Recently I have been flooded with Alipay’s Jifu character
I still haven’t seen Jingyefuค(TㅅT) and I’m heartbroken
I’ll bring it to you today The
effect of the scratch card on the mobile terminal is like this
Swipe your hand to trigger the scratch card
When the scratch card area reaches more than 70%, all gray layers will be automatically scratched 
Not a lot of code
That’s all the code
<!DOCTYPE html><html lang="en"><head>
<meta charset="UTF-8">
<meta content="width=device-width,maximum-scale=1.0,minimum-scale=1.0,initial-scale=1.0,user-scalable=no" name="viewport">
<title>Scrape</title>
<style>
#myCanvas {
background-repeat: no-repeat;
background-position: center;
background-size: 200px 200px;
}
</style></head><body>
<canvas id="myCanvas" width=300 height=300></canvas>
<script>
var canvas = document.getElementById('myCanvas'),
ctx = canvas.getContext('2d'),
w = canvas.width;
h = canvas.height;
area = w * h;
l = canvas.offsetLeft;
t = canvas.offsetTop,
img = new Image();
var randomImg = function(){
var random = Math.random();
if(random < 0.4){
img.src = './1.png';
}else if(random > 0.6){
img.src = './2.png';
}else{
img.src = './award.jpg';
}
}; var bindEvent = function(){
canvas.addEventListener('touchmove', moveFunc, false);
canvas.addEventListener('touchend', endFunc, false);
}; var moveFunc = function(e){
var touch = e.touches[0],
posX = touch.clientX - l,
posY = touch.clientY - t;
ctx.beginPath();
ctx.arc(posX, posY, 15, 0, Math.PI * 2, 0);
ctx.fill();
}; var endFunc = function(e){
var data = ctx.getImageData(0, 0, w, h).data,
scrapeNum = 0;
for(var i = 3, len = data.length; i < len; i += 4){
if(data[i] === 0){
scrapeNum++;
}
} if(scrapeNum > area * 0.7){
ctx.clearRect(0, 0, w, h);
canvas.removeEventListener('touchmove', moveFunc, false);
canvas.removeEventListener('touchend', endFunc, false);
}
} var init = (function(){
ctx.fillStyle = "#ccc";
ctx.fillRect(0, 0, w, h);
randomImg();
img.addEventListener('load', function(){
canvas.style.backgroundImage = 'url(' + img.src +')';
ctx.globalCompositeOperation = 'destination-out';
bindEvent();
});
})(); </script></body></html>Let me explain it briefly
First of all, in the page we only We need a canvas element
<canvas id="myCanvas" width=300 height=300></canvas>
In CSS we need to set the style of the canvas background image in advance
#myCanvas {
background-repeat: no-repeat;
background-position: center;
background-size: 200px 200px;}In the script we first need to declare the required variables
var canvas = document.getElementById('myCanvas'),
ctx = canvas.getContext('2d'),
w = canvas.width;
h = canvas.height;
area = w * h;
l = canvas.offsetLeft;
t = canvas.offsetTop,
img = new Image();Get the canvas object and its context object
area variable is prepared for the following pixel detection
img is used for image preloading
The most critical function is the init initialization function
var init = (function(){
ctx.fillStyle = "#ccc";
ctx.fillRect(0, 0, w, h);
randomImg();
img.addEventListener('load', function(){
canvas.style.backgroundImage = 'url(' + img.src +')';
ctx.globalCompositeOperation = 'destination-out';
bindEvent();
});
})();The process is as follows:
Cover the entire canvas with the gray layer
Random pictures
Picture preloading
After loading, set the picture as canvas background
Before scratching the card, set
ctx .globalCompositeOperation = 'destination-out';#Bind listening events for canvas, Tuca
This globalCompositeOperation is the scratch-off The key
For the usage of this attribute, you can click here
var randomImg = function(){
var random = Math.random(); if(random < 0.4){
img.src = './1.png';
}else if(random > 0.6){
img.src = './2.png';
}else{
img.src = './award.jpg';
}
};The function of the randomImg function is to randomize pictures
For random pictures, you need to use Math.random() random numbers
canvas we need to bind two functions
touchmove and touchend
var moveFunc = function(e){
var touch = e.touches[0],
posX = touch.clientX - l,
posY = touch.clientY - t;
ctx.beginPath();
ctx.arc(posX, posY, 15, 0, Math.PI * 2, 0);
ctx.fill();};To draw a circle when sliding the screen
Because destination-out is set, a scratch effect occurs
Also note that every time it is triggered All mustctx.beginPath();
Otherwisectx.fill();will connect the previously drawn circles and scrape over a large area
var endFunc = function(e){
var data = ctx.getImageData(0, 0, w, h).data,
scrapeNum = 0;
for(var i = 3, len = data.length; i < len; i += 4){
if(data[i] === 0){
scrapeNum++;
}
} if(scrapeNum > area * 0.7){
ctx.clearRect(0, 0, w, h);
canvas.removeEventListener('touchmove', moveFunc, false);
canvas.removeEventListener('touchend', endFunc, false);
}
} When lifted, touchend will be triggered
In this function, we used ctx.getImageData() to obtain the pixel information of the canvas
For the usage of this function, you can click here
When the gray layer is scratched, the background of the canvas is behind it
So we can judge whether the layer has been scratched by judging whether A in the pixel information RGBA is 0
scrapeNum represents the scraped pixel Click
So by scrapeNum > area * 0.7
When the scraped range is greater than 70% of the total range
Clear the entire gray layer
and above It is the content of mobile touch event and H5-Canvas pixel detection to realize scratch-off. For more related content, please pay attention to the PHP Chinese website (www.php.cn)!
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