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      1 /*
      2  * Copyright (C) 2016 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 #include "calibration/gyroscope/gyro_stillness_detect.h"
     18 #include <string.h>
     19 
     20 /////// FORWARD DECLARATIONS /////////////////////////////////////////
     21 
     22 // Enforces the limits of an input value [0,1].
     23 static float gyroStillDetLimit(float value);
     24 
     25 /////// FUNCTION DEFINITIONS /////////////////////////////////////////
     26 
     27 // Initialize the GyroStillDet structure.
     28 void gyroStillDetInit(struct GyroStillDet* gyro_still_det,
     29                       float var_threshold, float confidence_delta) {
     30   // Clear all data structure variables to 0.
     31   memset(gyro_still_det, 0, sizeof(struct GyroStillDet));
     32 
     33   // Set the delta about the variance threshold for calculation
     34   // of the stillness confidence score.
     35   if (confidence_delta < var_threshold) {
     36     gyro_still_det->confidence_delta = confidence_delta;
     37   } else {
     38     gyro_still_det->confidence_delta = var_threshold;
     39   }
     40 
     41   // Set the variance threshold parameter for the stillness
     42   // confidence score.
     43   gyro_still_det->var_threshold = var_threshold;
     44 
     45   // Signal to start capture of next stillness data window.
     46   gyro_still_det->start_new_window = true;
     47 }
     48 
     49 // Update the stillness detector with a new sample.
     50 void gyroStillDetUpdate(struct GyroStillDet* gyro_still_det,
     51                         uint64_t stillness_win_endtime, uint64_t sample_time,
     52                         float x, float y, float z) {
     53   // Using the method of the assumed mean to preserve some numerical
     54   // stability while avoiding per-sample divisions that the more
     55   // numerically stable Welford method would afford.
     56 
     57   // Reference for the numerical method used below to compute the
     58   // online mean and variance statistics:
     59   //   1). en.wikipedia.org/wiki/assumed_mean
     60 
     61   float delta = 0;
     62 
     63   // If the window end time is not valid then wait till it is.
     64   if (stillness_win_endtime <= 0) {
     65     return;
     66   }
     67 
     68   // Increment the number of samples.
     69   gyro_still_det->num_acc_samples++;
     70 
     71   // Online computation of mean for the running stillness period.
     72   gyro_still_det->mean_x += x;
     73   gyro_still_det->mean_y += y;
     74   gyro_still_det->mean_z += z;
     75 
     76   // Is this the first sample of a new window?
     77   if (gyro_still_det->start_new_window) {
     78     // Record the window start time.
     79     gyro_still_det->window_start_time = sample_time;
     80     gyro_still_det->start_new_window = false;
     81 
     82     // Update assumed mean values.
     83     gyro_still_det->assumed_mean_x = x;
     84     gyro_still_det->assumed_mean_y = y;
     85     gyro_still_det->assumed_mean_z = z;
     86 
     87     // Reset current window mean and variance.
     88     gyro_still_det->num_acc_win_samples = 0;
     89     gyro_still_det->win_mean_x = 0;
     90     gyro_still_det->win_mean_y = 0;
     91     gyro_still_det->win_mean_z = 0;
     92     gyro_still_det->acc_var_x = 0;
     93     gyro_still_det->acc_var_y = 0;
     94     gyro_still_det->acc_var_z = 0;
     95   } else {
     96     // Check to see if we have enough samples to compute a stillness
     97     // confidence score.
     98     gyro_still_det->stillness_window_ready =
     99         (sample_time >= stillness_win_endtime) &&
    100         (gyro_still_det->num_acc_samples > 1);
    101   }
    102 
    103   // Record the most recent sample time stamp.
    104   gyro_still_det->last_sample_time = sample_time;
    105 
    106   // Online window mean and variance ("one-pass" accumulation).
    107   gyro_still_det->num_acc_win_samples++;
    108 
    109   delta = (x - gyro_still_det->assumed_mean_x);
    110   gyro_still_det->win_mean_x += delta;
    111   gyro_still_det->acc_var_x += delta * delta;
    112 
    113   delta = (y - gyro_still_det->assumed_mean_y);
    114   gyro_still_det->win_mean_y += delta;
    115   gyro_still_det->acc_var_y += delta * delta;
    116 
    117   delta = (z - gyro_still_det->assumed_mean_z);
    118   gyro_still_det->win_mean_z += delta;
    119   gyro_still_det->acc_var_z += delta * delta;
    120 }
    121 
    122 // Calculates and returns the stillness confidence score [0,1].
    123 float gyroStillDetCompute(struct GyroStillDet* gyro_still_det) {
    124   float tmp_denom = 1.f;
    125   float tmp_denom_mean = 1.f;
    126 
    127   // Don't divide by zero (not likely, but a precaution).
    128   if (gyro_still_det->num_acc_win_samples > 1) {
    129     tmp_denom = 1.f / (gyro_still_det->num_acc_win_samples - 1);
    130     tmp_denom_mean = 1.f / gyro_still_det->num_acc_win_samples;
    131   } else {
    132     // Return zero stillness confidence.
    133     gyro_still_det->stillness_confidence = 0;
    134     return gyro_still_det->stillness_confidence;
    135   }
    136 
    137   // Update the final calculation of window mean and variance.
