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javascript - Canvas:了解 globalCompositeOperation 以删除部分图像覆盖

转载 作者:行者123 更新时间:2023-11-30 15:45:51 30 4
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我正在努力思考 globalCompositeOperation通过尝试组合这两个示例来获得属性:JSFiddleCodepen .

前者使用destination-out,后者使用source-over。是否可以在 Codepen 中使用火热光标,但也可以像在 Fiddle 中一样,让它删除用户点击的覆盖填充部分?

如有任何帮助,我们将不胜感激。如有必要,我可以结合 Codepen 上的演示使用相同的方法。

相关Fiddle代码:

function drawDot(mouseX,mouseY){
bridgeCanvas.beginPath();
bridgeCanvas.arc(mouseX, mouseY, brushRadius, 0, 2*Math.PI, true);
bridgeCanvas.fillStyle = '#000';
bridgeCanvas.globalCompositeOperation = "destination-out";
bridgeCanvas.fill();
}

相关Codepen代码:

Fire.prototype.clearCanvas = function(){
this.ctx.globalCompositeOperation = "source-over";
this.ctx.fillStyle = "rgba( 15, 5, 2, 1 )";
this.ctx.fillRect( 0, 0, window.innerWidth, window.innerHeight );

this.ctx.globalCompositeOperation = "lighter";
this.ctx.rect(0, 0, this.canvas.width, this.canvas.height);
this.ctx.fillStyle = this.pattern;
this.ctx.fill();/**/
}

最佳答案

正如我在评论中所说,您必须至少将代码分为两部分。
cropper 函数使用 "destination-out" 合成操作来移除 Canvas 上应该绘制新像素的已经绘制的像素。在您的版本中,它使用背景图像,一旦删除了前景像素,您就可以看到这个背景,因为在 Canvas 的现在透明区域中。

另一方面,火焰使用'"lighter"'、"color-dodge""soft-light" 混合操作。这将添加现有像素和新绘制像素的颜色。
至少第一个,如果用于透明区域,将与默认的 "source-over" 复合操作相同。因此,您需要将背景图像绘制到 Canvas 上才能在混合中使用它。

为此,您必须使用第二个屏幕外 Canvas ,您将只应用橡皮擦 “destination-out” 操作。然后,在可见的 Canvas 上,在每个新的橡皮擦帧上,您必须在可见的 Canvas 上绘制背景图像,然后是带有孔的橡皮擦,最后是混合,它会将所有的东西混合在一起。

这是一个快速代码转储,我在其中重写了一点橡皮擦,并修改了 Fire 一个,以使我们的主要功能处理事件和动画循环。

function MainDrawing(){
this.canvas = document.getElementById('main');
this.ctx = this.canvas.getContext('2d');
this.background = new Image();
this.background.src = "https://s3-us-west-2.amazonaws.com/s.cdpn.io/4273/calgary-bridge-1943.jpg"
this.eraser = new Eraser(this.canvas);
this.fire = new Fire(this.canvas);
this.attachEvents();
}
MainDrawing.prototype = {
anim: function(){
if(this.stopped)
return;
this.ctx.globalCompositeOperation = 'source-over';
this.ctx.drawImage(this.background, 0,0);
this.ctx.drawImage(this.eraser.canvas, 0,0);
this.fire.run();
requestAnimationFrame(this.anim.bind(this));
},
stop: function(){
this.stopped = true;
},
attachEvents: function(){
var mouseDown = false;
this.canvas.onmousedown = function(){
mouseDown = true;
};
this.canvas.onmouseup = function(){
mouseDown = false;
};
this.canvas.onmousemove = function(e){
if(mouseDown){
this.eraser.handleClick(e);
}
this.fire.updateMouse(e);
}.bind(this);
}
};

