Vulkan capture: preserve the game's sync mode, drop the capture throttle

The capture must never change vsync, and must not frame-limit itself.

- Removed the ~150 Hz capture throttle from VkCapture. It was wrong: vsync already
  paces capture (a 144 Hz FIFO game presents 144x/s, so we mirror 144x/s). The only
  limiter left is ring backpressure (skip a frame if the reaper is behind), which is
  correctness, not a cap, and never touches the game's present thread or sync mode.
- The layer/hook already pass VkSwapchainCreateInfoKHR straight through, so the
  present mode (= the sync mode) is untouched. Added a presentMode log to prove it.

Measured on Sphere Spectacle (direct launch, layer attached): presentMode=2 (FIFO),
steady 144.0 fps, and with the throttle gone the mirror now tracks it at 144/s
(was capped ~130). The earlier 400-600 fps reading was a direct-launch artifact --
a non-foreground windowed FIFO app isn't throttled by DWM -- not the layer, and not
the case through Steam (144). vk_validate now states the mirror follows the present
rate (no throttle) and still asserts a present-rate floor.

Full suite 21/21.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -110,10 +110,6 @@ default** and covers anything the hooked path doesn't.
 
 ### Current tasks
 
-- **The Vulkan capture must not change the swapchain's sync mode.** If the game vsyncs (e.g. FIFO at
-  144 Hz), it must still vsync with the layer/hook attached; if it didn't, it must not start. The
-  capture is forbidden from throttling or frame-limiting — that is vsync's job. Remove the ~150 Hz
-  capture throttle and prove (measure) the present mode/rate is identical with and without the layer.
 - **DX12 capture is measurably slower than the other backends — find out why and improve it.** The
   present-thread overhead guard shows DX12 ~0.34 ms vs DX11 ~0.05 ms / OpenGL ~0.09 ms (the D3D11On12
   bridge). Investigate and reduce it.
@@ -133,7 +129,10 @@ rate while mirroring (guarded by `vk_capture_perf_test`; every GPU backend's hoo
 present-thread overhead bound) — and the **inline-hook (suspended-inject) Vulkan path**, which now
 captures Sphere Spectacle correctly once the harness launches the exe with the right working
 directory and `vk_hook` intercepts present/swapchain resolved via `vkGetInstanceProcAddr` (not just
-`vkGetDeviceProcAddr`). Both Vulkan paths are verified on the real game. See **Lessons learned** and
+`vkGetDeviceProcAddr`). Both Vulkan paths are verified on the real game. The capture also **preserves
+the game's sync mode** — it passes the swapchain's present mode through untouched and no longer
+throttles the mirror (vsync paces it): Sphere Spectacle requests `FIFO` and holds a steady 144 Hz
+with the layer attached. See **Lessons learned** and
 the test suite for each. Open directions: per-game profiles, multi-guest virtual-pad mapping, and
 continuous raw-mouse *movement* forwarding (the MKB event stream is position-based today).
 
@@ -537,10 +536,19 @@ Non-obvious things that cost time and constrain the design:
   cadence; a hard FPS gate is meaningless"), which hid a real **144→3 FPS** stall: the read-back ran
   on the present thread and spent ~370 ms/frame doing a CPU read of write-combined staging memory.
   After moving the read-back to a reaper thread (`coop::hook::VkCapture`), `present_calls` (counted
-  every present, independent of the throttled mirror) reflects the game's true rate, so the tool now
-  **asserts** it stays healthy while capturing. The mirror is deliberately throttled to ~150 Hz, so
-  `copied < present_calls` is expected, not a drop. Lesson: a perf check must assert a bound — if you
-  find yourself explaining why a number is fine, make the test prove it.
+  every present) reflects the game's true rate, so the tool now **asserts** it stays healthy while
+  capturing. Lesson: a perf check must assert a bound — if you find yourself explaining why a number
+  is fine, make the test prove it.
+- **The capture must not touch the game's sync mode — and must not throttle itself.** The swapchain's
+  **present mode** *is* the sync mode (`FIFO` = vsync, `IMMEDIATE`/`MAILBOX` = off); `VkCapture` and
+  the layer pass `VkSwapchainCreateInfoKHR` straight through, so whatever the game asked for is what
+  it gets — proven by logging `ci->presentMode` (Sphere Spectacle = `FIFO`, and it holds a steady
+  144 Hz with the layer attached). An earlier ~150 Hz *capture* throttle was wrong and is removed:
+  capture follows the present rate, which **vsync** already paces (a 144 Hz FIFO game presents 144×/s,
+  so we mirror 144×/s). The only limiter left is ring backpressure — if the reaper can't keep up we
+  skip a frame rather than stall the game — which is correctness, not a frame cap. (A direct-launch
+  window that isn't composited/foreground can present uncapped because DWM doesn't throttle a
+  non-foreground windowed FIFO app — that's the OS, not us, and not the case under Steam.)
 - **A render client that predates our injection has no knowable format — measure it.**
   We inject into already-running games, so we usually never see the game's
   `IAudioClient::Initialize`; the render-hook then assumes the device mix format for that

