Drop verbose comments from the clip-cache change.

Crowd stress (128 sprites) showed Clip_Def string map_get as the top engine cost; cache on set_sprite_clip and use it in update/draw.

engine/app.odin

@@ -16,15 +16,8 @@ SPRITE_VERTS_SIZE :: SPRITE_VERT_COUNT * size_of(Vertex)
 VERTEX_BUFFER_SIZE :: MAX_SPRITES * SPRITE_VERTS_SIZE
 
 Queued_Sprite :: struct {
-	texture:     ^sdl.GPUTexture,
-	verts:       [SPRITE_VERT_COUNT]Vertex,
-	batch_group: u32, // 0 = strict order; nonzero = caller permits regrouping
-}
-
-Draw_Batch :: struct {
-	start:   int,
-	count:   int,
 	texture: ^sdl.GPUTexture,
+	verts:   [SPRITE_VERT_COUNT]Vertex,
 }
 
 App :: struct {
@@ -72,7 +65,7 @@ init :: proc(app: ^App, title: cstring, width, height: i32) -> bool {
 	}
 
 	requested: sdl.GPUShaderFormat = {.SPIRV, .DXIL, .MSL}
-	app.device = sdl.CreateGPUDevice(requested, false, nil)
+	app.device = sdl.CreateGPUDevice(requested, true, nil)
 	if app.device == nil {
 		fmt.eprintfln("CreateGPUDevice failed: %s", sdl.GetError())
 		return false
@@ -83,10 +76,6 @@ init :: proc(app: ^App, title: cstring, width, height: i32) -> bool {
 		return false
 	}
 
-	if !sdl.SetGPUAllowedFramesInFlight(app.device, 3) {
-		fmt.eprintfln("SetGPUAllowedFramesInFlight failed: %s", sdl.GetError())
-	}
-
 	// Prefer uncapped present for profiling; fall back if unsupported.
 	present := sdl.GPUPresentMode.VSYNC
 	if sdl.WindowSupportsGPUPresentMode(app.device, app.window, .IMMEDIATE) {
@@ -253,11 +242,7 @@ end_frame :: proc(app: ^App) {
 	}
 
 	n := len(app.draw_list)
-	batches: [dynamic]Draw_Batch
-	defer delete(batches)
 	if n > 0 {
-		prepare_draw_batches(app.draw_list[:], &batches)
-
 		map_ptr := sdl.MapGPUTransferBuffer(app.device, app.transfer_buffer, false)
 		if map_ptr == nil {
 			fmt.eprintfln("MapGPUTransferBuffer failed: %s", sdl.GetError())
@@ -301,20 +286,26 @@ end_frame :: proc(app: ^App) {
 	app.render_pass = sdl.BeginGPURenderPass(cmd, &color_info, 1, nil)
 	sdl.BindGPUGraphicsPipeline(app.render_pass, app.pipeline)
 
-	for batch in batches {
+	i := 0
+	for i < n {
+		run := texture_run_len(app.draw_list[:], i)
+		q0 := app.draw_list[i]
+
 		sampler_binding := sdl.GPUTextureSamplerBinding {
-			texture = batch.texture,
+			texture = q0.texture,
 			sampler = app.sampler,
 		}
 		sdl.BindGPUFragmentSamplers(app.render_pass, 0, &sampler_binding, 1)
 
 		vb_binding := sdl.GPUBufferBinding {
 			buffer = app.vertex_buffer,
-			offset = u32(batch.start * SPRITE_VERTS_SIZE),
+			offset = u32(i * SPRITE_VERTS_SIZE),
 		}
 		sdl.BindGPUVertexBuffers(app.render_pass, 0, &vb_binding, 1)
 
-		sdl.DrawGPUPrimitives(app.render_pass, u32(batch.count * SPRITE_VERT_COUNT), 1, 0, 0)
+		sdl.DrawGPUPrimitives(app.render_pass, u32(run * SPRITE_VERT_COUNT), 1, 0, 0)
+
+		i += run
 	}
 	sdl.EndGPURenderPass(app.render_pass)
 	app.render_pass = nil
@@ -463,60 +454,3 @@ texture_run_len :: proc(list: []Queued_Sprite, start: int) -> int {
 
