext = canvas.getContext('2d');
canvasContext.canvas.width = window.innerWidth;
canvasContext.canvas.height = window.innerHeight * 0.95;
setInterval(mainloop, 1000 / 50);
}
var potatoes = 0;
var potatoClick = new Image();
potatoClick.src = 'images/potato.jfif'
function mainloop() { // runs once every second
colorRect(0, 0, canvas.width, canvas.height, 'black')
canvasContext.font = "15px Comic Sans MS";
canvasContext.fillStyle = "white";
canvas.style.backgroundColor = 'rgba(255, 255, 255, 0)';
drawImg(potatoClick, 0, (canvas.height / 2) - 125, 500, 250)
if (potatoes > 0) {
canvasContext.fillText(`you have ${potatoes} potatoes`, 0, 30);
}
}
function colorRect(x, y, w, h, c) {
canvasContext.fillStyle = c;
canvasContext.fillRect(x, y, w, h);
}
function drawImg(src, x, y, w, h) {
canvasContext.drawImage(src, x, y, w, h);
}
function printMousePos(event) {}
I want to make the white in potatoClick transparent, but I can't figure out how. I've already tried assigning an alpha to it, and have also tried changing the opacity of the image, but neither have worked. If you could figure out how to make any color transparent, I would appreciate it. I wouldn't mind needing to change the way to draw the potato, but I think it would be cleaner and easier the way I have it now.
edit: I think my question was fundamentally flawed, sorry for the confusion
After drawing things at canvas, you can't edit them anymore.
Because, Image in canvas doesn't have any parameters after draw it.
It means if you want to edit image on canvas, you need to clear your canvas when the difference is occurred.
and if you want to change the image opacity you should set globalAlpha of ctx (draw image with opacity on to a canvas)
Usually, I use Fabric.js(http://fabricjs.com/) for solve that problem.
but If you want, you can make a your own renderer by defining simple class.
like this!
<body>
<img id="dog" src="https://cdn.pixabay.com/photo/2022/07/19/10/35/puppies-7331739__340.jpg">
<img id="woman" src="https://cdn.pixabay.com/photo/2021/05/04/20/57/woman-6229693__340.jpg">
<canvas id="canvas" width="500" height="500" style="border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.
</canvas>
<script>
var canvas;
var ctx;
var img;
window.onload = function () {
canvas = document.getElementById("canvas");
ctx = canvas.getContext("2d");
dog = document.getElementById("dog");
woman = document.getElementById("woman");
const exam1 = new Image(dog);
const exam2 = new Image(woman);
images = [];
images.push(exam1);
images.push(exam2);
render(images);
dog.onclick = function () {
exam1.opacity = 0.7;
exam2.opacity = 0.3;
exam1.y += 10;
exam2.x += 10;
render(images);
}
}
function render(images) {
ctx.clearRect(0, 0, canvas.width, canvas.height);
for (var i = 0; i < images.length; i++) {
images[i].render();
}
}
function Image(original) {
this.image = original;
this.width = this.image.width;
this.height = this.image.height;
this.opacity = 1.0;
this.x = 0;
this.y = 0;
}
Image.prototype.render = function () {
this.image.width = this.width;
this.image.height = this.height;
ctx.globalAlpha = this.opacity;
ctx.drawImage(this.image, this.x, this.y);
ctx.globalAlpha = 1.0;
}
</script>
</body>
I'm trying to loop my animation, but no matter what I do, it won't loop. I'm pretty new to canvas, javascript and code in general.
var canvas = document.getElementById("fabrication");
var ctx = canvas.getContext("2d");
var background = new Image();
background.src =
"C:/Users/dylan/Desktop/ProjectTwo/Images/fabricationbackground.jpg";
background.onload = function(){
}
//Loading all of my canvas
var posi =[];
posi[1] = 20;
posi[2] = 20;
var dx=10;
var dy=10;
var ballRadius = 4;
//Variables for drawing a ball and it's movement
function drawballleft(){
posi =xy(posi[1],posi[2])
}
function xy(x,y){
ctx.drawImage(background,0,0);
ctx.beginPath();
ctx.arc(x, y, ballRadius, 0, Math.PI*2);
ctx.fillStyle = "#FFFFFFF";
ctx.fill();
ctx.closePath();
var newpos=[];
newpos[1]= x +dx;
newpos[2]= y +dy;
return newpos;
//Drawing the ball, making it move off canvas.
