implement brush sampling and spacing into the Stroke class

engine/brush.h

@@ -23,20 +23,82 @@ public:
 class Stroke
 {
 public:
+    struct Sample
+    {
+        glm::vec2 pos;
+        float size;
+        float flow;
+        float angle;
+    };
     struct Keypoint
     {
         glm::vec2 pos;
         float pressure;
+        float dist;
     };
     int m_layer;
     float m_dist;
+    float m_step;
     ui::Brush m_brush;
     std::vector<Keypoint> m_keypoints;
+    std::vector<Sample> m_samples;
+    int m_last_kp;
+    std::minstd_rand prng;
+    void start(glm::vec2 pos, float pressure, const ui::Brush& brush)
+    {
+        m_last_kp = 0;
+        m_dist = 0.f;
+        m_step = glm::max(brush.m_tip_spacing * brush.m_tip_size * 30, 0.1f);
+        m_brush = brush;
+        add_point(pos, pressure);
+    }
     void add_point(glm::vec2 pos, float pressure)
     {
+        float dist = m_keypoints.empty() ? 0.f : 
+            m_keypoints.back().dist + glm::distance(m_keypoints.back().pos, pos);
         m_keypoints.emplace_back();
         m_keypoints.back().pos = pos;
         m_keypoints.back().pressure = pressure;
+        m_keypoints.back().dist = dist;
+    }
+    bool has_sample()
+    {
+        return m_keypoints.empty() ? false :                // no keypoints
+            (m_keypoints.back().dist > (m_dist + m_step));  // check if next kp is closer than spacing
+    }
+    std::vector<Sample> compute_samples()
+    {
+        int nsamples = (int)glm::floor((m_keypoints.back().dist - m_dist) / m_step);
+        std::vector<Sample> samples;
+        samples.reserve(nsamples); // preallocate the estimate number of samples
+        while (m_keypoints.back().dist > (m_dist + m_step))
+        {
+            m_dist += m_step;
+            while (m_dist > m_keypoints[m_last_kp + 1].dist)
+                m_last_kp++;
+            const auto& A = m_keypoints[m_last_kp];
+            const auto& B = m_keypoints[m_last_kp + 1]; // NOTE: this should be true when while is true
+            float t = (m_dist - A.dist) / (B.dist - A.dist); // NOTE: must be A != B
+            auto pos = glm::lerp(A.pos, B.pos, t);
+            float pressure = glm::lerp(A.pressure, B.pressure, t);
+            auto s = randomize_sample(pos, pressure);
+            samples.push_back(s);
+        }
+        return std::move(samples);
+    }
+    Sample randomize_sample(const glm::vec2& pos, float pressure)
+    {
+        auto rnd_nor = [&] { return float((double)prng() / (double)prng.max()); };              // normalized [0, +1]
+        auto rnd_neg = [&] { return float((double)prng() / (double)prng.max() * 2.0 - 1.0); };  // normalized [-1, +1]
+        auto rnd_rad = [&] { return float((double)prng() / (double)prng.max() * M_PI * 2.0); }; // normalized [0, 2pi]
+        auto rnd_vec = [&] { float rad = rnd_rad(); return glm::vec2(cosf(rad), sinf(rad)); };  // normalized direction vector
+
+        Sample s;
+        s.angle = (m_brush.m_tip_angle + rnd_nor() * m_brush.m_jitter_angle) * (float)(M_PI * 2.0);
+        s.pos = pos + (rnd_vec() * m_brush.m_jitter_spread * 100.f);
+        s.size = 100.f * m_brush.m_tip_size * (1.f - rnd_nor() * m_brush.m_jitter_scale);
+        s.flow = m_brush.m_tip_flow * (1.f - rnd_nor() * m_brush.m_jitter_flow);
+        return s;
     }
 };