 	return n
 }
-
-texture_run_count :: proc(list: []Queued_Sprite) -> int {
-	if len(list) == 0 do return 0
-	count := 0
-	i := 0
-	for i < len(list) {
-		run := texture_run_len(list, i)
-		count += 1
-		i += run
-	}
-	return count
-}
-
-prepare_draw_batches :: proc(list: []Queued_Sprite, batches: ^[dynamic]Draw_Batch) {
-	clear(batches)
-	group_texture_runs(list)
-	i := 0
-	for i < len(list) {
-		run := texture_run_len(list, i)
-		append(batches, Draw_Batch{start = i, count = run, texture = list[i].texture})
-		i += run
-	}
-}
-
-// Within each contiguous nonzero batch_group, stably sort by texture so
-// consecutive same-texture sprites become one draw. Group 0 and group
-// boundaries are never crossed.
-group_texture_runs :: proc(list: []Queued_Sprite) {
-	start := 0
-	for start < len(list) {
-		group := list[start].batch_group
-		if group == 0 {
-			start += 1
-			continue
-		}
-
-		end := start + 1
-		for end < len(list) && list[end].batch_group == group {
-			end += 1
-		}
-
-		// Stable insertion sort is sufficient while MAX_SPRITES is 128.
-		for i in start + 1 ..< end {
-			item := list[i]
-			j := i
-			for j > start {
-				if uintptr(list[j - 1].texture) <= uintptr(item.texture) {
-					break
-				}
-				list[j] = list[j - 1]
-				j -= 1
-			}
-			list[j] = item
-		}
-		start = end
-	}
-}

engine/batching_test.odin

@@ -7,18 +7,6 @@ fake_tex :: proc(id: uintptr) -> ^sdl.GPUTexture {
 	return cast(^sdl.GPUTexture)id
 }
 
