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              <style type="text/css">
html,body { margin: 0; padding: 0; width: 100%; height: 100%; overflow: hidden; }
</style>
<canvas id="canvas"></canvas>
            
          
!
            
              /*****************************************

This is code setup is just to demo a
camera following a player and screen shake

Take this code at your own risk!

note: when switching to the results tab, 
hit rerun to reload the game, and click
anywhere on the area to give focus for
inputs.

*****************************************/

// camera object
var camera = {
  cpos : { x : 0, y : 0 },
  ppos : { x : 0, y : 0 },
  acel : { x : 0, y : 0 },
  height : document.body.clientHeight,
  width  : document.body.clientWidth,
  maxHeight : 6 * document.body.clientHeight,
  maxWidth  : 6 * document.body.clientWidth,
  cullDist : 300,
  bound : true,
  target : { x : 0, y : 0 },
  strength : 0,
  damper : 0
}

// player object
var player = {
  cpos : { x : 0, y : 0 },
  ppos : { x : 0, y : 0 },
  acel : { x : 0, y : 0 }
}

// handle input
var inputs = {};
document.addEventListener("keydown", function(ev) {
  inputs[ev.keyCode] = true;
}, false);
document.addEventListener("keyup", function(ev) {
  inputs[ev.keyCode] = false;
}, false);

// init entities
var entities = [];
for (var i = 0; i < 200; i++) {
  var randx = Math.random() * 6 * document.body.clientWidth;
  var randy = Math.random() * 6 * document.body.clientHeight;
  entities.push({
    cpos : { x : randx, y : randy },
    ppos : { x : randx, y : randy },
    acel : { x: 0, y : 0 }
  });
}

// canvas stuff
var cvs = document.getElementById("canvas");
cvs.width = document.body.clientWidth;
cvs.height = document.body.clientHeight;
var ctx = cvs.getContext('2d');

function clearBlack() {
  ctx.beginPath();
  ctx.fillStyle = "black";
  ctx.fillRect(0, 0, cvs.width, cvs.height);
}
var anim;
function update() {
  anim = requestAnimationFrame(update);
  // clear screen
  clearBlack();
   
  // check inputs
  // up - w
  if (inputs[87]) {
    player.acel.y = -6;
  }

  // down - s
  else if (inputs[83]) {
    player.acel.y = 6;
  }

  // left - a
  if (inputs[65]) {
    player.acel.x = -6;
  }

  // right - d
  else if (inputs[68]) { 
    player.acel.x = 6;
  }
  
  // shift - screen shake
  if (inputs[16]) {
    camera.strength = 90;
    camera.damper = 5;
  }
  
  // get the target point
  camera.target.x = player.cpos.x - (camera.width / 2);
  camera.target.y = player.cpos.y - (camera.height / 2);
  
  // move camera towards target point
  camera.acel.x += (1 / 8) * (camera.target.x - camera.cpos.x);
  camera.acel.y += (1 / 8) * (camera.target.y - camera.cpos.y);
  
  // player drag
  player.ppos.x = player.cpos.x + (player.ppos.x - player.cpos.x) * 0.93;
  player.ppos.y = player.cpos.y + (player.ppos.y - player.cpos.y) * 0.93;
  
  // player verlet
  player.cpos.x += player.acel.x * 0.256;
  player.cpos.y += player.acel.y * 0.256;
  var px = 2 * player.cpos.x - player.ppos.x;
  var py = 2 * player.cpos.y - player.ppos.y;
  player.ppos.x = player.cpos.x;
  player.ppos.y = player.cpos.y;
  player.cpos.x = px;
  player.cpos.y = py;
  player.acel.x = 0;
  player.acel.y = 0;
  
  // camera shake update
  if (camera.strength > 0) {
    var randx = Math.random() * 2 * camera.strength - camera.strength;
    var randy = Math.random() * 2 * camera.strength - camera.strength;
    camera.acel.x += randx;
    camera.acel.y += randy;
    camera.strength -= camera.damper;
  }
  
  // camera drag
  camera.ppos.x = camera.cpos.x + (camera.ppos.x - camera.cpos.x) * 0.3;
  camera.ppos.y = camera.cpos.y + (camera.ppos.y - camera.cpos.y) * 0.3;
  
  // camera verlet
  camera.cpos.x += camera.acel.x * 0.256;
  camera.cpos.y += camera.acel.y * 0.256;
  var cx = 2 * camera.cpos.x - camera.ppos.x;
  var cy = 2 * camera.cpos.y - camera.ppos.y;
  camera.ppos.x = camera.cpos.x;
  camera.ppos.y = camera.cpos.y;
  camera.cpos.x = cx;
  camera.cpos.y = cy;
  camera.acel.x = 0;
  camera.acel.y = 0;

  // bind camera to world edges
  if (camera.bound) {

    // flush camera -- x bounds
    camera.cpos.x = Math.min(camera.cpos.x, camera.maxWidth - camera.width);
    camera.cpos.x = Math.max(camera.cpos.x, 0);

    // flush camera -- y bounds
    camera.cpos.y = Math.min(camera.cpos.y, camera.maxHeight - camera.height);
    camera.cpos.y = Math.max(camera.cpos.y, 0);
  }
  
  // draw player with corrected positions
  var playerx = player.cpos.x - camera.cpos.x;
  var playery = player.cpos.y - camera.cpos.y;
  ctx.beginPath();
  ctx.fillStyle = "white";
  ctx.fillRect(playerx, playery, 10, 10);
  
  // draw entities with corrected positions
  for (var i = 0; i < entities.length; i++) {
    var entity = entities[i];
    var ex = entity.cpos.x - camera.cpos.x;
    var ey = entity.cpos.y - camera.cpos.y;
    if ((ex >= -camera.cullDist && ex <= camera.width + camera.cullDist) &&
        (ey >= -camera.cullDist && ex <= camera.height + camera.cullDist) ) {
      ctx.beginPath();
      ctx.fillStyle="gold";
      ctx.arc(ex,ey, 10, 0, 2*Math.PI, false);
      ctx.fill();
    }
  }
}


// start everything and handle stopping
anim = requestAnimationFrame(update);
document.addEventListener("keydown", function(ev) {
  if (ev.keyCode === 27) {
    cancelAnimationFrame(anim);
  }
});
            
          
!
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