function Eraser(canvas){
this.main = canvas;
this.canvas = canvas.cloneNode();
var ctx = this.ctx = this.canvas.getContext('2d');
this.img = new Image();
this.img.onload = function(){
ctx.drawImage(this, 0, 0);
ctx.globalCompositeOperation = 'destination-out';
};
this.img.src = "https://s3-us-west-2.amazonaws.com/s.cdpn.io/4273/calgary-bridge-2013.jpg";
this.getRect();
}
Eraser.prototype = {
getRect: function(){
this.rect = this.main.getBoundingClientRect();
},
handleClick: function(evt){
var x = evt.clientX - this.rect.left;
var y = evt.clientY - this.rect.top;
this.draw(x,y);
},
draw: function(x, y){
this.ctx.beginPath();
this.ctx.arc(x, y, 30, 0, Math.PI*2);
this.ctx.fill();
}
};


var Fire = function(canvas){

this.canvas = canvas;
this.ctx = this.canvas.getContext('2d');

this.aFires = [];
this.aSpark = [];
this.aSpark2 = [];



this.mouse = {
x : this.canvas.width * .5,
y : this.canvas.height * .75,
}

}

Fire.prototype.run = function(){

this.update();
this.draw();

}
Fire.prototype.start = function(){

this.bRuning = true;
this.run();

}
Fire.prototype.stop = function(){

this.bRuning = false;

}
Fire.prototype.update = function(){

this.aFires.push( new Flame( this.mouse ) );
this.aSpark.push( new Spark( this.mouse ) );
this.aSpark2.push( new Spark( this.mouse ) );



for (var i = this.aFires.length - 1; i >= 0; i--) {

if( this.aFires[i].alive )
this.aFires[i].update();
else
this.aFires.splice( i, 1 );

}

for (var i = this.aSpark.length - 1; i >= 0; i--) {

if( this.aSpark[i].alive )
this.aSpark[i].update();
else
this.aSpark.splice( i, 1 );

}


for (var i = this.aSpark2.length - 1; i >= 0; i--) {

if( this.aSpark2[i].alive )
this.aSpark2[i].update();
else
this.aSpark2.splice( i, 1 );

}

}

Fire.prototype.draw = function(){

this.drawHalo();

this.ctx.globalCompositeOperation = "overlay";//or lighter or soft-light

for (var i = this.aFires.length - 1; i >= 0; i--) {

this.aFires[i].draw( this.ctx );

}

this.ctx.globalCompositeOperation = "soft-light";//"soft-light";//"color-dodge";

for (var i = this.aSpark.length - 1; i >= 0; i--) {

if( ( i % 2 ) === 0 )
this.aSpark[i].draw( this.ctx );

}

this.ctx.globalCompositeOperation = "color-dodge";//"soft-light";//"color-dodge";

for (var i = this.aSpark2.length - 1; i >= 0; i--) {

this.aSpark2[i].draw( this.ctx );

}


}

Fire.prototype.updateMouse = function( e ){

this.mouse.x = e.clientX;
this.mouse.y = e.clientY;

}


Fire.prototype.drawHalo = function(){

var r = rand( 300, 350 );
this.ctx.globalCompositeOperation = "lighter";
this.grd = this.ctx.createRadialGradient( this.mouse.x, this.mouse.y,r,this.mouse.x, this.mouse.y, 0 );
this.grd.addColorStop(0,"transparent");
this.grd.addColorStop(1,"rgb( 50, 2, 0 )");
this.ctx.beginPath();
this.ctx.arc( this.mouse.x, this.mouse.y - 100, r, 0, 2*Math.PI );
this.ctx.fillStyle= this.grd;
this.ctx.fill();

}


var Flame = function( mouse ){

this.cx = mouse.x;
this.cy = mouse.y;
this.x = rand( this.cx - 25, this.cx + 25);
this.y = rand( this.cy - 5, this.cy + 5);
this.lx = this.x;
this.ly = this.y;
this.vy = rand( 1, 3 );
this.vx = rand( -1, 1 );
this.r = rand( 30, 40 );
this.life = rand( 2, 7 );
this.alive = true;
this.c = {

h : Math.floor( rand( 2, 40) ),
s : 100,
l : rand( 80, 100 ),
a : 0,
ta : rand( 0.8, 0.9 )

