add: GravityFilter

pasada-lib/step_detector.cpp

@@ -4,6 +4,8 @@
 
 #include "step_detector.h"
 
+#include "pd_signal.h"
+
 StepDetector::StepDetector(double fps, StepListener *listener, bool debug) :
     listener(listener),
     f_neg(1, 0, 0, std::vector<double> {-1.0}),
@@ -13,6 +15,36 @@ StepDetector::StepDetector(double fps, StepListener *listener, bool debug) :
     debug(debug)
 {}
 
+static int gravity_num_taps(double fps) {
+    return 5.0 * fps;
+}
+
+// 5 secs buffer, prime y with direction of gravity (for tests & faster init)
+GravityFilter::GravityFilter(double fps) :
+    N(gravity_num_taps(fps)),
+    gauss_taps(pd_signal::gauss(N, N/2, N/4)),
+    gx(N, 0, 0, gauss_taps),
+    gy(N, 0, 0, gauss_taps),
+    gz(N, 0, 0, gauss_taps)
+{
+    gy.prime(-9.81);
+}
+
+double GravityFilter::filter(std::vector<float> values) {
+    gx.push(values[0]);
+    gy.push(values[1]);
+    gz.push(values[2]);
+    double x = gx.peek(), y = gy.peek(), z = gz.peek();
+    double g = sqrt(x * x + y * y + z * z);
+    // e = mean(a)
+    double ex = x / g, ey = y / g, ez = z / g;
+    // e \in a
+    double vx = values[0] * ex;
+    double vy = values[1] * ey;
+    double vz = values[2] * ez;
+    return vx + vy + vz;
+}
+
 void StepDetector::filter(std::vector<float> values) {
     // TODO: later on, we should use a vector projection towards gravity
     auto s1 = (double) values[1]; // take y-axis value for now