diff --git a/index.js b/index.js
index 7ff512d..d919dc8 100644
--- a/index.js
+++ b/index.js
@@ -339,10 +339,13 @@ function renderWallsToImageData(imageData, player, scene) {
             else {
                 u = t.x;
             }
-            for (let dy = 0; dy < Math.ceil(stripHeight); ++dy) {
+            const y1 = Math.floor((SCREEN_HEIGHT - stripHeight) * 0.5);
+            const y2 = Math.floor(y1 + stripHeight);
+            const by1 = Math.max(0, y1);
+            const by2 = Math.min(SCREEN_HEIGHT - 1, y2);
+            for (let y = by1; y <= by2; ++y) {
                 const tx = Math.floor(u * cell.width);
-                const ty = Math.floor(dy / Math.ceil(stripHeight) * cell.height);
-                const y = Math.floor((SCREEN_HEIGHT - stripHeight) * 0.5) + dy;
+                const ty = Math.floor((y - y1) / Math.ceil(stripHeight) * cell.height);
                 imageData.data[(y * SCREEN_WIDTH + x) * 4 + 0] = cell.data[(ty * cell.width + tx) * 4 + 0] / v.dot(d);
                 imageData.data[(y * SCREEN_WIDTH + x) * 4 + 1] = cell.data[(ty * cell.width + tx) * 4 + 1] / v.dot(d);
                 imageData.data[(y * SCREEN_WIDTH + x) * 4 + 2] = cell.data[(ty * cell.width + tx) * 4 + 2] / v.dot(d);
diff --git a/index.ts b/index.ts
index 9cd4bcf..a22d88b 100644
--- a/index.ts
+++ b/index.ts
@@ -365,11 +365,13 @@ function renderWallsToImageData(imageData: ImageData, player: Player, scene: Sce
                 u = t.x;
             }
 
-            for (let dy = 0; dy < Math.ceil(stripHeight); ++dy) {
+            const y1 = Math.floor((SCREEN_HEIGHT - stripHeight)*0.5);
+            const y2 = Math.floor(y1 + stripHeight);
+            const by1 = Math.max(0, y1);
+            const by2 = Math.min(SCREEN_HEIGHT-1, y2);
+            for (let y = by1; y <= by2; ++y) {
                 const tx = Math.floor(u*cell.width);
-                const ty = Math.floor(dy/Math.ceil(stripHeight)*cell.height);
-
-                const y = Math.floor((SCREEN_HEIGHT - stripHeight)*0.5) + dy;
+                const ty = Math.floor((y - y1)/Math.ceil(stripHeight)*cell.height);
                 imageData.data[(y*SCREEN_WIDTH + x)*4 + 0] = cell.data[(ty*cell.width + tx)*4 + 0]/v.dot(d);
                 imageData.data[(y*SCREEN_WIDTH + x)*4 + 1] = cell.data[(ty*cell.width + tx)*4 + 1]/v.dot(d);
                 imageData.data[(y*SCREEN_WIDTH + x)*4 + 2] = cell.data[(ty*cell.width + tx)*4 + 2]/v.dot(d);