Reimplement Chromium's OneEuroFilter to InputConsumer

Reimplemented Chromium's OneEuroFilter usage to InputConsumerNoResampling.
There are a few differences between Chromium's work and this CL. We
reimplemented One Euro filter an adaptive cutoff frequency low pass
made in this implementation as in the Chromium's implementation. The
class FilteredResampler filters the output of LegacyResampler using the
One Euro filter approach.

Here's the link to Chromium's to One Euro filter:
https://source.chromium.org/chromium/chromium/src/+/main:ui/base/prediction/one_euro_filter.h

Bug: 297226446
Flag: EXEMPT bugfix
Test: TEST=libinput_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Change-Id: I0316cb1e81c73b1dc28dc809f55dee3a1cc0ebd2

Author: Paul Ramirez

include/input/CoordinateFilter.h

@@ -0,0 +1,54 @@
+/**
+ * Copyright 2024 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.
+ */
+
+#pragma once
+
+#include <chrono>
+
+#include <input/Input.h>
+#include <input/OneEuroFilter.h>
+
+namespace android {
+
+/**
+ * Pair of OneEuroFilters that independently filter X and Y coordinates. Both filters share the same
+ * constructor's parameters. The minimum cutoff frequency is the base cutoff frequency, that is, the
+ * resulting cutoff frequency in the absence of signal's speed. Likewise, beta is a scaling factor
+ * of the signal's speed that sets how much the signal's speed contributes to the resulting cutoff
+ * frequency. The adaptive cutoff frequency criterion is f_c = f_c_min + β|̇x_filtered|
+ */
+class CoordinateFilter {
+public:
+    explicit CoordinateFilter(float minCutoffFreq, float beta);
+
+    /**
+     * Filters in place only the AXIS_X and AXIS_Y fields from coords. Each call to filter must
+     * provide a timestamp strictly greater than the timestamp of the previous call. The first time
+     * this method is invoked no filtering takes place. Subsequent calls do overwrite `coords` with
+     * filtered data.
+     *
+     * @param timestamp The timestamps at which to filter. It must be greater than the one passed in
+     * the previous call.
+     * @param coords Coordinates to be overwritten by the corresponding filtered coordinates.
+     */
+    void filter(std::chrono::duration<float> timestamp, PointerCoords& coords);
+
+private:
+    OneEuroFilter mXFilter;
+    OneEuroFilter mYFilter;
+};
+
+} // namespace android

include/input/OneEuroFilter.h

@@ -0,0 +1,101 @@
+/**
+ * Copyright 2024 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.
+ */
+
+#pragma once
+
+#include <chrono>
+#include <optional>
+
+#include <input/Input.h>
+
+namespace android {
+
+/**
+ * Low pass filter with adaptive low pass frequency based on the signal's speed. The signal's cutoff
+ * frequency is determined by f_c = f_c_min + β|̇x_filtered|. Refer to
+ * https://dl.acm.org/doi/10.1145/2207676.2208639 for details on how the filter works and how to
+ * tune it.
+ */
+class OneEuroFilter {
+public:
+    /**
+     * Default cutoff frequency of the filtered signal's speed. 1.0 Hz is the value in the filter's
+     * paper.
+     */
+    static constexpr float kDefaultSpeedCutoffFreq = 1.0;
+
+    OneEuroFilter() = delete;
+
+    explicit OneEuroFilter(float minCutoffFreq, float beta,
+                           float speedCutoffFreq = kDefaultSpeedCutoffFreq);
+
+    OneEuroFilter(const OneEuroFilter&) = delete;
+    OneEuroFilter& operator=(const OneEuroFilter&) = delete;
+    OneEuroFilter(OneEuroFilter&&) = delete;
+    OneEuroFilter& operator=(OneEuroFilter&&) = delete;
+
+    /**
+     * Returns the filtered value of rawPosition. Each call to filter must provide a timestamp
+     * strictly greater than the timestamp of the previous call. The first time the method is
+     * called, it returns the value of rawPosition. Any subsequent calls provide a filtered value.
+     *
+     * @param timestamp The timestamp at which to filter. It must be strictly greater than the one
+     * provided in the previous call.
+     * @param rawPosition Position to be filtered.
+     */
+    float filter(std::chrono::duration<float> timestamp, float rawPosition);
+
+private:
+    /**
+     * Minimum cutoff frequency. This is the constant term in the adaptive cutoff frequency
+     * criterion. Units are Hertz.
+     */
+    const float mMinCutoffFreq;
+
+    /**
+     * Slope of the cutoff frequency criterion. This is the term scaling the absolute value of the
+     * filtered signal's speed. The data member is dimensionless, that is, it does not have units.
+     */
+    const float mBeta;
+
+    /**
+     * Cutoff frequency of the signal's speed. This is the cutoff frequency applied to the filtering
+     * of the signal's speed. Units are Hertz.
+     */
+    const float mSpeedCutoffFreq;
+
+    /**
+     * The timestamp from the previous call. Units are seconds.
+     */
+    std::optional<std::chrono::duration<float>> mPrevTimestamp;
+
+    /**
+     * The raw position from the previous call.
+     */
+    std::optional<float> mPrevRawPosition;
+
+    /**
+     * The filtered velocity from the previous call. Units are position per second.
+     */
+    std::optional<float> mPrevFilteredVelocity;
+
+    /**
+     * The filtered position from the previous call.
+     */
+    std::optional<float> mPrevFilteredPosition;
+};
+
+} // namespace android