if (newpos[1] > canvas.width) {
newpos[1] = 20;
}
if (newpos[2] > canvas.height) {
newpos[2] = 20;
}
//If statement to detect if the ball moves off the canvas, to make it return to original spot
}
setInterval(drawballleft, 20);
//Looping the function
Please let me know if I've done something wrong, I really want to learn what I'm doing here. The ball is supposed to go off the canvas, and loop back onto itself, but it goes off the canvas and ends.
Thanks in advance!
I have made a few changes to your code.
First I am using requestAnimationFrame instead of setInterval. http://www.javascriptkit.com/javatutors/requestanimationframe.shtml
Second I am not using an image because I didn't want to run into a CORS issue. But you can put your background image back.
I simplified your posi array to use indexes 0 and 1 instead of 1 and 2 to clean up how you create your array.
I moved your return from before the two ifs to after so the ball will move back to the left or top when it goes off the side. I think that was the real problem you were seeing
var canvas = document.getElementById("fabrication");
var ctx = canvas.getContext("2d");
//Loading all of my canvas
var posi =[20,20];
var dx=10;
var dy=10;
var ballRadius = 4;
//Variables for drawing a ball and it's movement
function drawballleft(){
posi = xy(posi[0],posi[1])
requestAnimationFrame(drawballleft);
}
function xy(x,y){
ctx.fillStyle = '#FFF';
ctx.fillRect(0,0,400,300);
ctx.beginPath();
ctx.arc(x, y, ballRadius, 0, Math.PI*2);
ctx.fillStyle = "#000";
ctx.fill();
ctx.closePath();
var newpos=[x+dx,y+dy];
//Drawing the ball, making it move off canvas.
if (newpos[0] > canvas.width) {
newpos[0] = 20;
}
if (newpos[1] > canvas.height) {
newpos[1] = 20;
}
//If statement to detect if the ball moves off the canvas, to make it return to original spot
return newpos;
}
requestAnimationFrame(drawballleft);
canvas {
outline: 1px solid red;
}
<canvas width="400" height="300" id="fabrication"></canvas>
To make it all even simpler...
Use an external script for handling the canvas.
A really good one ;) :
https://github.com/GustavGenberg/handy-front-end#canvasjs
Include it with
<script type="text/javascript" src="https://gustavgenberg.github.io/handy-front-end/Canvas.js"></script>
Then it's this simple:
// Setup canvas
const canvas = new Canvas('my-canvas', 400, 300).start(function (ctx, handyObject, now) {
// init
handyObject.Ball = {};
handyObject.Ball.position = { x: 20, y: 20 };
handyObject.Ball.dx = 10;
handyObject.Ball.dy = 10;
handyObject.Ball.ballRadius = 4;
});
// Update loop, runs before draw loop
canvas.on('update', function (handyObject, delta, now) {
handyObject.Ball.position.x += handyObject.Ball.dx;
handyObject.Ball.position.y += handyObject.Ball.dy;
if(handyObject.Ball.position.x > canvas.width)
handyObject.Ball.position.x = 20;
if(handyObject.Ball.position.y > canvas.height)
handyObject.Ball.position.y = 20;
});
// Draw loop
canvas.on('draw', function (ctx, handyObject, delta, now) {
ctx.clear();
ctx.beginPath();
ctx.arc(handyObject.Ball.position.x, handyObject.Ball.position.y, handyObject.Ball.ballRadius, 0, Math.PI * 2);
ctx.fillStyle = '#000';
ctx.fill();
ctx.closePath();
});
I restructured your code and used the external script, and now it looks much cleaner and easier to read and toubleshoot!
JSFiddle: https://jsfiddle.net/n7osvt7y/
I am trying to create multiple animated rectangles using Html Canvas with requestAnimationFrame. As for now, I managed to do exactly what I wanted with only one animated rectangle, but I can't find out how to create more rectangles that would simply be in line and follow each other with an equal distance.
