diff --git a/index.html b/index.html
index f4d29e248f25ca11a1a1ab00a699992001f7c70a..78692eeacdc1ca2dc57abedee03d8789d3d926d1 100644
--- a/index.html
+++ b/index.html
@@ -46,15 +46,15 @@
             vec2 currentVelocity = texture2D(u_velocity, fragCoord/u_textureSize).xy;
 
             //implicitly solve advection
-            
+
             if (length(currentVelocity) == 0.0) {//no velocity
                 gl_FragColor = vec4(texture2D(u_material, fragCoord/u_textureSize).x, 0, 0, 0);
                 return;
             }
 
             vec2 pxCenter = vec2(0.5, 0.5);
-            vec2 pos = fragCoord - pxCenter - u_dt*currentVelocity;
-            if (pos.x < 0.0 || pos.x >= u_textureSize.x || pos.y < 0.0 || pos.y >= u_textureSize.y){
+            vec2 pos = fragCoord - pxCenter + u_dt*currentVelocity;
+            if (pos.x < 0.0 || pos.x >= u_textureSize.x-1.0 || pos.y < 0.0 || pos.y >= u_textureSize.y-1.0){
                 //boundary
                 gl_FragColor = vec4(0);
                 return;