include/input/Resampler.h

@@ -19,11 +19,13 @@
 #include <array>
 #include <chrono>
 #include <iterator>
+#include <map>
 #include <optional>
 #include <vector>
 
 #include <android-base/logging.h>
 #include <ftl/mixins.h>
+#include <input/CoordinateFilter.h>
 #include <input/Input.h>
 #include <input/InputTransport.h>
 #include <input/RingBuffer.h>
@@ -293,4 +295,43 @@ class LegacyResampler final : public Resampler {
     inline static void addSampleToMotionEvent(const Sample& sample, MotionEvent& motionEvent);
 };
 
+/**
+ * Resampler that first applies the LegacyResampler resampling algorithm, then independently filters
+ * the X and Y coordinates with a pair of One Euro filters.
+ */
+class FilteredLegacyResampler final : public Resampler {
+public:
+    /**
+     * Creates a resampler, using the given minCutoffFreq and beta to instantiate its One Euro
+     * filters.
+     */
+    explicit FilteredLegacyResampler(float minCutoffFreq, float beta);
+
+    void resampleMotionEvent(std::chrono::nanoseconds requestedFrameTime, MotionEvent& motionEvent,
+                             const InputMessage* futureMessage) override;
+
+    std::chrono::nanoseconds getResampleLatency() const override;
+
+private:
+    LegacyResampler mResampler;
+
+    /**
+     * Minimum cutoff frequency of the value's low pass filter. Refer to OneEuroFilter class for a
+     * more detailed explanation.
+     */
+    const float mMinCutoffFreq;
+
+    /**
+     * Scaling factor of the adaptive cutoff frequency criterion. Refer to OneEuroFilter class for a
+     * more detailed explanation.
+     */
+    const float mBeta;
+
+    /*
+     * Note: an associative array with constant insertion and lookup times would be more efficient.
+     * When this was implemented, there was no container with these properties.
+     */
+    std::map<int32_t /*pointerId*/, CoordinateFilter> mFilteredPointers;
+};
+
 } // namespace android

libs/input/Android.bp

@@ -217,6 +217,7 @@ cc_library {
     ],
     srcs: [
         "AccelerationCurve.cpp",
+        "CoordinateFilter.cpp",
         "Input.cpp",
         "InputConsumer.cpp",
         "InputConsumerNoResampling.cpp",
@@ -230,6 +231,7 @@ cc_library {
         "KeyLayoutMap.cpp",
         "MotionPredictor.cpp",
         "MotionPredictorMetricsManager.cpp",
+        "OneEuroFilter.cpp",
         "PrintTools.cpp",
         "PropertyMap.cpp",
         "Resampler.cpp",

libs/input/CoordinateFilter.cpp

@@ -0,0 +1,31 @@
+/**
+ * Copyright 2024 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.
+ */
+
+#define LOG_TAG "CoordinateFilter"
+
+#include <input/CoordinateFilter.h>
+
+namespace android {
+
+CoordinateFilter::CoordinateFilter(float minCutoffFreq, float beta)
+      : mXFilter{minCutoffFreq, beta}, mYFilter{minCutoffFreq, beta} {}
+
+void CoordinateFilter::filter(std::chrono::duration<float> timestamp, PointerCoords& coords) {
+    coords.setAxisValue(AMOTION_EVENT_AXIS_X, mXFilter.filter(timestamp, coords.getX()));
+    coords.setAxisValue(AMOTION_EVENT_AXIS_Y, mYFilter.filter(timestamp, coords.getY()));
+}
+
+} // namespace android