Also, there's a random data (I, II or III) inside each rectangle.
Here's my actual code:
//Referencing canvas
var canvas = document.getElementById("my-canvas");
var ctx = canvas.getContext("2d");
//Make Canvas fullscreen and responsive
function resize() {
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
}
window.addEventListener('resize', resize, false); resize();
//FPS
var framesPerSecond = 60;
//Default Y pos to center;
var yPos = canvas.height / 2;
//Default X pos offset
var xPos = -150;
//Speed (increment)
var speed = 2;
//Our array to store rectangles objects
var rectangles = [] ;
//Dynamic Number from database
var quote = ["I", "II", "III"];
//Random number for testing purpose
var rand = quote[Math.floor(Math.random() * quote.length)];
//Draw Rectangle
function drawRectangle () {
setTimeout(function() {
requestAnimationFrame(drawRectangle);
ctx.clearRect(0, 0, canvas.width, canvas.height);
//Background color
ctx.fillStyle = "yellow";
//Position, size.
var rectWidth = 70;
var rectHeigth = 55;
ctx.fillRect(xPos,yPos,rectWidth,rectHeigth);
ctx.font = "32px Arial";
ctx.textAlign = "center";
ctx.textBaseline = "middle";
ctx.fillStyle = "black";
//Data Layer
var dataLayer = ctx.fillText(rand,xPos+(rectWidth/2),yPos+(rectHeigth/2));
xPos += speed;
//Infinite loop for test
if (xPos > 1080) {
xPos = -150;
}
}, 1000 / framesPerSecond);
}
drawRectangle ();
canvas {background-color: #131217}
body { margin: 0; overflow: hidden; }
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Moving Blocks</title>
<style>
canvas {background-color: #131217}
body { margin: 0; overflow: hidden; }
</style>
</head>
<body>
<canvas id="my-canvas"></canvas>
</body>
</html>
Animating arrays of objects.
For animations you are best of using a single render function that renders all the objects once a frame, rather than create a separate render frame per object.
As for the squares there are many ways that you can get them to do what you want. It is a little difficult to answer as what you want is not completely clear.
This answer will use a rectangle object that has everything needed to be rendered and move. The rectangles will be kept in an array and the main render function will update and render each rectangle in turn.
There will be a spawn function that creates rectangles untill the limit has been reached.
// constants up the top
const quote = ["I", "II", "III"];
// function selects a random Item from an array
const randItem = (array) => array[(Math.random() * array.length) | 0];
// array to hold all rectangles
const rectangles = [];
var maxRectangles = 20;
const spawnRate = 50; // number of frames between spawns
var spawnCountdown = spawnRate;
//Referencing canvas
const ctx = canvas.getContext("2d");
var w, h; // global canvas width and height.
resizeCanvas(); // size the canvas to fit the page
requestAnimationFrame(mainLoop); // this will start when all code below has been run
function mainLoop() {
// resize in the rendering frame as using the resize
// event has some issuse and this is more efficient.
