viscous_fluid.h

/*
 * AUI Framework - Declarative UI toolkit for modern C++20
 * Copyright (C) 2020-2025 Alex2772 and Contributors
 *
 * SPDX-License-Identifier: MPL-2.0
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 */
 
#pragma once
 
#include <glm/glm.hpp>
 
namespace aui::animation_curves {
   class ViscousFluid {
   public:
       float operator()(float input);
 
   private:
       // controls the viscous fluid effect (how much of it).
       constexpr static float VISCOUS_FLUID_SCALE = 8.0f;
 
       constexpr static float viscousFluid(float x) {
           x *= VISCOUS_FLUID_SCALE;
           if (x < 1.0f) {
               x -= (1.0f - (float)glm::exp(-x));
           } else {
               float start = 0.36787944117f;   // 1/e == exp(-1)
               x = 1.0f - (float)glm::exp(1.0f - x);
               x = start + x * (1.0f - start);
           }
           return x;
       }
   };
 
   inline float ViscousFluid::operator()(float input) {
 
       // must be set to 1.0 (used in viscousFluid())
       const float VISCOUS_FLUID_NORMALIZE = 1.0f / viscousFluid(1.0f);
 
       // account for very small floating-point error
       const float VISCOUS_FLUID_OFFSET = 1.0f - VISCOUS_FLUID_NORMALIZE * viscousFluid(1.0f);
 
       float interpolated = VISCOUS_FLUID_NORMALIZE * viscousFluid(input);
       if (interpolated > 0) {
           return interpolated + VISCOUS_FLUID_OFFSET;
       }
       return interpolated;
   }
}

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