egedraw image paste

function rgbatohex(r,g,b,a=255) {
  if (isNaN(r) || isNaN(g) || isNaN(b) || isNaN(a)) return;
  r = parseInt(r);
  g = parseInt(g);
  b = parseInt(b);
  a = parseInt(a);
  r = ("00000" + r.toString(16)).slice(-2);
  g = ("00000" + g.toString(16)).slice(-2);
  b = ("00000" + b.toString(16)).slice(-2);
  a = ("00000" + a.toString(16)).slice(-2);
  if (a == 'ff') return "#"+r+g+b; else return "#"+r+g+b+a;
};

var upload = document.createElement("input");
var mousedown = canvas.onmousedown;
var mousemove = canvas.onmousemove;
var paste = document.createElement("button");
paste.innerHTML = "paste";
paste.onclick = function () {
  rct = canvas.getBoundingClientRect();
  if (imagea == null) return;
  this.innerHTML = "now click where you want to paste it!";
  canvas.onmousedown = function (e) {
    createClientMessage("Pasting..");
    paste.innerHTML = "paste";
    this.onmousedown = mousedown;
    this.onmousemove = mousemove;
    const img = new Image();
    img.crossOrigin = "anonymous";
    img.src = imagea;
    img.onload = function () {
      const c = new OffscreenCanvas(0,0);
      const ctx = c.getContext('2d');
      c.width = img.width;
      c.height = img.height;
      ctx.drawImage(img,0,0);
      var imgdata = ctx.getImageData(0,0,c.width,c.height).data;
      stopdraw();
      for (let i = 0; i < c.height; i++) {
        for (let j = 0; j < c.width; j++) {
          var k = (i * c.width + j) * 4;
          var pixeldata = [imgdata[k], imgdata[k + 1], imgdata[k + 2], imgdata[k + 3]];
          if (!pixeldata[3]) continue;
          drawLine(e.clientX-rct.left+j+0.5,e.clientY-rct.top+i+0.5,e.clientX-rct.left+j+0.5,e.clientY-rct.top+i+0.5,rgbatohex(...pixeldata), 1);
        };
      };
    };
  };
};
document.body.appendChild(paste);
let imagea = null;
upload.setAttribute("id", "FileUpload");
upload.setAttribute("type", "file");
upload.setAttribute("accept", "image/png, image/jpeg");
document.body.appendChild(upload);
document.getElementById("FileUpload").onchange = function (e) {
  var tgt = e.target,
    files = tgt.files;
  if (FileReader && files && files.length) {
    var fr = new FileReader();
    fr.onload = function () {
      imagea = fr.result;
    };
    fr.readAsDataURL(files[0]);
  };
};