if (w !== innerWidth || h !== innerHeight) {
resizeCanvas();
}
ctx.clearRect(0, 0, w, h);
spawnRectangle(); // spawns rectangles
updateAllRectangles(); // moves all active rectangles
drawAllRectangles(); // I will let you gues what this does... :P
requestAnimationFrame(mainLoop);
}
function resizeCanvas() {
w = canvas.width = innerWidth;
h = canvas.height = innerHeight;
// and reset any canvas constants
ctx.font = "32px Arial";
ctx.textAlign = "center";
ctx.textBaseline = "middle";
}
// function to spawn a rectangle
function spawnRectangle() {
if (rectangles.length < maxRectangles) {
if (spawnCountdown) {
spawnCountdown -= 1;
} else {
rectangles.push(
createRectangle({
y: canvas.height / 2, // set at center
text: randItem(quote),
dx: 2, // set the x speed
})
);
spawnCountdown = spawnRate;
}
}
}
// define the default rectangle
const rectangle = {
x: -40, // this is the center of the rectangle
y: 0,
dx: 0, // delta x and y are the movement per frame
dy: 0,
w: 70, // size
h: 55,
color: "yellow",
text: null,
textColor: "black",
draw() { // function to draw this rectangle
ctx.fillStyle = this.color;
ctx.fillRect(this.x - this.w / 2, this.y - this.h / 2, this.w, this.h);
ctx.fillStyle = this.textColor;
ctx.fillText(this.text, this.x, this.y);
},
update() { // moves the rectangle
this.x += this.dx;
this.y += this.dy;
if (this.x > canvas.width + this.w / 2) {
this.x = -this.w / 2;
// if the rectangle makes it to the other
// side befor all rectangles are spawnd
// then reduce the number so you dont get any
// overlap
if (rectangles.length < maxRectangles) {
maxRectangles = rectangles.length;
}
}
}
}
// creats a new rectangle. Setting can hold any unique
// data for the rectangle
function createRectangle(settings) {
return Object.assign({}, rectangle, settings);
}
function updateAllRectangles() {
var i;
for (i = 0; i < rectangles.length; i++) {
rectangles[i].update();
}
}
function drawAllRectangles() {
var i;
for (i = 0; i < rectangles.length; i++) {
rectangles[i].draw();
}
}
canvas {
position: absolute;
top: 0px;
left: 0px;
background-color: #131217;
}
body {
margin: 0;
overflow: hidden;
}
<canvas id="canvas"></canvas>
I've got some code which draws a rectangle on a canvas, but I want that rectangle to change color when I hover the mouse over it.
The problem is after I've drawn the rectangle I'm not sure how I select it again to make the adjustment.
What I want to do:
var c=document.getElementById("myCanvas");
var ctx=c.getContext("2d");
ctx.rect(20,20,150,100);
ctx.stroke();
$('c.[rectangle]').hover(function(this){
this.fillStyle = 'red';
this.fill();
});
You can't do this out-of-the-box with canvas. Canvas is just a bitmap, so the hover logic has to be implemented manually.
Here is how:
Store all the rectangles you want as simple object
For each mouse move on the canvas element:
Get mouse position
Iterate through the list of objects
use isPointInPath() to detect a "hover"
Redraw both states
Example
var canvas = document.querySelector("canvas"),
ctx = canvas.getContext("2d"),
rects = [
{x: 10, y: 10, w: 200, h: 50},
{x: 50, y: 70, w: 150, h: 30} // etc.
], i = 0, r;
// render initial rects.
while(r = rects[i++]) ctx.rect(r.x, r.y, r.w, r.h);
ctx.fillStyle = "blue"; ctx.fill();
canvas.onmousemove = function(e) {
// important: correct mouse position:
var rect = this.getBoundingClientRect(),
x = e.clientX - rect.left,
y = e.clientY - rect.top,
i = 0, r;
ctx.clearRect(0, 0, canvas.width, canvas.height); // for demo
while(r = rects[i++]) {
// add a single rect to path:
ctx.beginPath();
ctx.rect(r.x, r.y, r.w, r.h);
// check if we hover it, fill red, if not fill it blue
ctx.fillStyle = ctx.isPointInPath(x, y) ? "red" : "blue";
ctx.fill();
}
};
<canvas/>
This is a stable code in base of #K3N answer. The basic problem of his code is because when one box is over the another the two may get mouse hover at same time. My answer perfectly solves that adding a 'DESC' to 'ASC' loop.
var canvas = document.getElementById("canvas"),
ctx = canvas.getContext("2d");
var map = [
{x: 20, y: 20, w: 60, h: 60},
{x: 30, y: 50, w: 76, h: 60}
];
var hover = false, id;
var _i, _b;
function renderMap() {
for(_i = 0; _b = map[_i]; _i ++) {
ctx.fillStyle = (hover && id === _i) ? "red" : "blue";
ctx.fillRect(_b.x, _b.y, _b.w, _b.h);
}
}
// Render everything
renderMap();
canvas.onmousemove = function(e) {
// Get the current mouse position
var r = canvas.getBoundingClientRect(),
x = e.clientX - r.left, y = e.clientY - r.top;
hover = false;
ctx.clearRect(0, 0, canvas.width, canvas.height);
for(var i = map.length - 1, b; b = map[i]; i--) {
if(x >= b.x && x <= b.x + b.w &&
y >= b.y && y <= b.y + b.h) {
// The mouse honestly hits the rect
hover = true;
id = i;
break;
}
}
// Draw the rectangles by Z (ASC)
renderMap();
}
<canvas id="canvas"></canvas>
You may have to track the mouse on the canvas using JavaScript and see when it is over your rectangle and change the color then. See code below from my blog post
<!DOCTYPE html>
<html>
<body>
<canvas id="myCanvas" width="700" height="500" style="border:1px solid #c3c3c3;">
Your browser does not support the HTML5 canvas tag.
