Refactor game loop to use a semi-fixed timestep. (#5848)

Fix headless game being jumpy/stuttering.
Bump up network version.

src/openrct2/Context.cpp

@@ -82,7 +82,7 @@ namespace OpenRCT2
 
         bool _isWindowMinimised = false;
         uint32 _lastTick = 0;
-        uint32 _uncapTick = 0;
+        uint32 _accumulator = 0;
 
         /** If set, will end the OpenRCT2 game loop. Intentially private to this module so that the flag can not be set back to false. */
         bool _finished = false;
@@ -344,6 +344,15 @@ namespace OpenRCT2
             RunGameLoop();
         }
 
+        bool ShouldRunVariableFrame()
+        {
+            if (!gConfigGeneral.uncap_fps) return false;
+            if (gGameSpeed > 4) return false;
+            if (gOpenRCT2Headless) return false;
+            if (_uiContext->IsMinimised()) return false;
+            return true;
+        }
+
         /**
         * Run the main game loop until the finished flag is set.
         */
@@ -351,9 +360,21 @@ namespace OpenRCT2
         {
             log_verbose("begin openrct2 loop");
             _finished = false;
+
+            bool variableFrame = ShouldRunVariableFrame();
+            bool useVariableFrame;
+
             do
             {
-                if (ShouldRunVariableFrame())
+                useVariableFrame = ShouldRunVariableFrame();
+                // Make sure we catch the state change and reset it.
+                if (variableFrame != useVariableFrame)
+                {
+                    _lastTick = 0;
+                    variableFrame = useVariableFrame;
+                }
+
+                if (useVariableFrame)
                 {
                     RunVariableFrame();
                 }
@@ -365,32 +386,32 @@ namespace OpenRCT2
             log_verbose("finish openrct2 loop");
         }
 
-        bool ShouldRunVariableFrame()
-        {
-            if (!gConfigGeneral.uncap_fps) return false;
-            if (gGameSpeed > 4) return false;
-            if (gOpenRCT2Headless) return false;
-            if (_uiContext->IsMinimised()) return false;
-            return true;
-        }
-
         void RunFixedFrame()
         {
-            _uncapTick = 0;
             uint32 currentTick = platform_get_ticks();
-            uint32 ticksElapsed = currentTick - _lastTick;
-            if (ticksElapsed < UPDATE_TIME_MS)
-            {
-                platform_sleep(UPDATE_TIME_MS - ticksElapsed);
-                _lastTick += UPDATE_TIME_MS;
-            }
-            else
+
+            if (_lastTick == 0)
             {
                 _lastTick = currentTick;
             }
+
+            uint32 elapsed = currentTick - _lastTick;
+
+            _lastTick = currentTick;
+            _accumulator += elapsed;
+
             GetContext()->GetUiContext()->ProcessMessages();
+
+            if (_accumulator < UPDATE_TIME_MS)
+            {
+                platform_sleep(UPDATE_TIME_MS - _accumulator - 1);
+                return;
+            }
+
+            _accumulator -= UPDATE_TIME_MS;
+
             rct2_update();
-            if (!_isWindowMinimised)
+            if (!_isWindowMinimised && !gOpenRCT2Headless)
             {
                 platform_draw();
             }
@@ -399,43 +420,50 @@ namespace OpenRCT2
         void RunVariableFrame()
         {
             uint32 currentTick = platform_get_ticks();
-            if (_uncapTick == 0)
+
+            bool draw = !_isWindowMinimised && !gOpenRCT2Headless;
+
+            if (_lastTick == 0)
             {
-                _uncapTick = currentTick;
                 sprite_position_tween_reset();
+                _lastTick = currentTick;
             }
 
-            // Limit number of updates per loop (any long pauses or debugging can make this update for a very long time)
-            if (currentTick - _uncapTick > UPDATE_TIME_MS * 60)
-            {
-                _uncapTick = currentTick - UPDATE_TIME_MS - 1;
-            }
+            uint32 elapsed = currentTick - _lastTick;
+
+            if (elapsed > UPDATE_TIME_MS)
+                elapsed = UPDATE_TIME_MS;
+
+            _lastTick = currentTick;
+            _accumulator += elapsed;
 
             GetContext()->GetUiContext()->ProcessMessages();
 
-            while (_uncapTick <= currentTick && currentTick - _uncapTick > UPDATE_TIME_MS)
+            while (_accumulator >= UPDATE_TIME_MS)
             {
                 // Get the original position of each sprite
+                if(draw)
                     sprite_position_tween_store_a();
 
-                // Update the game so the sprite positions update
                 rct2_update();
 
-                // Get the next position of each sprite
-                sprite_position_tween_store_b();
+                _accumulator -= UPDATE_TIME_MS;
 
-                _uncapTick += UPDATE_TIME_MS;
+                // Get the next position of each sprite
+                if(draw)
+                    sprite_position_tween_store_b();
             }
 
-            // Tween the position of each sprite from the last position to the new position based on the time between the last
-            // tick and the next tick.
-            float nudge = 1 - ((float)(currentTick - _uncapTick) / UPDATE_TIME_MS);
-            sprite_position_tween_all(nudge);
+            if (draw)
+            {
+                const float alpha = (float)_accumulator / UPDATE_TIME_MS;
+                sprite_position_tween_all(alpha);
 
                 platform_draw();
 
                 sprite_position_tween_restore();
             }
+        }
 
         bool OpenParkAutoDetectFormat(IStream * stream)
         {

src/openrct2/network/network.h

@@ -55,7 +55,7 @@ extern "C" {
 // This define specifies which version of network stream current build uses.
 // It is used for making sure only compatible builds get connected, even within
 // single OpenRCT2 version.
-#define NETWORK_STREAM_VERSION "30"
+#define NETWORK_STREAM_VERSION "31"
 #define NETWORK_STREAM_ID OPENRCT2_VERSION "-" NETWORK_STREAM_VERSION
 
 #ifdef __cplusplus

src/openrct2/world/sprite.c

@@ -758,8 +758,10 @@ void sprite_position_tween_store_b()
     store_sprite_locations(_spritelocations2);
 }
 
-void sprite_position_tween_all(float nudge)
+void sprite_position_tween_all(float alpha)
 {
+    const float inv = (1.0f - alpha);
+
     for (uint16 i = 0; i < MAX_SPRITES; i++) {
         rct_sprite * sprite = get_sprite(i);
         if (sprite_should_tween(sprite)) {
@@ -767,9 +769,9 @@ void sprite_position_tween_all(float nudge)
             rct_xyz16 posB = _spritelocations2[i];
 
             sprite_set_coordinates(
-                posB.x + (sint16)((posA.x - posB.x) * nudge),
-                posB.y + (sint16)((posA.y - posB.y) * nudge),
-                posB.z + (sint16)((posA.z - posB.z) * nudge),
+                posB.x * alpha + posA.x * inv,
+                posB.y * alpha + posA.y * inv,
+                posB.z * alpha + posA.z * inv,
                 sprite
             );
             invalidate_sprite_2(sprite);