-queued :: proc(tex_id: uintptr, group: u32, marker: f32) -> Queued_Sprite {
-	q: Queued_Sprite
-	q.texture = fake_tex(tex_id)
-	q.batch_group = group
-	q.verts[0].pos = {marker, 0}
-	return q
-}
-
-marker_of :: proc(q: Queued_Sprite) -> f32 {
-	return q.verts[0].pos.x
-}
-
 @(test)
 texture_run_len_empty_or_oob :: proc(t: ^testing.T) {
 	testing.expect_value(t, texture_run_len(nil, 0), 0)
@@ -74,293 +62,3 @@ texture_run_len_all_different :: proc(t: ^testing.T) {
 	testing.expect_value(t, texture_run_len(list, 1), 1)
 	testing.expect_value(t, texture_run_len(list, 2), 1)
 }
-
-@(test)
-group_texture_runs_strict_order_unchanged :: proc(t: ^testing.T) {
-	// Group 0 alternates textures; sorting must not reorder (alpha order).
-	list := []Queued_Sprite {
-		queued(2, 0, 1),
-		queued(1, 0, 2),
-		queued(2, 0, 3),
-		queued(1, 0, 4),
-	}
-	before_runs := texture_run_count(list)
-	group_texture_runs(list)
-	testing.expect_value(t, texture_run_count(list), before_runs)
-	testing.expect_value(t, marker_of(list[0]), f32(1))
-	testing.expect_value(t, marker_of(list[1]), f32(2))
-	testing.expect_value(t, marker_of(list[2]), f32(3))
-	testing.expect_value(t, marker_of(list[3]), f32(4))
-}
-
-@(test)
-group_texture_runs_reduces_runs_inside_group :: proc(t: ^testing.T) {
-	list := []Queued_Sprite {
-		queued(2, 1, 1),
-		queued(1, 1, 2),
-		queued(2, 1, 3),
-		queued(1, 1, 4),
-	}
-	testing.expect_value(t, texture_run_count(list), 4)
-	group_texture_runs(list)
-	testing.expect_value(t, texture_run_count(list), 2)
-	testing.expect(t, list[0].texture == fake_tex(1), "lower texture pointer first")
-	testing.expect(t, list[1].texture == fake_tex(1), "same texture run")
-	testing.expect(t, list[2].texture == fake_tex(2), "second texture run")
-	testing.expect(t, list[3].texture == fake_tex(2), "second texture run cont")
-}
-
-@(test)
-group_texture_runs_stable_same_texture :: proc(t: ^testing.T) {
-	list := []Queued_Sprite {
-		queued(2, 1, 10),
-		queued(1, 1, 20),
-		queued(2, 1, 30),
-		queued(1, 1, 40),
-	}
-	group_texture_runs(list)
-	// Same-texture relative order preserved (stable sort).
-	testing.expect_value(t, marker_of(list[0]), f32(20))
-	testing.expect_value(t, marker_of(list[1]), f32(40))
-	testing.expect_value(t, marker_of(list[2]), f32(10))
-	testing.expect_value(t, marker_of(list[3]), f32(30))
-}
-
-@(test)
-group_texture_runs_respects_group_boundaries :: proc(t: ^testing.T) {
-	list := []Queued_Sprite {
-		queued(2, 1, 1),
-		queued(1, 1, 2),
-		queued(2, 0, 3), // strict barrier
-		queued(1, 2, 4),
-		queued(2, 2, 5),
-	}
-	group_texture_runs(list)
-	testing.expect_value(t, marker_of(list[2]), f32(3)) // barrier stays put
-	testing.expect(t, list[0].texture == fake_tex(1))
-	testing.expect(t, list[1].texture == fake_tex(2))
-	testing.expect(t, list[3].texture == fake_tex(1))
-	testing.expect(t, list[4].texture == fake_tex(2))
-	testing.expect_value(t, list[0].batch_group, u32(1))
-	testing.expect_value(t, list[1].batch_group, u32(1))
-	testing.expect_value(t, list[2].batch_group, u32(0))
-	testing.expect_value(t, list[3].batch_group, u32(2))
-	testing.expect_value(t, list[4].batch_group, u32(2))
-}
-
-@(test)
-group_texture_runs_does_not_merge_across_different_groups :: proc(t: ^testing.T) {
-	// Adjacent nonzero groups with different ids must not merge runs across.
-	list := []Queued_Sprite {
-		queued(1, 1, 1),
-		queued(2, 1, 2),
-		queued(1, 2, 3),
-		queued(2, 2, 4),
-	}
-	group_texture_runs(list)
-	testing.expect_value(t, texture_run_count(list), 4)
-	testing.expect_value(t, list[0].batch_group, u32(1))
-	testing.expect_value(t, list[1].batch_group, u32(1))
-	testing.expect_value(t, list[2].batch_group, u32(2))
-	testing.expect_value(t, list[3].batch_group, u32(2))
-}
-
-@(test)
-group_texture_runs_empty :: proc(t: ^testing.T) {
-	list := []Queued_Sprite{}
-	testing.expect_value(t, texture_run_count(list), 0)
-	group_texture_runs(list)
-	testing.expect_value(t, texture_run_count(list), 0)
-}
-
-@(test)
-group_texture_runs_already_optimal :: proc(t: ^testing.T) {
-	list := []Queued_Sprite {
-		queued(1, 1, 10),
-		queued(1, 1, 20),
-		queued(2, 1, 30),
-		queued(2, 1, 40),
-	}
-	testing.expect_value(t, texture_run_count(list), 2)
-	group_texture_runs(list)
-	testing.expect_value(t, texture_run_count(list), 2)
-	testing.expect_value(t, marker_of(list[0]), f32(10))
-	testing.expect_value(t, marker_of(list[1]), f32(20))
-	testing.expect_value(t, marker_of(list[2]), f32(30))
-	testing.expect_value(t, marker_of(list[3]), f32(40))
-}
-
-@(test)
-group_texture_runs_noncontiguous_same_group_id :: proc(t: ^testing.T) {
-	// Same nonzero id split by group 0: each window regroups alone.
-	list := []Queued_Sprite {
-		queued(2, 1, 1),
-		queued(1, 1, 2),
-		queued(2, 0, 3),
-		queued(2, 1, 4),
-		queued(1, 1, 5),
-	}
-	group_texture_runs(list)
-	testing.expect_value(t, marker_of(list[2]), f32(3))
-	testing.expect(t, list[0].texture == fake_tex(1))
