commit 181de1baaf9eb42471fc6a405c56cc7c70d3f89a
Author: Yannik Schmidt <yannik.schmidt@thermoscan.de>
Date:   Thu Dec 13 18:26:53 2018 +0100

    Initial

diff --git a/lazyload.js b/lazyload.js
new file mode 100755
index 0000000..4e151f7
--- /dev/null
+++ b/lazyload.js
@@ -0,0 +1,87 @@
+/* determine which size of image to load */
+function getSize(){
+    trueRes = screen.width/window.devicePixelRatio
+	if(trueRes > 1920)
+        return ''
+    else if(trueRes <= 1920 && trueRes >= 1200)
+        return '-640x480'
+    else
+        return '-320x240'
+}
+
+
+/* check if browser is capable of webp */
+function supportsWebp() {
+  return /Chrome/.test(navigator.userAgent) && /Google Inc/.test(navigator.vendor);
+  //if (!self.createImageBitmap) return false;
+  //const webpData = 'data:image/webp;base64,UklGRh4AAABXRUJQVlA4TBEAAAAvAAAAAAfQ//73v/+BiOh/AAA=';
+  //return createImageBitmap(webpData).then(() => true, () => false);
+}
+
+/* cache */
+var webP = supportsWebp()
+var elements = null
+var counter = 0
+
+/* garantuee initall call evaluates to true */
+var viewbox_y = -Infinity
+
+/* function to load images */
+function changeSrc(offset){
+    /* check if there was a relevant change */
+    var cur_viewbox = -document.getElementById("navbar").getBoundingClientRect()
+    if(cur_viewbox - viewbox_y < 100){
+        return;
+    }
+    
+    /* cache viewbox */
+    viewbox_y = cur_viewbox
+    
+    /* cache */
+    if(elements == null){
+        elements = document.querySelectorAll("*[rrealsrc]");
+    }
+
+    for (var i = counter; i < elements.length; i++) {
+            var boundingClientRect = elements[i].getBoundingClientRect();
+            if (elements[i].hasAttribute("rrealsrc") && boundingClientRect.top < window.innerHeight + offset) {
+				        var newSrc = elements[i].getAttribute("rrealsrc")
+                /* remove url( ... ) */
+				        newSrc = newSrc.substring(4,newSrc.length-1)
+                
+                if(newSrc.indexOf(".jpg") > -1){
+                    /* get correct size */
+                    newSrc += getSize()
+
+				    /* load webP if supported */
+				    if(webP){
+				    	newSrc = newSrc  + '.webp'
+				    }
+                }
+                elements[i].setAttribute("src", newSrc);
+                elements[i].style.backgroundImage = 'url(' + newSrc +')';
+                elements[i].removeAttribute("rrealsrc");
+            }else{
+                /* DOM is parsed top down and images are inserted in that order too */
+                /* meaing that once we reach pic that insnt in viewbox none following will be*/
+                counter = i; 
+                return;
+            }
+        }
+    
+    /* if we got here we are done */
+    counter = elements.length
+
+}
+refresh_handler = function(e) {
+    /* images directly in view first (offset 0)*/
+	changeSrc(0)
+	/* then load images almost in view */
+	changeSrc(500)
+};
+
+/* add listeners */
+document.getElementById("main_scrollable").addEventListener('scroll', refresh_handler);
+window.addEventListener('resize', refresh_handler);
+window.addEventListener('load', refresh_handler);
+getSize()