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+/************************************************************************************
+
+Filename    :   CAPI_DistortionTiming.h
+Content     :   Implements timing for the distortion renderer
+Created     :   Dec 16, 2014
+Authors     :   Chris Taylor
+
+Copyright   :   Copyright 2014 Oculus VR, LLC All Rights reserved.
+
+Licensed under the Oculus VR Rift SDK License Version 3.2 (the "License"); 
+you may not use the Oculus VR Rift SDK except in compliance with the License, 
+which is provided at the time of installation or download, or which 
+otherwise accompanies this software in either electronic or hard copy form.
+
+You may obtain a copy of the License at
+
+http://www.oculusvr.com/licenses/LICENSE-3.2 
+
+Unless required by applicable law or agreed to in writing, the Oculus VR SDK 
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+************************************************************************************/
+
+#ifndef OVR_CAPI_DistortionTiming_h
+#define OVR_CAPI_DistortionTiming_h
+
+#include "CAPI_HMDRenderState.h"
+#include "CAPI_FrameLatencyTracker.h"
+#include "CAPI_FrameTimeManager3.h"
+#include "CAPI_HMDRenderState.h"
+#include <Kernel/OVR_Lockless.h>
+#include <OVR_CAPI.h> // ovrFrameTiming
+
+/*
+    -----------------------------
+    Distortion Timing Terminology
+    -----------------------------
+
+    To fix on one set of terminology, a frame life-cycle is defined the following:
+
+    (1) Get prediction time for app left/right eye rendering.
+    (2) App renders left/right eyes.
+    (3) Get prediction time for timewarp.
+    (4) SDK renders distortion/timewarp/chroma, perhaps measuring the time it takes.
+    (5) SDK presents frame and waits for end of frame to occur.
+    (6) End of frame occurs at Vsync.
+    (7) App goes back to step 1 and starts rendering the next frame.
+    (8) Scanout starts some time later for frame from step 6.
+    (9) Display panel emits photons some time later for scanout from step 8.
+
+    "Frame interval" is the time interval between Vsyncs, whether or not Vsync is on.
+    "Frame end" time means the time at which the scanout starts at scanline 0.
+    "Visible midpoint" is when the middle scanline is half-visible to the user's eye.
+
+    "Start of scanout" is when the hardware begins scanout.  The pixels may not be fully
+    illuminated at this point.  A hardware-specific rise time on the order of a millisecond
+    or two must be added to get photons time.
+
+    All timing is done in units of seconds.
+
+    We approximate the scanline start-end interval with the frame interval.
+*/
+
+namespace OVR { namespace CAPI {
+
+
+//-----------------------------------------------------------------------------
+// AppTiming
+//
+// This structure provides the measurements for the current app frame.
+struct AppTiming
+{
+    // When half of the frame image data has been visible to the eye.
+    double VisibleMidpointTime;
+
+    // When the Rift starts scanning out, not including ScreenSwitchingDelay.
+    double ScanoutStartTime;
+
+    // Time between frames.
+    double FrameInterval;
+
+    void Clear()
+    {
+        FrameInterval       = 0.013; // A value that should not break anything.
+        ScanoutStartTime    = 0.; // Predict to current time.
+        VisibleMidpointTime = 0.; // Predict to current time.
+    }
+};
+
+
+//-----------------------------------------------------------------------------
+// TimewarpTiming
+//
+// This structure provides the measurements for the current frame timewarp.
+struct TimewarpTiming
+{
+    // Time at which scanout is predicted to start.
+    double ScanoutTime;
+
+    // The time when Just-In-Time timewarp should be started.
+    // The app should busy/idle-wait until this time before doing timewarp.
+    double JIT_TimewarpTime;
+
+    // Left and right eye start and end render times, respectively.
+    double EyeStartEndTimes[2][2];
+};
+
+
+//-----------------------------------------------------------------------------
+// LocklessAppTimingBase
+//
+// Base timing info shared via lockless data structure.
+// The AppDistortionTimer can use a copy of this data to derive an AppTiming
+// object for a given frame index.
+struct LocklessAppTimingBase
+{
+    // Is the data valid?
+    // 0 = Not valid.
+    uint32_t IsValid;
+
+    // Frame index of the last EndFrame() call to update timing.
+    uint32_t LastEndFrameIndex;
+
+    // Frame start time targetted by the last EndFrame() call to update timing.
+    double LastStartFrameTime;
+
+    // Last known Vsync time from distortion timer.