-	testing.expect(t, list[1].texture == fake_tex(2))
-	testing.expect_value(t, list[2].batch_group, u32(0))
-	testing.expect(t, list[3].texture == fake_tex(1))
-	testing.expect(t, list[4].texture == fake_tex(2))
-	testing.expect_value(t, list[0].batch_group, u32(1))
-	testing.expect_value(t, list[1].batch_group, u32(1))
-	testing.expect_value(t, list[3].batch_group, u32(1))
-	testing.expect_value(t, list[4].batch_group, u32(1))
-}
-
-make_test_draw_app :: proc() -> App {
-	app: App
-	app.cmd = cast(^sdl.GPUCommandBuffer)uintptr(1)
-	app.swapchain_texture = fake_tex(99)
-	app.swapchain_w = 800
-	app.swapchain_h = 600
-	app.camera = camera_default()
-	app.draw_list = make([dynamic]Queued_Sprite)
-	return app
-}
-
-destroy_test_draw_app :: proc(app: ^App) {
-	if app == nil do return
-	delete(app.draw_list)
-	app^ = {}
-}
-
-make_test_draw_character :: proc() -> Character_Data {
-	data: Character_Data
-	data.texture = fake_tex(42)
-	data.width = 100
-	data.height = 100
-	data.def.pivot = {0.5, 1.0}
-	data.def.clips = make(map[string]Clip_Def)
-	frames := make([]Frame_Def, 1)
-	frames[0] = Frame_Def {
-		rect        = {0, 0, 10, 10},
-		source_size = {10, 10},
-		trim_offset = {0, 0},
-	}
-	data.def.clips["idle"] = Clip_Def {
-		loop   = true,
-		fps    = 10,
-		frames = frames,
-	}
-	return data
-}
-
-destroy_test_draw_character :: proc(data: ^Character_Data) {
-	if data == nil do return
-	keys := make([dynamic]string, context.temp_allocator)
-	for key, clip in data.def.clips {
-		delete(clip.frames)
-		append(&keys, key)
-	}
-	for key in keys {
-		delete_key(&data.def.clips, key)
-	}
-	delete(data.def.clips)
-	data^ = {}
-}
-
-@(test)
-draw_sprite_stamps_batch_group_zero :: proc(t: ^testing.T) {
-	app := make_test_draw_app()
-	defer destroy_test_draw_app(&app)
-	data := make_test_draw_character()
-	defer destroy_test_draw_character(&data)
-
-	sprite := spawn_sprite(&data, {100, 200}, "idle", 0)
-	draw_sprite(&app, &sprite)
-
-	testing.expect_value(t, len(app.draw_list), 1)
-	testing.expect_value(t, app.draw_list[0].batch_group, u32(0))
-	testing.expect(t, app.draw_list[0].texture == data.texture)
-}
-
-@(test)
-draw_sprite_batched_stamps_batch_group :: proc(t: ^testing.T) {
-	app := make_test_draw_app()
-	defer destroy_test_draw_app(&app)
-	data := make_test_draw_character()
-	defer destroy_test_draw_character(&data)
-
-	sprite := spawn_sprite(&data, {100, 200}, "idle", 0)
-	draw_sprite_batched(&app, &sprite, 7)
-
-	testing.expect_value(t, len(app.draw_list), 1)
-	testing.expect_value(t, app.draw_list[0].batch_group, u32(7))
-	testing.expect(t, app.draw_list[0].texture == data.texture)
-}
-
-@(test)
-draw_sprite_batched_guards_leave_list_unchanged :: proc(t: ^testing.T) {
-	app := make_test_draw_app()
-	defer destroy_test_draw_app(&app)
-	data := make_test_draw_character()
-	defer destroy_test_draw_character(&data)
-	sprite := spawn_sprite(&data, {100, 200}, "idle", 0)
-
-	app.cmd = nil
-	draw_sprite_batched(&app, &sprite, 1)
-	testing.expect_value(t, len(app.draw_list), 0)
-	app.cmd = cast(^sdl.GPUCommandBuffer)uintptr(1)
-
-	app.swapchain_texture = nil
-	draw_sprite_batched(&app, &sprite, 1)
-	testing.expect_value(t, len(app.draw_list), 0)
-	app.swapchain_texture = fake_tex(99)
-
-	draw_sprite_batched(&app, nil, 1)
-	testing.expect_value(t, len(app.draw_list), 0)
-
-	no_data := sprite
-	no_data.data = nil
-	draw_sprite_batched(&app, &no_data, 1)
-	testing.expect_value(t, len(app.draw_list), 0)
-
-	no_tex := sprite
-	tex_data := data
-	tex_data.texture = nil
-	no_tex.data = &tex_data
-	draw_sprite_batched(&app, &no_tex, 1)
-	testing.expect_value(t, len(app.draw_list), 0)
-}
-
-@(test)
-prepare_draw_batches_regroups_then_plans_runs :: proc(t: ^testing.T) {
-	list := []Queued_Sprite {
-		queued(2, 1, 1),
-		queued(1, 1, 2),
-		queued(2, 1, 3),
-		queued(1, 1, 4),
-		queued(3, 0, 5),
-		queued(1, 0, 6),
-	}
-	batches := make([dynamic]Draw_Batch)
-	defer delete(batches)
-
-	prepare_draw_batches(list, &batches)
-
-	testing.expect_value(t, len(batches), 4)
-	testing.expect_value(t, batches[0].start, 0)
-	testing.expect_value(t, batches[0].count, 2)
-	testing.expect(t, batches[0].texture == fake_tex(1))
-	testing.expect_value(t, batches[1].start, 2)
-	testing.expect_value(t, batches[1].count, 2)
-	testing.expect(t, batches[1].texture == fake_tex(2))
-	testing.expect_value(t, batches[2].start, 4)
-	testing.expect_value(t, batches[2].count, 1)
-	testing.expect(t, batches[2].texture == fake_tex(3))
-	testing.expect_value(t, batches[3].start, 5)
-	testing.expect_value(t, batches[3].count, 1)
-	testing.expect(t, batches[3].texture == fake_tex(1))
-
-	// Group 0 submission order preserved.
-	testing.expect_value(t, marker_of(list[4]), f32(5))
-	testing.expect_value(t, marker_of(list[5]), f32(6))
-	testing.expect_value(t, list[4].batch_group, u32(0))
-	testing.expect_value(t, list[5].batch_group, u32(0))
-}