libs/input/OneEuroFilter.cpp

@@ -0,0 +1,79 @@
+/**
+ * Copyright 2024 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.
+ */
+
+#define LOG_TAG "OneEuroFilter"
+
+#include <chrono>
+#include <cmath>
+
+#include <android-base/logging.h>
+#include <input/CoordinateFilter.h>
+
+namespace android {
+namespace {
+
+inline float cutoffFreq(float minCutoffFreq, float beta, float filteredSpeed) {
+    return minCutoffFreq + beta * std::abs(filteredSpeed);
+}
+
+inline float smoothingFactor(std::chrono::duration<float> samplingPeriod, float cutoffFreq) {
+    return samplingPeriod.count() / (samplingPeriod.count() + (1.0 / (2.0 * M_PI * cutoffFreq)));
+}
+
+inline float lowPassFilter(float rawPosition, float prevFilteredPosition, float smoothingFactor) {
+    return smoothingFactor * rawPosition + (1 - smoothingFactor) * prevFilteredPosition;
+}
+
+} // namespace
+
+OneEuroFilter::OneEuroFilter(float minCutoffFreq, float beta, float speedCutoffFreq)
+      : mMinCutoffFreq{minCutoffFreq}, mBeta{beta}, mSpeedCutoffFreq{speedCutoffFreq} {}
+
+float OneEuroFilter::filter(std::chrono::duration<float> timestamp, float rawPosition) {
+    LOG_IF(FATAL, mPrevFilteredPosition.has_value() && (timestamp <= *mPrevTimestamp))
+            << "Timestamp must be greater than mPrevTimestamp";
+
+    const std::chrono::duration<float> samplingPeriod = (mPrevTimestamp.has_value())
+            ? (timestamp - *mPrevTimestamp)
+            : std::chrono::duration<float>{1.0};
+
+    const float rawVelocity = (mPrevFilteredPosition.has_value())
+            ? ((rawPosition - *mPrevFilteredPosition) / samplingPeriod.count())
+            : 0.0;
+
+    const float speedSmoothingFactor = smoothingFactor(samplingPeriod, mSpeedCutoffFreq);
+
+    const float filteredVelocity = (mPrevFilteredVelocity.has_value())
+            ? lowPassFilter(rawVelocity, *mPrevFilteredVelocity, speedSmoothingFactor)
+            : rawVelocity;
+
+    const float positionCutoffFreq = cutoffFreq(mMinCutoffFreq, mBeta, filteredVelocity);
+
+    const float positionSmoothingFactor = smoothingFactor(samplingPeriod, positionCutoffFreq);
+
+    const float filteredPosition = (mPrevFilteredPosition.has_value())
+            ? lowPassFilter(rawPosition, *mPrevFilteredPosition, positionSmoothingFactor)
+            : rawPosition;
+
+    mPrevTimestamp = timestamp;
+    mPrevRawPosition = rawPosition;
+    mPrevFilteredVelocity = filteredVelocity;
+    mPrevFilteredPosition = filteredPosition;
+
+    return filteredPosition;
+}
+
+} // namespace android

libs/input/Resampler.cpp

@@ -389,4 +389,34 @@ void LegacyResampler::resampleMotionEvent(nanoseconds frameTime, MotionEvent& mo
     mLastRealSample = *(mLatestSamples.end() - 1);
 }
 
+// --- FilteredLegacyResampler ---
+
+FilteredLegacyResampler::FilteredLegacyResampler(float minCutoffFreq, float beta)
+      : mResampler{}, mMinCutoffFreq{minCutoffFreq}, mBeta{beta} {}
+
+void FilteredLegacyResampler::resampleMotionEvent(std::chrono::nanoseconds requestedFrameTime,
+                                                  MotionEvent& motionEvent,
+                                                  const InputMessage* futureSample) {
+    mResampler.resampleMotionEvent(requestedFrameTime, motionEvent, futureSample);
+    const size_t numSamples = motionEvent.getHistorySize() + 1;
+    for (size_t sampleIndex = 0; sampleIndex < numSamples; ++sampleIndex) {
+        for (size_t pointerIndex = 0; pointerIndex < motionEvent.getPointerCount();
+             ++pointerIndex) {
+            const int32_t pointerId = motionEvent.getPointerProperties(pointerIndex)->id;
+            const nanoseconds eventTime =
+                    nanoseconds{motionEvent.getHistoricalEventTime(sampleIndex)};
+            // Refer to the static function `setMotionEventPointerCoords` for a justification of
+            // casting away const.
+            PointerCoords& pointerCoords = const_cast<PointerCoords&>(
+                    *(motionEvent.getHistoricalRawPointerCoords(pointerIndex, sampleIndex)));
+            const auto& [iter, _] = mFilteredPointers.try_emplace(pointerId, mMinCutoffFreq, mBeta);
+            iter->second.filter(eventTime, pointerCoords);
+        }
+    }
+}
+
+std::chrono::nanoseconds FilteredLegacyResampler::getResampleLatency() const {
+    return mResampler.getResampleLatency();
+}
+
 } // namespace android

libs/input/tests/Android.bp

@@ -25,6 +25,7 @@ cc_test {
         "InputVerifier_test.cpp",
         "MotionPredictor_test.cpp",
         "MotionPredictorMetricsManager_test.cpp",
+        "OneEuroFilter_test.cpp",
         "Resampler_test.cpp",
         "RingBuffer_test.cpp",
         "TestInputChannel.cpp",