add abr color dynamics

src/brush.h

@@ -54,6 +54,7 @@ public:
     float m_jitter_hue = 0;
     float m_jitter_sat = 0;
     float m_jitter_val = 0;
+    bool  m_jitter_hsv_eachsample = false;
     float m_jitter_aspect = 0;
     bool  m_jitter_aspect_bothaxis = false;
     int   m_blend_mode = 0;
@@ -172,6 +173,7 @@ public:
     float m_step = 0;
     float m_max_size = FLT_MAX;
     bool m_filter_points = true;
+    glm::vec3 m_tip_color;
     Camera m_camera;
     std::shared_ptr<Brush> m_brush;
     cbuffer<float> m_direction{ 1 };
@@ -182,11 +184,18 @@ public:
     std::vector<std::pair<glm::vec3, float>> m_hold_points;
     std::vector<StrokeSample> m_samples;
     int m_last_kp;
-    std::minstd_rand prng;
+    std::mt19937 prng;
     void start(const std::shared_ptr<Brush>& brush);
     void add_point(glm::vec3 pos, float pressure);
     void reset(bool clear_keypoints = false);
     bool has_sample();
     std::vector<StrokeSample> compute_samples();
     StrokeSample randomize_sample(const glm::vec3& pos, float pressure, float curve_angle);
+
+    void randomize_prng();
+    float rnd_nor() { return float((double)prng() / (double)prng.max()); };                     // normalized [0, +1]
+    float rnd_neg() { return float((double)prng() / (double)prng.max() * 2.0 - 1.0); };         // normalized [-1, +1]
+    float rnd_rad() { return float((double)prng() / (double)prng.max() * M_PI * 2.0); };        // normalized [0, 2pi]
+    glm::vec3 rnd_vec() { float rad = rnd_rad(); return glm::vec3(cosf(rad), sinf(rad), 0); };  // normalized direction vector
+    float rnd_bneg() { return prng() % 2 == 0 ? -1.f : 1.f; };                                  // -1 or 1
 };