From f76d13920776ca5be95f1fe1c59420011b804dc5 Mon Sep 17 00:00:00 2001
From: Christoph Oelckers <coelckers@users.noreply.github.com>
Date: Sun, 2 Feb 2020 09:51:44 +0100
Subject: [PATCH] - fixed SW's interpolation math.

---
 source/build/include/clockticks.hpp |  4 ++++
 source/sw/src/draw.cpp              | 15 ++++++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/source/build/include/clockticks.hpp b/source/build/include/clockticks.hpp
index c4de5584d..d4a6948dd 100644
--- a/source/build/include/clockticks.hpp
+++ b/source/build/include/clockticks.hpp
@@ -44,6 +44,10 @@ public:
     {
         return ticksS32 >> 16;
     }
+    double toScale16F() const
+    {
+        return ticksS32 * (1/(65536.*65536.));
+    }
     ClockTicks& setFromScale16(int64_t const ticksScale16)
     {
         ticksS32 = ticksScale16 << 16;
diff --git a/source/sw/src/draw.cpp b/source/sw/src/draw.cpp
index dc1ed5609..0f3d7a2da 100644
--- a/source/sw/src/draw.cpp
+++ b/source/sw/src/draw.cpp
@@ -1994,6 +1994,19 @@ SWBOOL PicInView(short, SWBOOL);
 void DoPlayerDiveMeter(PLAYERp pp);
 void MoveScrollMode2D(PLAYERp pp);
 
+
+static inline int32_t calc_smoothratio_demo(ClockTicks totalclk, ClockTicks ototalclk)
+{
+    const int TICRATE = 120;
+    const int REALGAMETICSPERSEC = TICRATE / synctics;
+
+    int32_t rfreq = (refreshfreq != -1 ? refreshfreq : 60);
+    double elapsedTime = (totalclk - ototalclk).toScale16F();
+    double elapsedFrames = elapsedTime * rfreq * (1. / TICRATE);
+    double ratio = (elapsedFrames * REALGAMETICSPERSEC) / rfreq;
+    return clamp(xs_RoundToInt(ratio * 65536), 0, 65536);
+}
+
 void
 drawscreen(PLAYERp pp)
 {
@@ -2063,7 +2076,7 @@ drawscreen(PLAYERp pp)
     PreUpdatePanel();
 
 
-    smoothratio = min(max(((int32_t) totalclock - ototalclock) * (65536 / synctics),0),65536);
+    smoothratio = calc_smoothratio_demo(totalclock, ototalclock);
 
     if (!ScreenSavePic)
     {