engine/sprite.odin

@@ -5,12 +5,14 @@ import sdl "vendor:sdl3"
 Vec2 :: [2]f32
 
 Sprite :: struct {
-	data:     ^Character_Data,
-	position: Vec2,
-	clip:     string,
-	frame:    int,
-	time:     f32,
-	flip_x:   bool,
+	data:      ^Character_Data,
+	position:  Vec2,
+	clip:      string,
+	clip_def:  Clip_Def,
+	has_clip:  bool,
+	frame:     int,
+	time:      f32,
+	flip_x:    bool,
 }
 
 spawn_sprite :: proc(
@@ -24,16 +26,14 @@ spawn_sprite :: proc(
 		position = position,
 	}
 	set_sprite_clip(&sprite, clip)
-	if frame != 0 {
-		c, ok := character_clip(sprite.data, sprite.clip)
-		if ok {
-			if frame < 0 {
-				sprite.frame = 0
-			} else if frame >= len(c.frames) {
-				sprite.frame = len(c.frames) - 1
-			} else {
-				sprite.frame = frame
-			}
+	if frame != 0 && sprite.has_clip {
+		c := sprite.clip_def
+		if frame < 0 {
+			sprite.frame = 0
+		} else if frame >= len(c.frames) {
+			sprite.frame = len(c.frames) - 1
+		} else {
+			sprite.frame = frame
 		}
 	}
 	return sprite
@@ -42,9 +42,9 @@ spawn_sprite :: proc(
 update_sprite :: proc(sprite: ^Sprite, dt: f32) {
 	if sprite == nil || sprite.data == nil do return
 	if dt <= 0 do return
+	if !sprite.has_clip do return
 
-	clip, ok := character_clip(sprite.data, sprite.clip)
-	if !ok do return
+	clip := sprite.clip_def
 
 	frame_count := len(clip.frames)
 	if frame_count <= 0 do return
@@ -82,31 +82,7 @@ to_clip :: proc(px, py, sw, sh: f32) -> [2]f32 {
 	}
 }
 
-// Axis-aligned quad: two unique x and y values, so scale once and reuse.
-sprite_quad_to_clip :: proc(x0, y0, x1, y1, sw, sh: f32) -> [4]Vec2 {
-	sx := 2.0 / sw
-	sy := 2.0 / sh
-
-	left := x0 * sx - 1
-	right := x1 * sx - 1
-	top := 1 - y0 * sy
-	bottom := 1 - y1 * sy
-
-	return {
-		{left, top},
-		{right, top},
-		{right, bottom},
-		{left, bottom},
-	}
-}
-
 draw_sprite :: proc(app: ^App, sprite: ^Sprite) {
-	draw_sprite_batched(app, sprite, 0)
-}
-
-// Nonzero batch_group lets end_frame regroup consecutive same-group sprites by
-// texture. Group 0 keeps exact submission order for correct alpha overlap.
-draw_sprite_batched :: proc(app: ^App, sprite: ^Sprite, batch_group: u32) {
 	if app.cmd == nil || app.swapchain_texture == nil {
 		return
 	}
@@ -116,12 +92,13 @@ draw_sprite_batched :: proc(app: ^App, sprite: ^Sprite, batch_group: u32) {
 	if len(app.draw_list) >= MAX_SPRITES {
 		return
 	}
-
-	frame, ok := character_frame(sprite.data, sprite.clip, sprite.frame)
-	if !ok {
+	if !sprite.has_clip do return
+	if sprite.frame < 0 || sprite.frame >= len(sprite.clip_def.frames) {
 		return
 	}
 
