apply fix for helicopter screen Hz rate

Author: segrax

Source/Briefing.cpp

@@ -122,30 +122,30 @@ void cFodder::Briefing_Helicopter_Check(double dtSeconds)
 
     mBriefingHelicopter_TransitionCounterSeconds += dtSeconds;
 
-    auto doTransitionStepIfDue = [&]() -> bool {
+    auto consumeTransitionSteps = [&]() -> int {
         if (mBriefingHelicopter_TransitionCounterSeconds < kTransitionStepSeconds)
-            return false;
-        // keep remainder so it’s stable under varying dt
-        mBriefingHelicopter_TransitionCounterSeconds =
-            std::fmod(mBriefingHelicopter_TransitionCounterSeconds, kTransitionStepSeconds);
-        return true;
+            return 0;
+
+        const int steps = (int)(mBriefingHelicopter_TransitionCounterSeconds / kTransitionStepSeconds);
+        mBriefingHelicopter_TransitionCounterSeconds -= (double)steps * kTransitionStepSeconds;
+        return steps;
         };
 
     // ---- “Fade in until ready, then start heli movement” ----
     if (!mMouse_Exit_Loop || !mPhase_Aborted) {
 
         if (mBriefingHelicopter_ScreenX > 0x30) {
 
-            if (doTransitionStepIfDue()) {
+            int transitionSteps = consumeTransitionSteps();
+            while (transitionSteps-- > 0) {
                 // Original: 16 steps total for fade-in.
                 if (mBriefingHelicopter_Moving < 16) {
                     mBriefingHelicopter_Moving++;
                     mPaletteLevel++;
-                    return;
                 }
             }
 
-            // Once fade-in complete, advance helicopter sim (60Hz tick wrapper)
+            // Once fade-in complete, advance helicopter sim (50Hz tick wrapper)
             if (mBriefingHelicopter_Moving >= 16) {
                 Briefing_Update_Helicopter_Frame(dtSeconds);
             }
@@ -154,15 +154,19 @@ void cFodder::Briefing_Helicopter_Check(double dtSeconds)
     }
 
     // ---- “Fade out / exit path” ----
-    if (doTransitionStepIfDue()) {
+    int transitionSteps = consumeTransitionSteps();
+    while (transitionSteps-- > 0) {
         if (mBriefingHelicopter_Moving == 0 && mBriefingHelicopter_NotDone) {
             mBriefingHelicopter_NotDone = 0;
             mGraphics->mImageMissionIntro.CopyPalette(mGraphics->mPalette, 0x100, 0);
+            break;
         }
         else {
             if (mBriefingHelicopter_Moving > 0 || mPaletteLevel > 0) {
-                mBriefingHelicopter_Moving--;
-                mPaletteLevel--;
+                if (mBriefingHelicopter_Moving > 0)
+                    mBriefingHelicopter_Moving--;
+                if (mPaletteLevel > 0)
+                    mPaletteLevel--;
                 mSurface->paletteNew_SetToBlack();
             }
         }
@@ -448,7 +452,10 @@ void cFodder::Briefing_Intro_Helicopter_Start() {
 
     mBriefingHelicopter_TransitionCounterSeconds = 0;
     mBriefingHelicopter_AccumSeconds = 0.0;
-    mBriefingHelicopter_StepSeconds = 1.0 / 60.0;
+    mHeliIntro_AccumSeconds = 0.0;
+    // Original briefing helicopter simulation cadence is PAL 50Hz.
+    // Keep this in sync with offsets/countdowns that are defined in 50Hz ticks.
+    mBriefingHelicopter_StepSeconds = 1.0 / 50.0;
     mHeliParallax_SubPx = 0;
     mHeliText_SubPx = 0;
 

Source/Fodder.hpp

@@ -565,6 +565,7 @@ class cFodder {
     double          mBriefingHelicopter_AccumSeconds = 0.0;
     double          mBriefingHelicopter_StepSeconds = 1.0 / 50.0;
     double          mBriefingHelicopter_TransitionCounterSeconds = 0;
+    double          mHeliIntro_AccumSeconds = 0.0;
     double          mHeliParallax_SubPx = 0.0;     // accumulates fractional scroll in 16.16 units
     double          mHeliText_SubPx = 0.0;		// accumulates fractional text movement in pixels
 

Source/Graphics.cpp

@@ -165,46 +165,23 @@ void cGraphics::Sidebar_Copy_ScreenBuffer_Common(uint16 pRow, int16 pRows, int16
 
 void cGraphics::HeliIntro_TickParallaxAndText(double dtSeconds)
 {
-    // ----- Text slide (PAL) -----
-    //
-    // Original Amiga PAL behaviour:
-    //   Each step moves 4 pixels
-    // => 4 * 50 = 200 px/sec toward 0x0C.
-    const double textVelPxPerSec = 200.0;
-
-    if (Heli_TextPosBottom != 0x0C) {
-        g_Fodder->mHeliText_SubPx += textVelPxPerSec * dtSeconds;
-
-        const int32_t stepPx = (int32_t)std::floor(g_Fodder->mHeliText_SubPx);
-        if (stepPx > 0) {
-            g_Fodder->mHeliText_SubPx -= (double)stepPx;
-            Heli_TextPosBottom -= stepPx;
-
-            if (Heli_TextPosBottom <= 0x0C) {
-                Heli_TextPosBottom = 0x0C;
-                g_Fodder->mHeliText_SubPx = 0.0;
-            }
-        }
-    }
-
-    Heli_TextPos = 344 - Heli_TextPosBottom;
+    // Keep briefing intro parallax and text on fixed PAL 50Hz ticks.
+    // This mirrors original cadence and avoids variable-dt jitter.
+    const double tickSeconds = 1.0 / 50.0;
+    g_Fodder->mHeliIntro_AccumSeconds += dtSeconds;
 
-    // ----- Parallax scroll (PAL) -----
-    //
-    // Original behaviour per PAL frame (50Hz):
-    //   baseSpeed = 0x8000 (16.16 units) per frame
-    // => per-second speed = 0x8000 * 50 (16.16 units/sec)
-    const double baseSpeed_16_16_per_sec = (double)0x8000 * 50.0;
+    const int32_t wrap = 320 << 16;
 
-    g_Fodder->mHeliParallax_SubPx += baseSpeed_16_16_per_sec * dtSeconds;
+    while (g_Fodder->mHeliIntro_AccumSeconds >= tickSeconds) {
+        g_Fodder->mHeliIntro_AccumSeconds -= tickSeconds;
 
-    const int32_t baseStep = (int32_t)std::floor(g_Fodder->mHeliParallax_SubPx);
-    if (baseStep != 0) {
-        g_Fodder->mHeliParallax_SubPx -= (double)baseStep;
-
-        const int32_t wrap = 320 << 16;
+        if (Heli_TextPosBottom > 0x0C) {
+            Heli_TextPosBottom -= 4;
+            if (Heli_TextPosBottom < 0x0C)
+                Heli_TextPosBottom = 0x0C;
+        }
 
-        int32_t d0 = baseStep;
+        int32_t d0 = 0x8000;
 
         Heli_VeryBack -= d0;
         if (Heli_VeryBack < 0) Heli_VeryBack += wrap;
@@ -221,4 +198,6 @@ void cGraphics::HeliIntro_TickParallaxAndText(double dtSeconds)
         Heli_Front -= d0;
         if (Heli_Front < 0) Heli_Front += wrap;
     }
+
+    Heli_TextPos = 344 - Heli_TextPosBottom;
 }