Seabon Seabon - 20 days ago 8
HTML Question

Bad canvas context

I am currently working on a canvas animation, it works pretty well.

You can check out my code below:



var canvas = document.getElementById('bubble'),
ctx = canvas.getContext('2d');
ctx.save();

var wW = canvas.width = window.innerWidth,
wH = canvas.height = window.innerHeight;

var particles = [];
var particleIndex = 0;
var dx = dy = distance = 0;

function Particle(){
this.x = Math.random() * wW;
this.y = Math.random() * wH;
this.rad = 20;
this.color = "blue";
this.vX = Math.random() * (1.01 - (-1)) + (-1);
this.vY = Math.random() * (1.01 - (-1)) + (-1);
particles[particleIndex] = this;
particleIndex++;
}

Particle.prototype.draw = function(){
this.x += this.vX;
this.y += this.vY;

this.collision();

//outer
ctx.beginPath();
ctx.arc(this.x, this.y, this.rad, 0, Math.PI * 2);
ctx.stroke();

//center
ctx.beginPath();
ctx.arc(this.x, this.y, this.rad/10, 0, Math.PI * 2);
ctx.fillStyle="red";
ctx.fill();
}

Particle.prototype.collision = function(){
if(this.x + this.rad >= wW || this.x - this.rad <= 0){
this.vX *= -1;
}

if(this.y + this.rad >= wH || this.y - this.rad <= 0){
this.vY *= -1;
}

}

function line(){
for(var i=0; i < particles.length; i++){
for(var j=0; j < particles.length; j++){
dx = particles[i].x - particles[j].x;
dy = particles[i].y - particles[j].y;
distance = Math.sqrt(dx * dx + dy * dy);

if(distance <= 60){
ctx.beginPath();
ctx.moveTo(particles[i].x, particles[i].y);
ctx.lineTo(particles[j].x, particles[j].y);
ctx.strokeStyle="rgba(0,0,0,"+ 6/distance +")";
ctx.stroke();
}
}
}
}

for(var i=0; i<20; i++){
new Particle();
}

function anim(){
requestAnimationFrame(anim);
ctx.clearRect(0,0,wW,wH);

line();

for(var i=0; i < particles.length; i++){
particles[i].draw();
}
}

anim();

html, body{
padding:0;
margin:0;
}

body{
height: 100%;
width: 100%;
overflow:hidden;
}

<canvas id="bubble"></canvas>





I have a function line() which calculates the space between each circle and draws line between them.
In this function, i put this :

ctx.strokeStyle="rgba(0,0,0,"+ 6/distance +")";


I want to change the line opacity as a function of distance. However, it changes the circle opacity and not the line. I don't understand why, I tried to restore the context but it did not work..

Thank you !

Answer

You can store it to a variable before starting the line drawing and revert it afterwards

function line() {
  var originalStrokeStyle = ctx.strokeStyle;
  for (var i = 0; i < particles.length; i++) {
    for (var j = 0; j < particles.length; j++) {
      dx = particles[i].x - particles[j].x;
      dy = particles[i].y - particles[j].y;
      distance = Math.sqrt(dx * dx + dy * dy);

        if (distance <= 60) {
          ctx.beginPath();
          ctx.moveTo(particles[i].x, particles[i].y);
          ctx.lineTo(particles[j].x, particles[j].y);
          ctx.strokeStyle = "rgba(0,0,0," + 6 / distance + ")";
          ctx.stroke();
        }
    }
  }
  ctx.strokeStyle = originalStrokeStyle;
}

Updated demo at http://codepen.io/gpetrioli/pen/aBZpXJ

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