Pyramidal LK Optical Flow

Overview

This application tracks feature points on an input video, draws the features on each frame and saves them to disk. The user can define what backend will be used for processing.

Note

The output will be in grayscale as the algorithm currently doesn't support color inputs.

Instructions

The command line parameters are:

<backend> <input video> <pyramid levels> <output frames>

where

• backend: either cpu or cuda; it defines the backend that will perform the processing.
• input video: input video file name, it accepts all video types that OpenCV's cv::VideoCapture accepts.
• pyramid levels: specify the number of pyramid that used in the algorithm.
• output frames: the file name that will be used for the output frames. Example: output.png will generate frames output_0000.png, output_0001.png, output_0002.png, and so on.

Here's one example:

• C++

  ./vpi_sample_12_optflow_lk cuda ../assets/dashcam.mp4 5 frame.png

• Python

  python main.py cuda ../assets/dashcam.mp4 5 frame.png

This is using the CUDA backend and one of the provided sample videos with pyramid level equals 5.

Results

Frame 0009

Source Code

For convenience, here's the code that is also installed in the samples directory.

Language: C++ Python

import cv2
import sys
import vpi
import numpy as np
from os import path
from argparse import ArgumentParser
from contextlib import contextmanager
 
# --------------------------------------
# Some definitions and utility functions
 
# Maximum number of keypoints that will be tracked
MAX_KEYPOINTS = 100
 
def update_mask(mask, trackColors, prevFeatures, curFeatures, status = None):
    '''Draw keypoint path from previous frame to current one'''
 
    numTrackedKeypoints = 0
 
    def none_context(a=None): return contextmanager(lambda: (x for x in [a]))()
 
    with curFeatures.rlock(), \
         (status.rlock() if status else none_context()), \
         (prevFeatures.rlock() if prevFeatures else none_context()):
 
        for i in range(curFeatures.size):
            # keypoint is being tracked?
            if not status or status.cpu()[i] == 0:
                color = tuple(trackColors[i,0].tolist())
 
                # draw the tracks
                if prevFeatures:
                    cv2.line(mask, tuple(np.round(prevFeatures.cpu()[i])), tuple(curFeatures.cpu()[i]), color, 2)
 
                cv2.circle(mask, tuple(np.round(curFeatures.cpu()[i])), 5, color, -1)
 
                numTrackedKeypoints += 1
 
    return numTrackedKeypoints
 
def save_file_to_disk(frame, mask, baseFileName, frameCounter):
    '''Apply mask on frame and save it to disk'''
 
    frame = frame.convert(vpi.Format.BGR8, backend=vpi.Backend.CUDA)
    with frame.rlock():
        frame = cv2.add(frame.cpu(), mask)
 
    name, ext = path.splitext(baseFileName)
    fname = "{}_{:04d}{}".format(name, frameCounter, ext)
 
    cv2.imwrite(fname, frame, [cv2.IMWRITE_JPEG_QUALITY, 70])
 
# ----------------------------
# Parse command line arguments
 
parser = ArgumentParser()
parser.add_argument('backend', choices=['cpu', 'cuda'],
                    help='Backend to be used for processing')
 
parser.add_argument('input',
                    help='Input video to be processed')
 
parser.add_argument('pyramid_levels', type=int,
                    help='Number of levels in the pyramid used with the algorithm')
 
parser.add_argument('output',
                    help='Output file name')
 
args = parser.parse_args();
 
if args.backend == 'cuda':
    backend = vpi.Backend.CUDA
else:
    assert args.backend == 'cpu'
    backend = vpi.Backend.CPU
 
# adjust output file name to take into account backend used and python version
name, ext = path.splitext(args.output)
args.output = "{}_python{}_{}{}".format(name, sys.version_info[0], args.backend, ext)
 
# ----------------
# Open input video
 
inVideo = cv2.VideoCapture(args.input)
 
# Read first input frame
ok, cvFrame = inVideo.read()
if not ok:
    exit('Cannot read first input frame')
 
# ---------------------------
# Perform some pre-processing
 
# Retrieve features to be tracked from first frame using
# Harris Corners Detector
with vpi.Backend.CPU:
    frame = vpi.asimage(cvFrame, vpi.Format.BGR8).convert(vpi.Format.U8)
    curFeatures, scores = frame.harriscorners(strength=0.1, sensitivity=0.01)
 