hook/src/vk_capture.cpp

@@ -8,17 +8,6 @@
 namespace coop::hook
 {
 
-namespace
-{
-double now_ms()
-{
-	LARGE_INTEGER f, c;
-	QueryPerformanceFrequency(&f);
-	QueryPerformanceCounter(&c);
-	return 1000.0 * static_cast<double>(c.QuadPart) / static_cast<double>(f.QuadPart);
-}
-} // namespace
-
 VkCapture::~VkCapture()
 {
 	shutdown();
@@ -340,11 +329,10 @@ bool VkCapture::present(VkImage image, VkFormat fmt, std::uint32_t w, std::uint3
 	{
 		return false;
 	}
-	const double t = now_ms();
-	if (t - m_last_submit_ms < kMinCaptureIntervalMs)
-	{
-		return false; // throttle: mirror at ~150 Hz, not the game's (possibly 400+) present rate
-	}
+	// No time-based throttle here: capture follows the game's present rate, which vsync paces (if the
+	// game vsyncs at 144 Hz it presents 144x/s, so we capture 144x/s). The only limiter is ring
+	// backpressure below -- if the reaper can't keep up we skip a frame rather than block the game --
+	// which is correctness, not a frame cap (it never stalls the present thread or touches sync mode).
 	// Pick a slot whose previous capture the reaper has finished. None free -> the reaper is behind,
 	// so skip this frame (the game keeps its rate; the mirror just drops a frame).
 	int idx = -1;
@@ -408,7 +396,6 @@ bool VkCapture::present(VkImage image, VkFormat fmt, std::uint32_t w, std::uint3
 	{
 		return false;
 	}
-	m_last_submit_ms = t;
 	s.w = w;
 	s.h = h;
 	s.fmt = fmt;

hook/src/vk_capture.hpp

@@ -146,11 +146,6 @@ private:
 	VkCommandPool m_pool = VK_NULL_HANDLE;
 	Slot m_slots[kSlots];
 	int m_next = 0;
-	// Cap capture to ~150 Hz: a guest stream is at most the host's refresh (usually 60, at most 144),
-	// so mirroring every present of an uncapped 400+ FPS game is pure wasted reaper CPU. Throttling
-	// here -- not on the game -- keeps the mirror smooth while freeing the cores the game wants.
-	static constexpr double kMinCaptureIntervalMs = 1000.0 / 150.0;
-	double m_last_submit_ms = 0.0;
 
 	// --- D3D11 shared texture (reaper thread only) ---
 	ID3D11Device* m_d3d = nullptr;

tools/vk_validate/main.cpp

@@ -422,9 +422,9 @@ int main(int argc, char** argv)
 
 	// Performance gate (this is the check the earlier version refused to make -- it reported the rate
 	// and rationalized it, which hid the 144->3 FPS stall). The capture runs off the present thread,
-	// so the game must keep a healthy present rate while we mirror. present_calls counts EVERY present
-	// (not throttled); the mirror is intentionally throttled to ~150 Hz, so copied < present is normal
-	// and not a drop. A present rate that collapses (the bug was ~3/s) fails here.
+	// so the game must keep a healthy present rate while we mirror. The capture is NOT throttled --
+	// it follows the present rate, which vsync paces -- so the mirror rate tracks the present rate. A
+	// present rate that collapses (the bug was ~3/s) fails here.
 	if (alive && find_pid(L"sphere.exe") == pid)
 	{
 		const std::uint64_t p0 = block->video.present_calls;
@@ -437,7 +437,7 @@ int main(int argc, char** argv)
 		// so its delta is the true mirror rate even while we're just sleeping here.
 		const double mirror = static_cast<double>(g1 - g0) / 3.0;
 		std::printf("   present rate while capturing = %.1f /s; mirror rate = %.1f /s (capture is off the "
-		            "present thread + throttled)\n",
+		            "present thread; the mirror follows the present rate -- vsync paces it)\n",
 		            fps, mirror);
 		check(fps > 30.0, "game keeps a healthy present rate while capturing (no present-thread stall)");
 	}

vk_layer/coop_vk_layer.cpp

@@ -288,6 +288,11 @@ VKAPI_ATTR VkResult VKAPI_CALL layer_QueuePresentKHR(VkQueue queue, const VkPres
 VKAPI_ATTR VkResult VKAPI_CALL layer_CreateSwapchainKHR(VkDevice device, const VkSwapchainCreateInfoKHR* ci,
                                                        const VkAllocationCallbacks* a, VkSwapchainKHR* out)
 {
+	// Log the game's chosen present mode (= its sync mode) -- 0 IMMEDIATE, 1 MAILBOX, 2 FIFO (vsync),
+	// 3 FIFO_RELAXED. We pass `ci` straight through, so whatever the game asked for is what it gets;
+	// this proves the layer never changes vsync.
+	logvk("CreateSwapchain: presentMode=%d (0=IMMEDIATE 1=MAILBOX 2=FIFO 3=FIFO_RELAXED) %ux%u minImageCount=%u",
+	      static_cast<int>(ci->presentMode), ci->imageExtent.width, ci->imageExtent.height, ci->minImageCount);
 	const VkResult r = g_real_create_swapchain(device, ci, a, out);
 	if (g_active && r == VK_SUCCESS && out && g_get_swapchain_images)
 	{