fix(lod): stabilize distant transition and chunk coverage

assets/shaders/vulkan/g_pass.frag

@@ -35,25 +35,22 @@ layout(set = 0, binding = 0) uniform GlobalUniforms {
     vec4 lpv_origin;
 } global;
 
-// 4x4 Bayer matrix for dithered LOD transitions
-float bayerDither4x4(vec2 position) {
-    const float bayerMatrix[16] = float[](
-        0.0/16.0,  8.0/16.0,  2.0/16.0, 10.0/16.0,
-        12.0/16.0, 4.0/16.0, 14.0/16.0,  6.0/16.0,
-        3.0/16.0, 11.0/16.0,  1.0/16.0,  9.0/16.0,
-        15.0/16.0, 7.0/16.0, 13.0/16.0,  5.0/16.0
-    );
-    int x = int(mod(position.x, 4.0));
-    int y = int(mod(position.y, 4.0));
-    return bayerMatrix[x + y * 4];
+// World-space hash for LOD transition masking.
+// Using world-space noise avoids a fixed screen-space dot pattern.
+float lodTransitionNoise(vec2 worldXZ) {
+    vec2 p = floor(worldXZ * 0.25);
+    p = fract(p * vec2(0.1031, 0.1030));
+    p += dot(p, p.yx + 33.33);
+    return fract((p.x + p.y) * p.x);
 }
 
 void main() {
-    const float LOD_TRANSITION_WIDTH = 24.0;
+    const float LOD_TRANSITION_WIDTH = 16.0;
     if (vTileID < 0 && vMaskRadius > 0.0) {
         float distFromMask = length(vFragPosWorld.xz) - vMaskRadius;
         float fade = clamp(distFromMask / LOD_TRANSITION_WIDTH, 0.0, 1.0);
-        float ditherThreshold = bayerDither4x4(gl_FragCoord.xy);
+        vec2 worldXZ = vFragPosWorld.xz + global.cam_pos.xz;
+        float ditherThreshold = lodTransitionNoise(worldXZ);
         if (fade < ditherThreshold) discard;
     }
 

src/world/lod_manager.zig

@@ -55,6 +55,7 @@ const MeshMap = lod_gpu.MeshMap;
 const RegionMap = lod_gpu.RegionMap;
 
 const MAX_LOD_REGIONS = 2048;
+const CHUNK_COVERAGE_PADDING: i32 = 1;
 
 comptime {
     if (LODLevel.count < 2) {
@@ -780,14 +781,15 @@ pub const LODManager = struct {
         }
     }
 
-    /// Check if all chunks within the given world bounds are loaded and renderable
+    /// Check if all chunks within the given world bounds are loaded and renderable.
+    /// Includes a small chunk halo around bounds to avoid exposing border cut-faces.
     pub fn areAllChunksLoaded(self: *Self, bounds: LODChunk.WorldBounds, checker: ChunkChecker, ctx: *anyopaque) bool {
         _ = self;
         // Convert world bounds to chunk coordinates
-        const min_cx = @divFloor(bounds.min_x, CHUNK_SIZE_X);
-        const min_cz = @divFloor(bounds.min_z, CHUNK_SIZE_X);
-        const max_cx = @divFloor(bounds.max_x - 1, CHUNK_SIZE_X); // -1 because max is exclusive
-        const max_cz = @divFloor(bounds.max_z - 1, CHUNK_SIZE_X);
+        const min_cx = @divFloor(bounds.min_x, CHUNK_SIZE_X) - CHUNK_COVERAGE_PADDING;
+        const min_cz = @divFloor(bounds.min_z, CHUNK_SIZE_X) - CHUNK_COVERAGE_PADDING;
+        const max_cx = @divFloor(bounds.max_x - 1, CHUNK_SIZE_X) + CHUNK_COVERAGE_PADDING; // -1 because max is exclusive
+        const max_cz = @divFloor(bounds.max_z - 1, CHUNK_SIZE_X) + CHUNK_COVERAGE_PADDING;
 
         // Check every chunk in the region
         var cz = min_cz;

src/world/lod_renderer.zig

@@ -25,6 +25,8 @@ const AABB = @import("../engine/math/aabb.zig").AABB;
 const rhi_types = @import("../engine/graphics/rhi_types.zig");
 const log = @import("../engine/core/log.zig");
 
+const CHUNK_COVERAGE_PADDING: i32 = 1;
+
 /// Expected RHI interface for LODRenderer:
 /// - createBuffer(size: usize, usage: BufferUsage) !BufferHandle
 /// - destroyBuffer(handle: BufferHandle) void
@@ -142,17 +144,18 @@ pub fn LODRenderer(comptime RHI: type) type {
                     const bounds = chunk.worldBounds();
 
                     // Check if all underlying block chunks are loaded.
-                    // If they are, we skip rendering the LOD chunk to let blocks show through.
+                    // We require a 1-chunk halo around the LOD region to avoid exposing
+                    // block chunk cut-faces when neighbors are still missing.
                     if (chunk_checker) |checker| {
                         const side: i32 = @intCast(chunk.lod_level.chunksPerSide());
                         const start_cx = chunk.region_x * side;
                         const start_cz = chunk.region_z * side;
 
                         var all_loaded = true;
-                        var lcz: i32 = 0;
-                        while (lcz < side) : (lcz += 1) {
-                            var lcx: i32 = 0;
-                            while (lcx < side) : (lcx += 1) {
+                        var lcz: i32 = -CHUNK_COVERAGE_PADDING;
+                        while (lcz < side + CHUNK_COVERAGE_PADDING) : (lcz += 1) {
+                            var lcx: i32 = -CHUNK_COVERAGE_PADDING;
+                            while (lcx < side + CHUNK_COVERAGE_PADDING) : (lcx += 1) {
                                 if (!checker(start_cx + lcx, start_cz + lcz, checker_ctx.?)) {
                                     all_loaded = false;
                                     break;