+    double LastKnownVsyncTime;
+
+    // Vsync fuzz factor used to measure uncertainty in timing.
+    double VsyncFuzzFactor;
+
+    // Most updated measurement of the frame interval.
+    double FrameInterval;
+
+    // Scanout delay measured by the builtin latency tester.
+    double ScanoutDelay;
+
+    // Screen switching delay calculated in distortion timer.
+    double ScreenSwitchingDelay;
+};
+
+
+//-----------------------------------------------------------------------------
+// DistortionTimer
+//
+// This is a calculator for the app and timewarp/distortion timing.
+class DistortionTimer
+{
+    typedef OVR::CAPI::FTM3::FrameTimeManagerCore TimeMan;
+
+public:
+    DistortionTimer();
+    ~DistortionTimer();
+
+    // Returns false if distortion timing could not be initialized.
+    bool Initialize(HMDRenderState const* renderState,
+                    FrameLatencyTracker const* lagTester);
+    void Reset();
+
+    //-------------------------------------------------------------------------
+    // Timewarp Timing
+
+    // Calculate timing for current frame timewarp.
+    // Result can be retrieved via GetTimewarpTiming().
+    void CalculateTimewarpTiming(uint32_t frameIndex, double previousKnownVsyncTime = 0.);
+
+    // Called after CalculateTimewarpTiming() will retrieve the timewarp
+    // timing for this frame.
+    TimewarpTiming const* GetTimewarpTiming()
+    {
+        return &CurrentFrameTimewarpTiming;
+    }
+
+    // Add a distortion draw call timing measurement.
+    void AddDistortionTimeMeasurement(double distortionTimeSeconds);
+
+    // Returns true if more distortion timing measurements are needed.
+    bool NeedDistortionTimeMeasurement() const
+    {
+        // NOTE: Even when Vsync is off this measurement is still valid and useful.
+        return !DistortionRenderTimes.AtCapacity();
+    }
+
+    // Insert right after spin-wait for Present query to finish for the renderer.
+    void SetLastPresentTime()
+    {
+        // Update vsync time.  This is the post-present time, which is expected to be
+        // after the Vsync has completed and our query event put in after the Present
+        // has indicated that it is signaled.  However this is not reliable.
+        LastPresentTime = Timer::GetSeconds();
+    }
+
+    // Returns the time to use for the current frame for latency tester present time,
+    // which is not the same as the LastPresentTime.
+    double GetLatencyTesterPresentTime() const
+    {
+        return LatencyTesterPresentTime;
+    }
+
+    // Set/get the Timewarp IMU time, which is the time at which the IMU was sampled.
+    void SetTimewarpIMUTime(double t)
+    {
+        LastTimewarpIMUTime = t;
+    }
+
+    double GetTimewarpIMUTime() const
+    {
+        return LastTimewarpIMUTime;
+    }
+
+protected:
+    // Vsync/no-vsync versions split out for readability.
+    AppTiming getAppTimingWithVsync();
+    AppTiming getAppTimingNoVsync();
+
+protected:
+    // Last time that Present() was called for post-present latency measurement.
+    // Provided by SetLastPresentTime() cooperatively with the distortion renderer.
+    double LastPresentTime;
+
+    // The time to use for the latency tester for present time, which is the reference
+    // time used for calculating present-scanout delay.
+    double LatencyTesterPresentTime;
+
+    // Last known Vsync time, provided cooperatively by the distortion renderer
+    // via the GetTimewarpTiming() call or internal estimation.
+    double LastKnownVsyncTime;
+
+    // Time in seconds that the Vsync measurement may be in error.
+    // It is assumed to be pretty tight for D3D11 and Display Driver data but for
+    // end frame -based timing we need to add some buffer to avoid misprediction.
+    double LastKnownVsyncFuzzBuffer;
+
+    // The current app frame index, initially zero.
+    uint32_t AppFrameIndex;
+    // Updated in getAppTiming()
+    // Read in CalculateTimewarpTiming()
+
+protected:
+    // Calculator for the time it takes to render distortion.
+    mutable OVR::CAPI::FTM3::MedianCalculator DistortionRenderTimes;
+
+    // Current estimate for timewarp render time.
+    double EstimatedTimewarpRenderTime;
+
+protected:
+#ifdef OVR_OS_WIN32
+    ScopedFileHANDLE DeviceHandle;
+
+    // Attempt to use the display driver for getting a previous vsync
+    bool getDriverVsyncTime(double* previousKnownVsyncTime);
+#endif
+
+    // Get Vsync to next Vsync interval.
