diff --git a/in_one_weekend/src/color.zig b/in_one_weekend/src/color.zig
index bd04896..d15dadd 100644
--- a/in_one_weekend/src/color.zig
+++ b/in_one_weekend/src/color.zig
@@ -4,10 +4,10 @@ const Vec3 = @import("vec3.zig").Vec3;
 
 pub const Color = Vec3;
 
-pub fn averageColor(pixelColor: Color, samplesPerPixel: i32) Color {
+pub fn averageColor(pixelColor: *Color, samplesPerPixel: i32) Color {
     // Divide the color by the number of samples.
     // const scale = 1.0 / @intToFloat(f32, samplesPerPixel);
-    var scaledColor = pixelColor.div(@intToFloat(f32, samplesPerPixel));
+    var scaledColor = pixelColor.*.div(@intToFloat(f32, samplesPerPixel));
 
     return Color{
         .x = math.clamp(scaledColor.x, 0, 0.999),
diff --git a/in_one_weekend/src/main.zig b/in_one_weekend/src/main.zig
index e7c252b..abec5c8 100644
--- a/in_one_weekend/src/main.zig
+++ b/in_one_weekend/src/main.zig
@@ -85,35 +85,43 @@ pub fn main() anyerror!void {
     };
 
     var prng = std.rand.DefaultPrng.init(42);
+    var pixelAccu: [imageWidth * imageHeight]Color = undefined;
+    for (pixelAccu) |*pixel| {
+        pixel.* = Color{};
+    }
 
     {
         _ = sdl.c.SDL_LockSurface(surface);
 
-        var j: usize = 0;
-        while (j < imageHeight) {
-            var i: usize = 0;
-            while (i < imageWidth) {
-                var pixelColor = Color{};
-                var k: usize = 0;
-                while (k < samplesPerPixel) {
+        var k: usize = 0;
+        while (k < samplesPerPixel) {
+            _ = sdl.c.SDL_LockSurface(surface);
+
+            var j: usize = 0;
+            while (j < imageHeight) {
+                var i: usize = 0;
+                while (i < imageWidth) {
+                    var imageIndex = i + j * imageWidth;
+                    var pixelColor = &pixelAccu[imageIndex];
+
                     const u = (@intToFloat(f32, i) + prng.random.float(f32)) / @intToFloat(f32, (imageWidth - 1));
                     const v = (@intToFloat(f32, j) + prng.random.float(f32)) / @intToFloat(f32, (imageHeight - 1));
                     const r = camera.getRay(u, v);
                     const sample = rayColor(r, world);
-                    pixelColor = pixelColor.add(sample);
+                    pixelColor.* = pixelColor.*.add(sample);
 
-                    k += 1;
+                    const averageColor = color.averageColor(pixelColor, @intCast(i32, k));
+                    // SDL coordinate system is flipped compared to the raytracer
+                    sdl.setSurfacePixel(surface, i, imageHeight - 1 - j, averageColor);
+
+                    i += 1;
                 }
-
-                const averageColor = color.averageColor(pixelColor, samplesPerPixel);
-                // SDL coordinate system is flipped compared to the raytracer
-                sdl.setSurfacePixel(surface, i, imageHeight - 1 - j, averageColor);
-
-                i += 1;
+                j += 1;
             }
-            j += 1;
+            sdl.c.SDL_UnlockSurface(surface);
+            _ = sdl.c.SDL_UpdateWindowSurface(window);
+            k += 1;
         }
-        defer sdl.c.SDL_UnlockSurface(surface);
     }
 
     if (sdl.c.SDL_UpdateWindowSurface(window) != 0) {