impl/classifier/AnglesClassifier.java

/*
 * Copyright (C) 2016 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License
 */

package com.android.incallui.answer.impl.classifier;

import android.util.ArrayMap;
import android.view.MotionEvent;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

/**
 * A classifier which calculates the variance of differences between successive angles in a stroke.
 * For each stroke it keeps its last three points. If some successive points are the same, it
 * ignores the repetitions. If a new point is added, the classifier calculates the angle between the
 * last three points. After that, it calculates the difference between this angle and the previously
 * calculated angle. Then it calculates the variance of the differences from a stroke. To the
 * differences there is artificially added value 0.0 and the difference between the first angle and
 * PI (angles are in radians). It helps with strokes which have few points and punishes more strokes
 * which are not smooth.
 *
 * <p>This classifier also tries to split the stroke into two parts in the place in which the
 * biggest angle is. It calculates the angle variance of the two parts and sums them up. The reason
 * the classifier is doing this, is because some human swipes at the beginning go for a moment in
 * one direction and then they rapidly change direction for the rest of the stroke (like a tick).
 * The final result is the minimum of angle variance of the whole stroke and the sum of angle
 * variances of the two parts split up. The classifier tries the tick option only if the first part
 * is shorter than the second part.
 *
 * <p>Additionally, the classifier classifies the angles as left angles (those angles which value is
 * in [0.0, PI - ANGLE_DEVIATION) interval), straight angles ([PI - ANGLE_DEVIATION, PI +
 * ANGLE_DEVIATION] interval) and right angles ((PI + ANGLE_DEVIATION, 2 * PI) interval) and then
 * calculates the percentage of angles which are in the same direction (straight angles can be left
 * angels or right angles)
 */
class AnglesClassifier extends StrokeClassifier {
  private Map<Stroke, Data> mStrokeMap = new ArrayMap<>();

  public AnglesClassifier(ClassifierData classifierData) {
    mClassifierData = classifierData;
  }

  @Override
  public String getTag() {
    return "ANG";
  }

  @Override
  public void onTouchEvent(MotionEvent event) {
    int action = event.getActionMasked();

    if (action == MotionEvent.ACTION_DOWN) {
      mStrokeMap.clear();
    }

    for (int i = 0; i < event.getPointerCount(); i++) {
      Stroke stroke = mClassifierData.getStroke(event.getPointerId(i));

      if (mStrokeMap.get(stroke) == null) {
        mStrokeMap.put(stroke, new Data());
      }
      mStrokeMap.get(stroke).addPoint(stroke.getPoints().get(stroke.getPoints().size() - 1));
    }
  }

  @Override
  public float getFalseTouchEvaluation(Stroke stroke) {
    Data data = mStrokeMap.get(stroke);
    return AnglesVarianceEvaluator.evaluate(data.getAnglesVariance())
        + AnglesPercentageEvaluator.evaluate(data.getAnglesPercentage());
  }

  private static class Data {
    private static final float ANGLE_DEVIATION = (float) Math.PI / 20.0f;
    private static final float MIN_MOVE_DIST_DP = .01f;

    private List<Point> mLastThreePoints = new ArrayList<>();
    private float mFirstAngleVariance;
    private float mPreviousAngle;
    private float mBiggestAngle;
    private float mSumSquares;
    private float mSecondSumSquares;
    private float mSum;
    private float mSecondSum;
    private float mCount;
    private float mSecondCount;
    private float mFirstLength;
    private float mLength;
    private float mAnglesCount;
    private float mLeftAngles;
    private float mRightAngles;
    private float mStraightAngles;

    public Data() {
      mFirstAngleVariance = 0.0f;
      mPreviousAngle = (float) Math.PI;
      mBiggestAngle = 0.0f;
      mSumSquares = mSecondSumSquares = 0.0f;
      mSum = mSecondSum = 0.0f;
      mCount = mSecondCount = 1.0f;
      mLength = mFirstLength = 0.0f;
      mAnglesCount = mLeftAngles = mRightAngles = mStraightAngles = 0.0f;
    }

    public void addPoint(Point point) {
      // Checking if the added point is different than the previously added point
      // Repetitions and short distances are being ignored so that proper angles are calculated.
      if (mLastThreePoints.isEmpty()
          || (!mLastThreePoints.get(mLastThreePoints.size() - 1).equals(point)
              && (mLastThreePoints.get(mLastThreePoints.size() - 1).dist(point)
                  > MIN_MOVE_DIST_DP))) {
        if (!mLastThreePoints.isEmpty()) {
          mLength += mLastThreePoints.get(mLastThreePoints.size() - 1).dist(point);
        }
        mLastThreePoints.add(point);
        if (mLastThreePoints.size() == 4) {
          mLastThreePoints.remove(0);

          float angle =
              mLastThreePoints.get(1).getAngle(mLastThreePoints.get(0), mLastThreePoints.get(2));

          mAnglesCount++;
          if (angle < Math.PI - ANGLE_DEVIATION) {
            mLeftAngles++;
          } else if (angle <= Math.PI + ANGLE_DEVIATION) {
            mStraightAngles++;
          } else {
            mRightAngles++;
          }

          float difference = angle - mPreviousAngle;

          // If this is the biggest angle of the stroke so then we save the value of
          // the angle variance so far and start to count the values for the angle
          // variance of the second part.
          if (mBiggestAngle < angle) {
            mBiggestAngle = angle;
            mFirstLength = mLength;
            mFirstAngleVariance = getAnglesVariance(mSumSquares, mSum, mCount);
            mSecondSumSquares = 0.0f;
            mSecondSum = 0.0f;
            mSecondCount = 1.0f;
          } else {
            mSecondSum += difference;
            mSecondSumSquares += difference * difference;
            mSecondCount += 1.0f;
          }

          mSum += difference;
          mSumSquares += difference * difference;
          mCount += 1.0f;
          mPreviousAngle = angle;
        }
      }
    }

    public float getAnglesVariance(float sumSquares, float sum, float count) {
      return sumSquares / count - (sum / count) * (sum / count);
    }

    public float getAnglesVariance() {
      float anglesVariance = getAnglesVariance(mSumSquares, mSum, mCount);
      if (mFirstLength < mLength / 2f) {
        anglesVariance =
            Math.min(
                anglesVariance,
                mFirstAngleVariance
                    + getAnglesVariance(mSecondSumSquares, mSecondSum, mSecondCount));
      }
      return anglesVariance;
    }

    public float getAnglesPercentage() {
      if (mAnglesCount == 0.0f) {
        return 1.0f;
      }
      return (Math.max(mLeftAngles, mRightAngles) + mStraightAngles) / mAnglesCount;
    }
  }
}