+	frame := sprite.clip_def.frames[sprite.frame]
+
 	src_w := f32(frame.source_size[0])
 	src_h := f32(frame.source_size[1])
 	if src_w <= 0 do src_w = f32(frame.rect[2])
@@ -148,8 +125,10 @@ draw_sprite_batched :: proc(app: ^App, sprite: ^Sprite, batch_group: u32) {
 
 	sw := f32(app.swapchain_w)
 	sh := f32(app.swapchain_h)
-	points := sprite_quad_to_clip(x0_px, y0_px, x1_px, y1_px, sw, sh)
-	p0, p1, p2, p3 := points[0], points[1], points[2], points[3]
+	p0 := to_clip(x0_px, y0_px, sw, sh)
+	p1 := to_clip(x1_px, y0_px, sw, sh)
+	p2 := to_clip(x1_px, y1_px, sw, sh)
+	p3 := to_clip(x0_px, y1_px, sw, sh)
 
 	tex_w := f32(sprite.data.width)
 	tex_h := f32(sprite.data.height)
@@ -164,25 +143,20 @@ draw_sprite_batched :: proc(app: ^App, sprite: ^Sprite, batch_group: u32) {
 		{pos = p3, uv = {u0, v1}},
 	}
 
-	append(
-		&app.draw_list,
-		Queued_Sprite {
-			texture = sprite.data.texture,
-			verts = verts,
-			batch_group = batch_group,
-		},
-	)
+	append(&app.draw_list, Queued_Sprite{texture = sprite.data.texture, verts = verts})
 }
 
 set_sprite_clip :: proc(sprite: ^Sprite, clip: string) {
 	if sprite == nil || sprite.data == nil do return
 
-	if sprite.clip == clip do return
+	if sprite.clip == clip && sprite.has_clip do return
 
-	_, ok := character_clip(sprite.data, clip)
+	def, ok := character_clip(sprite.data, clip)
 	if !ok do return
 
 	sprite.clip = clip
+	sprite.clip_def = def
+	sprite.has_clip = true
 	sprite.frame = 0
 	sprite.time = 0
 }

examples/crowd/main.odin

@@ -1,9 +1,12 @@
 package main
 
+import "core:fmt"
 import eng "pkg:engine"
 
 // Many sprites sharing one Character_Data (Flyweight) — good batching demo.
-COUNT :: 24
+COUNT :: eng.MAX_SPRITES
+COLS :: 16
+FRAME_LOG_EVERY :: 60
 
 main :: proc() {
 	app: eng.App
@@ -16,25 +19,45 @@ main :: proc() {
 
 	sprites: [COUNT]eng.Sprite
 	for i in 0 ..< COUNT {
-		col := i % 8
-		row := i / 8
+		col := i % COLS
+		row := i / COLS
 		pos := eng.Vec2 {
-			f32(120 + col * 80),
-			f32(280 + row * 120),
+			f32(40 + col * 48),
+			f32(80 + row * 60),
 		}
 		clip := "idle" if (i % 2) == 0 else "walk"
 		sprites[i] = eng.spawn_sprite(&data, pos, clip, i % 5)
 	}
 
-	// Look at the middle of the grid
 	app.camera.position = {400, 400}
 
 	last := eng.now_seconds()
+	frame_i := 0
+	sum_ms: f64
+	peak_ms: f64
 
 	for eng.events() {
 		now := eng.now_seconds()
 		dt := f32(now - last)
 		last = now
+		frame_ms := f64(dt) * 1000.0
+		sum_ms += frame_ms
+		if frame_ms > peak_ms do peak_ms = frame_ms
+		frame_i += 1
+
+		if frame_i % FRAME_LOG_EVERY == 0 {
+			avg := sum_ms / f64(FRAME_LOG_EVERY)
+			fps := 1000.0 / avg if avg > 0 else 0
+			fmt.printfln(
+				"crowd frame: avg=%.2f ms (%.1f FPS) peak=%.2f ms over %d frames",
+				avg,
+				fps,
+				peak_ms,
+				FRAME_LOG_EVERY,
+			)
+			sum_ms = 0
+			peak_ms = 0
+		}
 
 		for &s in sprites {
 			eng.update_sprite(&s, dt)