Fixed potential render scene time precision issues.

The timestamp was simply converted from GetTimeAtCurrentTick to a float. Since this value is backed by QueryPerformanceCounter which is 0 at boot, you could see broken animations on the GPU when your system has been on for a long time. So I simplified the RPI's time API (removed unused code), and subtracted the application start time each frame before converting the time value to a float.
Also moved FindShaderInputConstantIndex("m_time") to be called only once, instead of every frame.

Signed-off-by: santorac <55155825+santorac@users.noreply.github.com>

Gems/Atom/RPI/Code/Include/Atom/RPI.Public/RPISystem.h

@@ -96,8 +96,7 @@ namespace AZ
             // SystemTickBus::OnTick
             void OnSystemTick() override;
 
-            // Fill system time and game time information for simulation or rendering
-            void FillTickTimeInfo();
+            float GetCurrentTime();
 
             // The set of core asset handlers registered by the system.
             AZStd::vector<AZStd::unique_ptr<Data::AssetHandler>> m_assetHandlers;
@@ -123,8 +122,9 @@ namespace AZ
             // The job policy used for feature processor's rendering prepare
             RHI::JobPolicy m_prepareRenderJobPolicy = RHI::JobPolicy::Parallel;
 
-            TickTimeInfo m_tickTime;
-
+            ScriptTimePoint m_startTime;
+			float m_currentSimulationTime = 0.0f;
+			
             RPISystemDescriptor m_descriptor;
 
             // Reference to the shader asset that is used

Gems/Atom/RPI/Code/Include/Atom/RPI.Public/RenderPipeline.h

@@ -32,7 +32,6 @@ namespace AZ
     namespace RPI
     {
         class Scene;
-        struct TickTimeInfo;
         class ShaderResourceGroup;
         class AnyAsset;
         class WindowContext;
@@ -203,7 +202,7 @@ namespace AZ
             void OnRemovedFromScene(Scene* scene);
 
             // Called when this pipeline is about to be rendered
-            void OnStartFrame(const TickTimeInfo& tick);
+            void OnStartFrame(float time);
 
             // Called when the rendering of current frame is finished.
             void OnFrameEnd();

Gems/Atom/RPI/Code/Include/Atom/RPI.Public/Scene.h

@@ -47,14 +47,6 @@ namespace AZ
         // Callback function to modify values of a ShaderResourceGroup
         using ShaderResourceGroupCallback = AZStd::function<void(ShaderResourceGroup*)>;
 
-        //! A structure for ticks which contains system time and game time.
-        struct TickTimeInfo
-        {
-            float m_currentGameTime;
-            float m_gameDeltaTime = 0;
-        };
-
-
         class Scene final
             : public SceneRequestBus::Handler
         {
@@ -173,12 +165,14 @@ namespace AZ
                         
             // Cpu simulation which runs all active FeatureProcessor Simulate() functions.
             // @param jobPolicy if it's JobPolicy::Parallel, the function will spawn a job thread for each FeatureProcessor's simulation.
-            void Simulate(const TickTimeInfo& tickInfo, RHI::JobPolicy jobPolicy);
+            // @param simulationTime the number of seconds since the application started
+            void Simulate(RHI::JobPolicy jobPolicy, float simulationTime);
 
             // Collect DrawPackets from FeatureProcessors
             // @param jobPolicy if it's JobPolicy::Parallel, the function will spawn a job thread for each FeatureProcessor's
             // PrepareRender.
-            void PrepareRender(const TickTimeInfo& tickInfo, RHI::JobPolicy jobPolicy);
+            // @param simulationTime the number of seconds since the application started; this is the same time value that was passed to Simulate()
+            void PrepareRender(RHI::JobPolicy jobPolicy, float simulationTime);
 
             // Function called when the current frame is finished rendering.
             void OnFrameEnd();
@@ -240,6 +234,7 @@ namespace AZ
             // Registry which allocates draw filter tag for RenderPipeline
             RHI::Ptr<RHI::DrawFilterTagRegistry> m_drawFilterTagRegistry;
 
+            RHI::ShaderInputConstantIndex m_timeInputIndex;
             float m_simulationTime;
         };