</canvas>
<script>
var myRect={x:150, y:75, w:50, h:50, color:"red"};
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.fillStyle = myRect.color;
ctx.fillRect(myRect.x, myRect.y, myRect.w, myRect.h);
c.addEventListener("mousemove", function(e){
if ((e.clientX>=myRect.x)&(e.clientX<=myRect.x+myRect.w)&(e.clientY>=myRect.y)&(e.clientY<=myRect.y+myRect.h)){
myRect.color = "green";}
else{
myRect.color = "red";}
updateCanvas();
}, false);
function updateCanvas(){
ctx.fillStyle = myRect.color;
ctx.fillRect(myRect.x, myRect.y, myRect.w, myRect.h);
}
</script>
</body>
</html>
I believe this is a slightly more in-depth answer that would work better for you, especially if you are interested in game design with the canvas element.
The main reason this would work better for you is because it focuses more on an OOP (object orientated programming) approach. This allows for objects to be defined, tracked and altered at a later time via some event or circumstance. It also allows for easy scaling of your code and in my opinion is just more readable and organized.
Essentially what you have here is two shapes colliding. The cursor and the individual point / object it hovers over. With basic squares, rectangles or circles this isn't too bad. But, if you are comparing two more unique shapes, you'll need to read up more on Separating Axis Theorem (SAT) and other collision techniques. At that point optimizing and performance will become a concern, but for now I think this is the optimal approach.
const canvas = document.querySelector('canvas');
const ctx = canvas.getContext('2d');
const width = canvas.width = window.innerWidth;
const height = canvas.height = window.innerHeight;
const cx = width / 2;
const cy = height / 2;
const twoPie = Math.PI * 2;
const points = []; // This will be the array we store our hover points in later
class Point {
constructor(x, y, r) {
this.x = x;
this.y = y;
this.r = r || 0;
}
}
class HoverPoint extends Point {
constructor(x, y, r, color, hoverColor) {
super(x, y, r);
this.color = color;
this.hoverColor = hoverColor;
this.hovered = false;
this.path = new Path2D();
}
draw() {
this.hovered ? ctx.fillStyle = this.hoverColor : ctx.fillStyle = this.color;
this.path.arc(this.x, this.y, this.r, 0, twoPie);
ctx.fill(this.path);
}
}
class Cursor extends Point {
constructor(x, y, r) {
super(x, y, r);
}
collisionCheck(points) {
// This is the method that will be called during the animate function that
// will check the cursors position against each of our objects in the points array.
document.body.style.cursor = "default";
points.forEach(point => {
point.hovered = false;
if (ctx.isPointInPath(point.path, this.x, this.y)) {
document.body.style.cursor = "pointer";
point.hovered = true;
}
});
}
}
function createPoints() {
// Create your points and add them to the points array.
points.push(new HoverPoint(cx, cy, 100, 'red', 'coral'));
points.push(new HoverPoint(cx + 250, cy - 100, 50, 'teal', 'skyBlue'));
// ....
}
function update() {
ctx.clearRect(0, 0, width, height);
points.forEach(point => point.draw());
}
function animate(e) {
const cursor = new Cursor(e.offsetX, e.offsetY);
update();
cursor.collisionCheck(points);
}
createPoints();
update();
canvas.onmousemove = animate;
There is one more thing that I would like to suggest. I haven't done tests on this yet but I suspect that using some simple trigonometry to detect if our circular objects collide would preform better over the ctx.IsPointInPath() method.
However if you are using more complex paths and shapes, then the ctx.IsPointInPath() method would most likely be the way to go. if not some other more extensive form of collision detection as I mentioned earlier.
