client.js

var xhr = new XMLHttpRequest();
xhr.open('GET', 'http://localhost:3333/www.fnordware.com/superpng/pnggrad8rgb.jpg', true);
xhr.responseType = 'blob';

xhr.onload = function(e) {
  var reader = new FileReader();

  reader.onloadend = function() {
      var img = document.createElement('img');

      img.onload = function () {
          var canvas = document.createElement("canvas");
          canvas.width = img.width;
          canvas.height = img.height;
          document.body.appendChild(canvas);
 
          var ctx=canvas.getContext("2d");
          ctx.drawImage(img,0,0);
          
          var imageData = ctx.getImageData(0,0,img.width,img.height);
          
          canvas = document.createElement("canvas");
          canvas.width = img.width;
          canvas.height = img.height;
          document.body.appendChild(canvas);
          
          ctx=canvas.getContext("2d");
          
          for (var i=0;i<imageData.data.length;i+=4)
          {
              imageData.data[i]   = 255-imageData.data[i]; // RED
              imageData.data[i+1] = 255-imageData.data[i+1]; // GREEN
              imageData.data[i+2] = 255-imageData.data[i+2]; // BLUE
              imageData.data[i+3] = 255; // ALPHA
          }
          
          ctx.putImageData(imageData,0,0);
          
          console.log(imageData);
      }
      
      img.src = this.result;
  }

  reader.readAsDataURL(this.response);
};

xhr.send();

nginx.conf

daemon off;
master_process off;
error_log /dev/stdout notice;

events { }

http {
  access_log  off;

  server {
    listen 3333;
    server_name _;
    resolver 8.8.8.8;

    location ~ "^/(.*)/(.*)" {
      proxy_pass http://$1/$2;
      proxy_buffering        off;
      proxy_redirect         off;
      proxy_ignore_headers   Set-Cookie;
      proxy_set_header       Authorization '';
      proxy_set_header       Origin '';
      proxy_set_header       X-Forwarded-For $proxy_add_x_forwarded_for;
      add_header             Access-Control-Allow-Origin "*";
      add_header             Access-Control-Allow-Methods "GET";
      add_header             Access-Control-Max-Age "1728000";
    }
  }
}

start with nginx -c /absolute/path/to/nginx.conf