}




}
Flame.prototype.update = function()
{

this.lx = this.x;
this.ly = this.y;

this.y -= this.vy;
this.vy += 0.08;


this.x += this.vx;

if( this.x < this.cx )
this.vx += 0.2;
else
this.vx -= 0.2;




if( this.r > 0 )
this.r -= 0.3;

if( this.r <= 0 )
this.r = 0;



this.life -= 0.12;

if( this.life <= 0 ){

this.c.a -= 0.05;

if( this.c.a <= 0 )
this.alive = false;

}else if( this.life > 0 && this.c.a < this.c.ta ){

this.c.a += .08;

}

}
Flame.prototype.draw = function( ctx ){

this.grd1 = ctx.createRadialGradient( this.x, this.y, this.r*3, this.x, this.y, 0 );
this.grd1.addColorStop( 0.5, "hsla( " + this.c.h + ", " + this.c.s + "%, " + this.c.l + "%, " + (this.c.a/20) + ")" );
this.grd1.addColorStop( 0, "transparent" );

this.grd2 = ctx.createRadialGradient( this.x, this.y, this.r, this.x, this.y, 0 );
this.grd2.addColorStop( 0.5, "hsla( " + this.c.h + ", " + this.c.s + "%, " + this.c.l + "%, " + this.c.a + ")" );
this.grd2.addColorStop( 0, "transparent" );


ctx.beginPath();
ctx.arc( this.x, this.y, this.r * 3, 0, 2*Math.PI );
ctx.fillStyle = this.grd1;
//ctx.fillStyle = "hsla( " + this.c.h + ", " + this.c.s + "%, " + this.c.l + "%, " + (this.c.a/20) + ")";
ctx.fill();


ctx.globalCompositeOperation = "overlay";
ctx.beginPath();
ctx.arc( this.x, this.y, this.r, 0, 2*Math.PI );
ctx.fillStyle = this.grd2;
ctx.fill();



ctx.beginPath();
ctx.moveTo( this.lx , this.ly);
ctx.lineTo( this.x, this.y);
ctx.strokeStyle = "hsla( " + this.c.h + ", " + this.c.s + "%, " + this.c.l + "%, 1)";
ctx.lineWidth = rand( 1, 2 );
ctx.stroke();
ctx.closePath();

}


var Spark = function( mouse ){

this.cx = mouse.x;
this.cy = mouse.y;
this.x = rand( this.cx -40, this.cx + 40);
this.y = rand( this.cy, this.cy + 5);
this.lx = this.x;
this.ly = this.y;
this.vy = rand( 1, 3 );
this.vx = rand( -4, 4 );
this.r = rand( 0, 1 );
this.life = rand( 4, 8 );
this.alive = true;
this.c = {

h : Math.floor( rand( 2, 40) ),
s : 100,
l : rand( 40, 100 ),
a : rand( 0.8, 0.9 )

}

}
Spark.prototype.update = function()
{

this.lx = this.x;
this.ly = this.y;

this.y -= this.vy;
this.x += this.vx;

if( this.x < this.cx )
this.vx += 0.2;
else
this.vx -= 0.2;

this.vy += 0.08;
this.life -= 0.1;

if( this.life <= 0 ){

this.c.a -= 0.05;

if( this.c.a <= 0 )
this.alive = false;

}

}
Spark.prototype.draw = function( ctx ){

ctx.beginPath();
ctx.moveTo( this.lx , this.ly);
ctx.lineTo( this.x, this.y);
ctx.strokeStyle = "hsla( " + this.c.h + ", " + this.c.s + "%, " + this.c.l + "%, " + (this.c.a / 2) + ")";
ctx.lineWidth = this.r * 2;
ctx.lineCap = 'round';
ctx.stroke();
ctx.closePath();

ctx.beginPath();
ctx.moveTo( this.lx , this.ly);
ctx.lineTo( this.x, this.y);
ctx.strokeStyle = "hsla( " + this.c.h + ", " + this.c.s + "%, " + this.c.l + "%, " + this.c.a + ")";
ctx.lineWidth = this.r;
ctx.stroke();
ctx.closePath();

}

rand = function( min, max ){ return Math.random() * ( max - min) + min; };

var app = new MainDrawing();
app.anim();
<canvas id="main" width="750" height="465"></canvas>

关于javascript - Canvas:了解 globalCompositeOperation 以删除部分图像覆盖,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/40078206/

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