The resulting change would look like this...
class Cursor extends Point {
constructor(x, y, r) {
super(x, y, r);
}
collisionCheck(points) {
document.body.style.cursor = "default";
points.forEach(point => {
let dx = point.x - this.x;
let dy = point.y - this.y;
let distance = Math.hypot(dx, dy);
let dr = point.r + this.r;
point.hovered = false;
// If the distance between the two objects is less then their combined radius
// then they must be touching.
if (distance < dr) {
document.body.style.cursor = "pointer";
point.hovered = true;
}
});
}
}
here is a link containing examples an other links related to collision detection
I hope you can see how easily something like this can be modified and used in games and whatever else. Hope this helps.
Below code adds shadow to canvas circle on hovering it.
<html>
<body>
<canvas id="myCanvas" width="1000" height="500" style="border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.</canvas>
</body>
<script>
var canvas = document.getElementById("myCanvas"),
ctx = canvas.getContext("2d"),
circle = [{
x: 60,
y: 50,
r: 40,
},
{
x: 100,
y: 150,
r: 50,
} // etc.
];
// render initial rects.
for (var i = 0; i < circle.length; i++) {
ctx.beginPath();
ctx.arc(circle[i].x, circle[i].y, circle[i].r, 0, 2 * Math.PI);
ctx.fillStyle = "blue";
ctx.fill();
}
canvas.onmousemove = function(e) {
var x = e.pageX,
y = e.pageY,
i = 0,
r;
ctx.clearRect(0, 0, canvas.width, canvas.height);
for (let i = 0; i < circle.length; i++) {
if ((x > circle[i].x - circle[i].r) && (y > circle[i].y - circle[i].r) && (x < circle[i].x + circle[i].r) && (y < circle[i].y + circle[i].r)) {
ctx.beginPath();
ctx.arc(circle[i].x, circle[i].y, circle[i].r, 0, 2 * Math.PI);
ctx.fillStyle = "blue";
ctx.fill();
ctx.shadowBlur = 10;
ctx.lineWidth = 3;
ctx.strokeStyle = 'rgb(255,255,255)';
ctx.shadowColor = 'grey';
ctx.stroke();
ctx.shadowColor = 'white';
ctx.shadowBlur = 0;
} else {
ctx.beginPath();
ctx.arc(circle[i].x, circle[i].y, circle[i].r, 0, 2 * Math.PI);
ctx.fillStyle = "blue";
ctx.fill();
ctx.shadowColor = 'white';
ctx.shadowBlur = 0;
}
}
};
</script>
</html>
I know this is old, but I am surprised no one has mentioned JCanvas. It adds to the simplicity of animating canvas on events. More documentation here https://projects.calebevans.me/jcanvas/docs/mouseEvents/
<html lang="en">
<head>
<!-- css and other -->
</head>
<body onload="draw();">
<canvas id = "canvas" width="500" height="500" style= border:1px solid #000000;"> </canvas>
<script>
function draw() {
$('canvas').drawRect({
layer: true,
fillStyle:'#333',
x:100, y: 200,
width: 600,
height: 400,
mouseover: function(layer) {
$(this).animateLayer(layer, {
fillStyle: 'green'
}, 1000, 'swing');
}
});
}
<script src="https://code.jquery.com/jquery-3.3.1.min.js" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jcanvas/21.0.1/jcanvas.js" crossorigin="anonymous"></script>
</body>
</html>
Consider this following code:
var c=document.getElementById("myCanvas");
var ctx=c.getContext("2d");
ctx.rect(20,20,150,100);
ctx.stroke();
c.addEventListener("mouseover", doMouseOver, false);//added event to canvas
function doMouseOver(e){
ctx.fillStyle = 'red';
ctx.fill();
}
DEMO
You could use canvas.addEventListener
var canvas = document.getElementById('canvas0');
canvas.addEventListener('mouseover', function() { /*your code*/ }, false);
It worked on google chrome
var c=document.getElementById("myCanvas");
var ctx=c.getContext("2d");
ctx.rect(20,20,150,100);
ctx.stroke();
$(c).hover(function(e){
ctx.fillStyle = 'red';
ctx.fill();
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<canvas id="myCanvas"/>
Okay, we need your help! We (with our informatics class) are building a digital scratchmap! Like this:
(source: megagadgets.nl)
With your mouse you should be able to scratch out the places you've been to. Now we're stuck. We have a canvas and we draw the image of a world map. Then when the user clicks and drags a stroke gets add on top of the world map.