    138   float tmp = gyro_still_det->win_mean_x;
    139   gyro_still_det->win_mean_x *= tmp_denom_mean;
    140   gyro_still_det->win_var_x =
    141       (gyro_still_det->acc_var_x - gyro_still_det->win_mean_x * tmp) *
    142       tmp_denom;
    143 
    144   tmp = gyro_still_det->win_mean_y;
    145   gyro_still_det->win_mean_y *= tmp_denom_mean;
    146   gyro_still_det->win_var_y =
    147       (gyro_still_det->acc_var_y - gyro_still_det->win_mean_y * tmp) *
    148       tmp_denom;
    149 
    150   tmp = gyro_still_det->win_mean_z;
    151   gyro_still_det->win_mean_z *= tmp_denom_mean;
    152   gyro_still_det->win_var_z =
    153       (gyro_still_det->acc_var_z - gyro_still_det->win_mean_z * tmp) *
    154       tmp_denom;
    155 
    156   // Adds the assumed mean value back to the total mean calculation.
    157   gyro_still_det->win_mean_x += gyro_still_det->assumed_mean_x;
    158   gyro_still_det->win_mean_y += gyro_still_det->assumed_mean_y;
    159   gyro_still_det->win_mean_z += gyro_still_det->assumed_mean_z;
    160 
    161   // Define the variance thresholds.
    162   float upper_var_thresh =
    163       (gyro_still_det->var_threshold + gyro_still_det->confidence_delta);
    164 
    165   float lower_var_thresh =
    166       (gyro_still_det->var_threshold - gyro_still_det->confidence_delta);
    167 
    168   // Compute the stillness confidence score.
    169   if ((gyro_still_det->win_var_x > upper_var_thresh) ||
    170       (gyro_still_det->win_var_y > upper_var_thresh) ||
    171       (gyro_still_det->win_var_z > upper_var_thresh)) {
    172     // Sensor variance exceeds the upper threshold (i.e., motion detected).
    173     // Set stillness confidence equal to 0.
    174     gyro_still_det->stillness_confidence = 0;
    175 
    176   } else {
    177     if ((gyro_still_det->win_var_x <= lower_var_thresh) &&
    178         (gyro_still_det->win_var_y <= lower_var_thresh) &&
    179         (gyro_still_det->win_var_z <= lower_var_thresh)) {
    180       // Sensor variance is below the lower threshold (i.e., stillness
    181       // detected).
    182       // Set stillness confidence equal to 1.
    183       gyro_still_det->stillness_confidence = 1.f;
    184 
    185     } else {
    186       // Motion detection thresholds not exceeded. Compute the stillness
    187       // confidence score.
    188 
    189       float var_thresh = gyro_still_det->var_threshold;
    190 
    191       // Compute the stillness confidence score.
    192       // Each axis score is limited [0,1].
    193       tmp_denom = 1.f / (upper_var_thresh - lower_var_thresh);
    194       gyro_still_det->stillness_confidence =
    195           gyroStillDetLimit(
    196               0.5f - (gyro_still_det->win_var_x - var_thresh) * tmp_denom) *
    197           gyroStillDetLimit(
    198               0.5f - (gyro_still_det->win_var_y - var_thresh) * tmp_denom) *
    199           gyroStillDetLimit(
    200               0.5f - (gyro_still_det->win_var_z - var_thresh) * tmp_denom);
    201     }
    202   }
    203 
    204   // Return the stillness confidence.
    205   return gyro_still_det->stillness_confidence;
    206 }
    207 
    208 // Resets the stillness detector and initiates a new detection window.
    209 // 'reset_stats' determines whether the stillness statistics are reset.
    210 void gyroStillDetReset(struct GyroStillDet* gyro_still_det,
    211                        bool reset_stats) {
    212   float tmp_denom = 1.f;
    213 
    214   // Reset the stillness data ready flag.
    215   gyro_still_det->stillness_window_ready = false;
    216 
    217   // Signal to start capture of next stillness data window.
    218   gyro_still_det->start_new_window = true;
    219 
    220   // Track the stillness confidence (current->previous).
    221   gyro_still_det->prev_stillness_confidence =
    222       gyro_still_det->stillness_confidence;
    223 
    224   // Track changes in the mean estimate.
    225   if (gyro_still_det->num_acc_samples > 1) {
    226     tmp_denom = 1.f / gyro_still_det->num_acc_samples;
    227   }
    228   gyro_still_det->prev_mean_x = gyro_still_det->mean_x * tmp_denom;
    229   gyro_still_det->prev_mean_y = gyro_still_det->mean_y * tmp_denom;
    230   gyro_still_det->prev_mean_z = gyro_still_det->mean_z * tmp_denom;
    231 
    232   // Reset the current statistics to zero.
    233   if (reset_stats) {
    234     gyro_still_det->num_acc_samples = 0;
    235     gyro_still_det->mean_x = 0;
    236     gyro_still_det->mean_y = 0;
    237     gyro_still_det->mean_z = 0;
    238     gyro_still_det->acc_var_x = 0;
    239     gyro_still_det->acc_var_y = 0;
    240     gyro_still_det->acc_var_z = 0;
    241   }
    242 }
    243 
    244 // Enforces the limits of an input value [0,1].
    245 float gyroStillDetLimit(float value) {
    246   // Fix limits [0,1].
    247   if (value < 0) {
    248     value = 0;
    249   } else {
    250     if (value > 1.f) {
    251       value = 1.f;
    252     }
    253   }
    254 
    255   return value;
    256 }
    257