feat: culling

engine/render/Sky.ts

@@ -58,8 +58,13 @@ export namespace Sky {
    * Per pixel it reconstructs the view ray from the camera basis, shades a
    * horizon->zenith gradient by the ray's elevation, brightens toward `sun` near
    * `sunDir`, then lays crisp-edged cumulus over the top.
+   *
+   * `step` (>= 1) renders the sky at 1/step resolution: the expensive shading
+   * (the per-pixel cloud fbm dominates the frame) runs once per step x step
+   * block and is copied across it. The sky is low-frequency, so 2 is nearly free
+   * visually and quarters the cloud cost; 1 is full resolution.
    */
-  export function render(fb: Framebuffer, camera: Camera, sky: SkyConfig, time: number): void {
+  export function render(fb: Framebuffer, camera: Camera, sky: SkyConfig, time: number, step = 1): void {
     const { width, height, color, depth } = fb
     const forward = Camera.forward(camera)
     const right = Vec3.normalize(Vec3.cross(forward, UP))
@@ -70,10 +75,15 @@ export namespace Sky {
     const cosSun = Math.cos(sky.sunSize)
     const clouds = sky.clouds
     const cloud: CloudSample = { cover: 0, shade: 1 }
-    for (let y = 0; y < height; y++) {
-      const ndcY = 1 - ((y + 0.5) / height) * 2
-      for (let x = 0; x < width; x++) {
-        const ndcX = ((x + 0.5) / width) * 2 - 1
+    const s = Math.max(1, step | 0)
+    for (let by = 0; by < height; by += s) {
+      // Shade at the block center, then flood the whole block with that color.
+      const sampleY = Math.min(height - 1, by + (s >> 1))
+      const ndcY = 1 - ((sampleY + 0.5) / height) * 2
+      const yEnd = Math.min(height, by + s)
+      for (let bx = 0; bx < width; bx += s) {
+        const sampleX = Math.min(width - 1, bx + (s >> 1))
+        const ndcX = ((sampleX + 0.5) / width) * 2 - 1
         // View ray = forward + right*ndcX*tanX + up*ndcY*tanY, then normalized.
         let dx = forward.x + right.x * ndcX * tanX + up.x * ndcY * tanY
         let dy = forward.y + right.y * ndcX * tanX + up.y * ndcY * tanY
@@ -100,9 +110,14 @@ export namespace Sky {
             c = Color.lerp(c, Color.scale(clouds.color, cloud.shade), cloud.cover)
           }
         }
-        const i = y * width + x
-        color[i] = c
-        depth[i] = 0
+        const xEnd = Math.min(width, bx + s)
+        for (let y = by; y < yEnd; y++) {
+          const o = y * width
+          for (let x = bx; x < xEnd; x++) {
+            color[o + x] = c
+            depth[o + x] = 0
+          }
+        }
       }
     }
   }