Now we want to convert the (green drawn) strokes to transparency so we can reveal the image behind it. (Just like scratching out the places you've been to and revealing the map behind it (in colour)).
This is our html:
<body>
<h1>Scratchmap</h1>
<hr>
<canvas id="ball" width="600px" height ="600px">
</canvas>
<canvas id="ball2" width="600px" height ="600px">
</canvas>
</body>
And this is our javascript:
// Set variables
var a_canvas = document.getElementById("ball");
var context = a_canvas.getContext("2d");
var a_canvas2 = document.getElementById("ball2");
var context2 = a_canvas2.getContext("2d");
var img = new Image();
img.onload = function () {
context.drawImage(img, img_x, img_y);
}
img.src = "worldmap.png"
var mouse_pos_x = [];
var mouse_pos_y = [];
var thickness = 0;
var arraycount = 0;
var mouse_down = false;
var mouse_skip = [];
function update() {}
document.body.onmousedown = function () {
mouse_down = true;
var mouseX, mouseY;
if (event.offsetX) {
mouseX = event.offsetX;
mouseY = event.offsetY;
} else if (event.layerX) {
mouseX = event.layerX;
mouseY = event.layerY;
}
mouse_pos_x.push(mouseX);
mouse_pos_y.push(mouseY);
arraycount += 1;
}
document.body.onmouseup = function () {
if (mouse_down) {
mouse_down = false;
mouse_skip.push(arraycount);
}
}
document.body.onmousemove = function () {
if (mouse_down) {
var mouseX, mouseY;
if (event.offsetX) {
mouseX = event.offsetX;
mouseY = event.offsetY;
} else if (event.layerX) {
mouseX = event.layerX;
mouseY = event.layerY;
}
context.drawImage(img, 0, 0);
mouse_pos_x.push(mouseX);
mouse_pos_y.push(mouseY);
context.lineWidth = 2.5;
context.strokeStyle = "#00FF00";
context.moveTo(mouse_pos_x[arraycount - 1], mouse_pos_y[arraycount - 1]);
context.lineTo(mouse_pos_x[arraycount], mouse_pos_y[arraycount]);
context.stroke();
arraycount += 1;
var imgdata = context.getImageData(0, 0, a_canvas.width, a_canvas.height);
var l = imgdata.data.length / 4;
for (var i = 0; i < l; i++) {
var r = imgdata.data[i * 4 + 0];
var g = imgdata.data[i * 4 + 1];
var b = imgdata.data[i * 4 + 2];
if (g < 255) {
imgdata.data[i * 4 + 3] = 0;
}
}
context2.putImageData(imgdata, 0, 0);
}
}
setInterval(update, 10);
Now when we remove the draw_image() the green color becomes yellow on the other canvas. But with the draw_image() nothing gets drawn on the second canvas.
What's going wrong? Or do you have a way to do this with other Javascript or not in javascript at all?
Any help would be appreciated!
Luud Janssen & Friends
You can do this with a slightly different approach:
Set the hidden image as CSS background
Draw the cover image on top using context
Change composite mode to destination-out
Anything now drawn will erase instead of draw revealing the (CSS set) image behind
Live demo
The key code (see demo linked above for details):
function start() {
/// draw top image - background image is already set with CSS
ctx.drawImage(this, 0, 0, canvas.width, canvas.height);
/// KEY: this will earse where next drawing is drawn
ctx.globalCompositeOperation = 'destination-out';
canvas.onmousedown = handleMouseDown;
canvas.onmousemove = handleMouseMove;
window.onmouseup = handleMouseUp;
}
Then it's just a matter of tracking the mouse position and draw any shape to erase that area, for example a circle:
function erase(x, y) {
ctx.beginPath();
ctx.arc(x, y, radius, 0, pi2);
ctx.fill();
}
Random images for illustrative purposes