+    // NOTE: Technically the Vsync-Vsync frame interval is not the same as the scanout
+    // start to end interval because there is a back porch that implies some blanking time
+    double getFrameInterval() const;
+
+    // Get Frame End to Scanout delay.  Measured by DK2 Latency Tester if available.
+    // This works for Vsync on or off.
+    double getScanoutDelay();
+
+protected:
+    // Update the TimeManager during timewarp calculation
+    void submitDisplayFrame(double frameEndTime, double frameInterval);
+
+    // Update LastKnownVsyncTime.
+    // Pass zero if no Vsync timing information is available.
+    void updateLastKnownVsyncTime(double previousKnownVsyncTime = 0.);
+
+    double getJITTimewarpTime(double frameEndTime);
+
+protected:
+    // DK2 Latency Tester object
+    FrameLatencyTracker const* LatencyTester;
+
+    // Render state parameters from HMD
+    HMDRenderState const* RenderState;
+
+    // Constant screen switching delay calculated from the shutter info.
+    // This is the time it takes between pixels starting to scan out and
+    // for the visible light to rise to half the expected brightness value.
+    // For OLEDs on the DK2 this is about 1 millisecond.
+    double ScreenSwitchingDelay;
+
+    // Time Manager
+    TimeMan TimeManager;
+
+    // The last predicted vsync time from the previous frame.
+    double LastTimewarpFrameEndTime;
+
+    // Has the timing object already been initialized?
+    bool AlreadyInitialized;
+
+protected:
+    // Updated by CalculateTimewarpTiming(). 
+    TimewarpTiming CurrentFrameTimewarpTiming;
+
+    // Time when sensor was sampled for timewarp pose.
+    double LastTimewarpIMUTime;
+
+protected:
+    // Lockless data used by application for eye pose timing via the
+    // provided AppDistortionTimer class.
+    LocklessUpdater<LocklessAppTimingBase> LocklessAppTimingBaseUpdater;
+
+    void ClearAppTimingUpdater()
+    {
+        LocklessAppTimingBase cleared = LocklessAppTimingBase();
+        LocklessAppTimingBaseUpdater.SetState(cleared);
+    }
+
+public:
+    LocklessUpdater<LocklessAppTimingBase>* GetUpdater()
+    {
+        return &LocklessAppTimingBaseUpdater;
+    }
+};
+
+
+// TODO: This header needs to be split up.
+
+
+//-----------------------------------------------------------------------------
+// AppRenderTimer
+//
+// This is an app-side calculator for predicted render times based on frame
+// indices provided by the app.
+class AppRenderTimer
+{
+public:
+    AppRenderTimer();
+    ~AppRenderTimer();
+
+    void SetUpdater(LocklessUpdater<LocklessAppTimingBase>* updater)
+    {
+        AppTimingBaseUpdater = updater;
+    }
+
+    bool IsValid() const
+    {
+        return AppTimingBaseUpdater != nullptr;
+    }
+
+    // Returns true on success.
+    // Pass in 0 for frameIndex to use the next scanout time,
+    // or a non-zero incrementing number for each frame to support queue ahead.
+    void GetAppTimingForIndex(AppTiming& result, bool vsyncOn, uint32_t frameIndex = 0);
+
+protected:
+    LocklessUpdater<LocklessAppTimingBase>* AppTimingBaseUpdater;
+};
+
+
+//-----------------------------------------------------------------------------
+// AppTimingHistory
+//
+// Keep a history of recent application render timing data, to keep a record of
+// when frame indices are expected to scanout.  This is used later to compare
+// with when those frames scan out to self-test the timing code.
+//
+// This class is not thread-safe.
+class AppTimingHistory
+{
+public:
+    AppTimingHistory();
+    ~AppTimingHistory();
+
+    void Clear();
+    void SetScanoutTimeForFrame(uint32_t frameIndex, double scanoutTime);
+
+    // Returns 0.0 if not found.
+    double LookupScanoutTime(uint32_t frameIndex);
+
+protected:
+    static const int kFramesMax = 8;
+
+    struct Record
+    {
+        uint32_t FrameIndex;
+        double   ScanoutTime;
+    };
+
+    int    LastWriteIndex;
+    Record History[kFramesMax];
+};
+
+
+}} // namespace OVR::CAPI
+
+#endif // OVR_CAPI_DistortionTiming_h