fix black sample

engine/brush.cpp

@@ -183,6 +183,8 @@ std::vector<ui::StrokeSample> ui::Stroke::compute_samples()
             s.angle += curve_angle;
         }
         m_prev_sample = s;
+        if (!s.valid())
+            LOG("Invalid sample");
         samples.push_back(s);
     }
     return std::move(samples);
@@ -215,6 +217,8 @@ void ui::Stroke::add_point(glm::vec2 pos, float pressure)
         m_keypoints.back().dist + glm::distance(m_keypoints.back().pos, pos);
     if (m_keypoints.empty())
         m_prev_sample.origin = pos;
+    else if (m_keypoints.back().pos == pos)
+        return; // skip same point, leading to black samples (NaN values)
     Keypoint kp;
     kp.pos = pos;
     kp.pressure = pressure;