diff --git a/index.php b/index.php
index b7dd9de..efdae73 100644
--- a/index.php
+++ b/index.php
@@ -101,8 +101,8 @@
                 const ctx = canvas.getContext("2d");
 
                 const copy = ctx.createImageData(128, 64);
-                for (let i = 0; i < pen.data.length; i++) {
-                    copy.data[(i*4)+3] = 255;
+                for (let i = 0; i < pen.data.length; i += 4) {
+                    copy.data[i+3] = 255;
                 }
 
                 ctx.putImageData(copy, 0, 0);