better smooth-linear function

src/net/torvald/terrarum/modulebasegame/clut/Skybox.kt

@@ -1,6 +1,5 @@
 package net.torvald.terrarum.modulebasegame.clut
 
-import com.badlogic.gdx.graphics.Color
 import com.badlogic.gdx.graphics.Pixmap
 import com.badlogic.gdx.graphics.Texture
 import com.badlogic.gdx.utils.Disposable
@@ -13,7 +12,6 @@ import net.torvald.terrarum.App
 import net.torvald.terrarum.App.printdbg
 import net.torvald.terrarum.abs
 import net.torvald.terrarum.modulebasegame.worldgenerator.HALF_PI
-import java.lang.Math.pow
 import kotlin.math.*
 
 /**
@@ -128,7 +126,23 @@ object Skybox : Disposable {
     private fun Double.coerceInSmoothly(low: Double, high: Double): Double {
         val x = this.coerceIn(low, high)
         val x2 = ((x - low) * (high - low).pow(-1.0))
-        return FastMath.interpolateLinear(polynomialDecay2(0.5, 2, x2), low, high)
+//        return FastMath.interpolateLinear(polynomialDecay2(0.5, 2, x2), low, high)
+        return FastMath.interpolateLinear(smoothLinear(x2), low, high)
+    }
+
+    /**
+     * To get the idea what the fuck is going on here, please refer to https://www.desmos.com/calculator/d2otu5deub
+     */
+    private fun smoothLinear(x: Double): Double {
+        val k = 0.11269 // a magic number
+        val d = 1.127016653 // also a magic number
+        val a = -1.0 / 0.2
+        return if (x < k)
+            -a.pow(1) * x.pow(2)
+        else if (x > 1.0 - k)
+            a.pow(1) * (1.0 - x).pow(2) + 1.0
+        else
+            d * (x - 0.5) + 0.5
     }
 
     private fun getTexturmaps(albedo: Double): Array<Texture> {