Implement a slider widget for volume control that returns an exponentially scaled value

OpenRA.Mods.Common/Widgets/ExponentialSliderWidget.cs

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+#region Copyright & License Information
+/*
+ * Copyright 2007-2018 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System;
+using System.Drawing;
+using OpenRA.Graphics;
+using OpenRA.Widgets;
+
+namespace OpenRA.Mods.Common.Widgets
+{
+	public class ExponentialSliderWidget : SliderWidget
+	{
+		// Values are according to https://www.dr-lex.be/info-stuff/volumecontrols.html
+		public double ExpA = 1.0e-3;
+		public double ExpB = 6.908;
+
+		public ExponentialSliderWidget() :
+			base() { }
+		public ExponentialSliderWidget(ExponentialSliderWidget other) :
+			base(other) { }
+
+		float ExpFromLinear(float x)
+		{
+			if (x <= 0)
+				return 0;
+
+			return (float)(ExpA * Math.Exp(ExpB * x)).Clamp(0.0, 1.0);
+		}
+
+		float LinearFromExp(float x)
+		{
+			return (float)(Math.Log(x / ExpA) / ExpB).Clamp(0.0, 1.0);
+		}
+
+		protected override float ValueFromPx(int x)
+		{
+			return MinimumValue + (MaximumValue - MinimumValue) * ExpFromLinear((x - 0.5f * RenderBounds.Height) / (RenderBounds.Width - RenderBounds.Height));
+		}
+
+		protected override int PxFromValue(float x)
+		{
+			return (int)(0.5f * RenderBounds.Height + (RenderBounds.Width - RenderBounds.Height) * LinearFromExp((x - MinimumValue) / (MaximumValue - MinimumValue)));
+		}
+	}
+}