From 24accdcab05b99e1b01a8f71d0f41609738d5d3c Mon Sep 17 00:00:00 2001
From: TheOmyNomy <theomynomy@nomy.sh>
Date: Mon, 9 Aug 2021 18:56:47 +1000
Subject: [PATCH] Add LegacyUtils class with non linear colour interpolation
 method

---
 .../Skinning/Legacy/LegacySliderBody.cs       | 12 ++--
 osu.Game/Utils/LegacyUtils.cs                 | 65 +++++++++++++++++++
 2 files changed, 69 insertions(+), 8 deletions(-)
 create mode 100644 osu.Game/Utils/LegacyUtils.cs

diff --git a/osu.Game.Rulesets.Osu/Skinning/Legacy/LegacySliderBody.cs b/osu.Game.Rulesets.Osu/Skinning/Legacy/LegacySliderBody.cs
index a8bb69c080..92941665e0 100644
--- a/osu.Game.Rulesets.Osu/Skinning/Legacy/LegacySliderBody.cs
+++ b/osu.Game.Rulesets.Osu/Skinning/Legacy/LegacySliderBody.cs
@@ -3,9 +3,9 @@
 
 using System;
 using osu.Framework.Extensions.Color4Extensions;
-using osu.Framework.Utils;
 using osu.Game.Rulesets.Osu.Objects;
 using osu.Game.Rulesets.Osu.Skinning.Default;
+using osu.Game.Utils;
 using osuTK.Graphics;
 
 namespace osu.Game.Rulesets.Osu.Skinning.Legacy
@@ -37,14 +37,10 @@ namespace osu.Game.Rulesets.Osu.Skinning.Legacy
 
                 position -= realBorderPortion;
 
-                // Stable interpolates slider body colour directly in sRGB space, and because
-                // Interpolation.ValueAt() uses linear space, we have to counteract applying it
-                // by calling ToSRGB() on the input colours, and ToLinear() on the resulting colour.
+                Color4 outerColour = AccentColour.Darken(0.1f);
+                Color4 innerColour = lighten(AccentColour, 0.5f);
 
-                Color4 outerColour = AccentColour.Darken(0.1f).ToSRGB();
-                Color4 innerColour = lighten(AccentColour, 0.5f).ToSRGB();
-
-                return Interpolation.ValueAt(position / realGradientPortion, outerColour, innerColour, 0, 1).ToLinear();
+                return LegacyUtils.InterpolateNonLinear(position / realGradientPortion, outerColour, innerColour, 0, 1);
             }
 
             /// <summary>
diff --git a/osu.Game/Utils/LegacyUtils.cs b/osu.Game/Utils/LegacyUtils.cs
new file mode 100644
index 0000000000..9351125acd
--- /dev/null
+++ b/osu.Game/Utils/LegacyUtils.cs
@@ -0,0 +1,65 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Transforms;
+using osuTK.Graphics;
+
+namespace osu.Game.Utils
+{
+    public static class LegacyUtils
+    {
+        public static Color4 InterpolateNonLinear(double time, Color4 startColour, Color4 endColour, double startTime, double endTime, Easing easing = Easing.None)
+            => InterpolateNonLinear(time, startColour, endColour, startTime, endTime, new DefaultEasingFunction(easing));
+
+        public static Colour4 InterpolateNonLinear(double time, Colour4 startColour, Colour4 endColour, double startTime, double endTime, Easing easing = Easing.None)
+            => InterpolateNonLinear(time, startColour, endColour, startTime, endTime, new DefaultEasingFunction(easing));
+
+        /// <summary>
+        /// Interpolates between two sRGB <see cref="Color4"/>s directly in sRGB space.
+        /// </summary>
+        public static Color4 InterpolateNonLinear<TEasing>(double time, Color4 startColour, Color4 endColour, double startTime, double endTime, TEasing easing) where TEasing : IEasingFunction
+        {
+            if (startColour == endColour)
+                return startColour;
+
+            double current = time - startTime;
+            double duration = endTime - startTime;
+
+            if (duration == 0 || current == 0)
+                return startColour;
+
+            float t = Math.Max(0, Math.Min(1, (float)easing.ApplyEasing(current / duration)));
+
+            return new Color4(
+                startColour.R + t * (endColour.R - startColour.R),
+                startColour.G + t * (endColour.G - startColour.G),
+                startColour.B + t * (endColour.B - startColour.B),
+                startColour.A + t * (endColour.A - startColour.A));
+        }
+
+        /// <summary>
+        /// Interpolates between two sRGB <see cref="Colour4"/>s directly in sRGB space.
+        /// </summary>
+        public static Colour4 InterpolateNonLinear<TEasing>(double time, Colour4 startColour, Colour4 endColour, double startTime, double endTime, in TEasing easing) where TEasing : IEasingFunction
+        {
+            if (startColour == endColour)
+                return startColour;
+
+            double current = time - startTime;
+            double duration = endTime - startTime;
+
+            if (duration == 0 || current == 0)
+                return startColour;
+
+            float t = Math.Max(0, Math.Min(1, (float)easing.ApplyEasing(current / duration)));
+
+            return new Colour4(
+                startColour.R + t * (endColour.R - startColour.R),
+                startColour.G + t * (endColour.G - startColour.G),
+                startColour.B + t * (endColour.B - startColour.B),
+                startColour.A + t * (endColour.A - startColour.A));
+        }
+    }
+}