Add texture blitting optimization

TODO.md

@@ -29,9 +29,15 @@ findings (AMD Radeon test):
 
 based on phase 2 results:
 
-- [ ] batch rendering, instancing (render-bound confirmed)
-- [ ] ~~if cpu-bound: SIMD, struct-of-arrays, multithreading~~ (not needed)
-- [ ] re-test after each change
+- [x] batch rendering via texture blitting (10x improvement)
+- [x] ~~if cpu-bound: SIMD, struct-of-arrays, multithreading~~ (not needed)
+- [x] re-test after each change
+
+findings:
+- texture blitting: pre-render circle to texture, drawTexture() per entity
+- baseline: 60fps @ ~5k entities
+- optimized: 60fps @ ~50k entities, 30fps @ 100k entities
+- see journal.txt for detailed benchmarks
 
 ## phase 4: add collision
 

build.zig.zon

@@ -1,7 +1,7 @@
 .{
     .name = .lofivor,
     .version = "0.0.1",
-    .fingerprint = 0x9de74c19d031cdc9,
+    .fingerprint = 0x74960d9e2a8050e2,
     .dependencies = .{
         .raylib_zig = .{
             .url = "git+https://github.com/raylib-zig/raylib-zig?ref=devel#a4d18b2d1cf8fdddec68b5b084535fca0475f466",

journal.txt

@@ -20,7 +20,40 @@ analysis: linear scaling, each drawCircle = separate GPU draw call
 
 ---
 
-optimization 1: [pending]
+optimization 1: texture blitting
+--------------------------------
+technique: pre-render circle to 16x16 texture, drawTexture() per entity
+code: sandbox_main.zig:109-124, 170-177
+
+benchmark2.log results:
+- 60fps stable: ~50,000 entities
+- 60fps breaks: ~52,000-55,000 entities (18-21ms frame)
+- 23k entities: 16.7ms frame (still vsync-locked)
+- 59k entities: 20.6ms frame
+
+extended benchmark (benchmark3):
+- 50k entities: 16.7ms (vsync-locked, briefly touches 19ms)
+- 60k entities: 20.7ms
+- 70k entities: 23.7ms
+- 80k entities: 30.1ms
+- 100k entities: 33-37ms (~30fps)
+
+comparison to baseline:
+- baseline broke 60fps at ~5,000 entities
+- texture blitting breaks at ~50,000 entities
+- ~10x improvement in entity ceiling
+
+analysis: raylib batches texture draws internally when using same texture.
+individual drawCircle() = separate draw call each. drawTexture() with same
+texture = batched into fewer GPU calls.
+
+notes: render_ms stays ~16-18ms up to ~50k, then scales roughly linearly.
+at 100k entities we're at ~30fps which is still playable. update loop
+remains negligible (<0.6ms even at 100k).
+
+---
+
+optimization 2: [pending]
 -------------------------
 technique:
 results:

src/sandbox_main.zig

@@ -12,6 +12,10 @@ const SCREEN_HEIGHT = sandbox.SCREEN_HEIGHT;
 const BG_COLOR = rl.Color{ .r = 10, .g = 10, .b = 18, .a = 255 };
 const CYAN = rl.Color{ .r = 0, .g = 255, .b = 255, .a = 255 };
 
+// entity rendering
+const ENTITY_RADIUS: f32 = 4.0;
+const TEXTURE_SIZE: i32 = 16; // must be >= 2 * radius
+
 // logging thresholds
 const TARGET_FRAME_MS: f32 = 16.7; // 60fps
 const THRESHOLD_MARGIN: f32 = 2.0; // hysteresis margin to avoid bounce
@@ -102,11 +106,35 @@ const BenchmarkLogger = struct {
     }
 };
 
+fn createCircleTexture() ?rl.Texture2D {
+    // create a render texture to draw circle into
+    const target = rl.loadRenderTexture(TEXTURE_SIZE, TEXTURE_SIZE) catch return null;
+
+    rl.beginTextureMode(target);
+    rl.clearBackground(rl.Color{ .r = 0, .g = 0, .b = 0, .a = 0 }); // transparent
+    rl.drawCircle(
+        @divTrunc(TEXTURE_SIZE, 2),
+        @divTrunc(TEXTURE_SIZE, 2),
+        ENTITY_RADIUS,
+        CYAN,
+    );
+    rl.endTextureMode();
+
+    return target.texture;
+}
+
 pub fn main() !void {
     rl.initWindow(@intCast(SCREEN_WIDTH), @intCast(SCREEN_HEIGHT), "lofivor sandbox");
     defer rl.closeWindow();
     rl.setTargetFPS(60);
 
+    // create circle texture for batched rendering
+    const circle_texture = createCircleTexture() orelse {
+        std.debug.print("failed to create circle texture\n", .{});
+        return;
+    };
+    defer rl.unloadTexture(circle_texture);
+
     var entities = sandbox.Entities.init();
     var prng = std.Random.DefaultPrng.init(@intCast(std.time.timestamp()));
     var rng = prng.random();
@@ -140,13 +168,14 @@ pub fn main() !void {
         rl.beginDrawing();
         rl.clearBackground(BG_COLOR);
 
-        // draw entities as filled circles
+        // draw entities using pre-rendered circle texture
+        const half_size = @as(f32, @floatFromInt(TEXTURE_SIZE)) / 2.0;
         for (entities.items[0..entities.count]) |entity| {
-            rl.drawCircle(
-                @intFromFloat(entity.x),
-                @intFromFloat(entity.y),
-                4,
-                CYAN,
+            rl.drawTexture(
+                circle_texture,
+                @intFromFloat(entity.x - half_size),
+                @intFromFloat(entity.y - half_size),
+                rl.Color.white, // tint (white = use original colors)
             );
         }