# Limit the number of features we'll track and calculate their colors on the
# output image
with curFeatures.lock(), scores.rlock():
    # Sort features in descending scores order and keep the first MAX_KEYPOINTS
    ind = np.argsort(scores.cpu(), kind='mergesort')[::-1]
    curFeatures.cpu()[:] = np.take(curFeatures.cpu(), ind, axis=0)
    curFeatures.size = min(curFeatures.size, MAX_KEYPOINTS)
 
    # Keypoints' have different hues, calculated from their position in the first frame
    trackColors = np.array([[(int(p[0]) ^ int(p[1])) % 180,255,255] for p in curFeatures.cpu()], np.uint8).reshape(-1,1,3)
    # Convert colors from HSV to RGB
    trackColors = cv2.cvtColor(trackColors, cv2.COLOR_HSV2BGR).astype(int)
 
with backend:
    optflow = vpi.OpticalFlowPyrLK(frame, curFeatures, args.pyramid_levels)
 
# Counter for the frames
idFrame = 0
 
# Create mask with features' tracks over time
mask = np.zeros((frame.height, frame.width, 3), np.uint8)
numTrackedKeypoints = update_mask(mask, trackColors, None, curFeatures)
 
while True:
    # Apply mask to frame and save it to disk
    save_file_to_disk(frame, mask, args.output, idFrame)
 
    print("Frame id={}: {} points tracked.".format(idFrame, numTrackedKeypoints))
 
    prevFeatures = curFeatures
 
    # Read one input frame
    ret, cvFrame = inVideo.read()
    if not ret:
        print("Video ended.")
        break
    idFrame += 1
 
    # Convert frame to grayscale
    with vpi.Backend.CUDA:
        frame = vpi.asimage(cvFrame, vpi.Format.BGR8).convert(vpi.Format.U8);
 
    # Calculate where keypoints are in current frame
    curFeatures, status = optflow(frame)
 
    # Update the mask with the current keypoints' position
    numTrackedKeypoints = update_mask(mask, trackColors, prevFeatures, curFeatures, status)
 
    # No more keypoints to track?
    if numTrackedKeypoints == 0:
        print("No keypoints to track.")
        break # nothing else to do
 
# vim: ts=8:sw=4:sts=4:et:ai

#include <opencv2/core/version.hpp>
#if CV_MAJOR_VERSION >= 3
#    include <opencv2/imgcodecs.hpp>
#    include <opencv2/videoio.hpp>
#else
#    include <opencv2/highgui/highgui.hpp>
#endif
 
#include <opencv2/imgproc/imgproc.hpp>
#include <vpi/OpenCVInterop.hpp>
 
#include <vpi/Array.h>
#include <vpi/Image.h>
#include <vpi/Pyramid.h>
#include <vpi/Status.h>
#include <vpi/Stream.h>
#include <vpi/algo/ConvertImageFormat.h>
#include <vpi/algo/GaussianPyramid.h>
#include <vpi/algo/HarrisCorners.h>
#include <vpi/algo/OpticalFlowPyrLK.h>
 
#include <algorithm>
#include <cstring> // for memset
#include <fstream>
#include <iostream>
#include <map>
#include <numeric>
#include <sstream>
#include <vector>
 
// Max number of corners detected by harris corner algo
constexpr int MAX_HARRIS_CORNERS = 8192;
 
// Max number of keypoints to be tracked
constexpr int MAX_KEYPOINTS = 100;
 
#define CHECK_STATUS(STMT)                                      \
    do                                                          \
    {                                                           \
        VPIStatus status__ = (STMT);                            \
        if (status__ != VPI_SUCCESS)                            \
        {                                                       \
            char buffer[VPI_MAX_STATUS_MESSAGE_LENGTH];         \
            vpiGetLastStatusMessage(buffer, sizeof(buffer));    \
            std::ostringstream ss;                              \
            ss << vpiStatusGetName(status__) << ": " << buffer; \
            throw std::runtime_error(ss.str());                 \
        }                                                       \
    } while (0);
 
static void SaveFileToDisk(VPIImage img, cv::Mat cvMask, std::string baseFileName, int32_t frameCounter)
{
    VPIImageData imgData;
    CHECK_STATUS(vpiImageLock(img, VPI_LOCK_READ, &imgData));
 
    cv::Mat cvImage;
    try
    {
        cv::Mat tmp;
        CHECK_STATUS(vpiImageDataExportOpenCVMat(imgData, &tmp));
        cvtColor(tmp, cvImage, cv::COLOR_GRAY2BGR);
 
        CHECK_STATUS(vpiImageUnlock(img));
    }
    catch (...)
    {
        CHECK_STATUS(vpiImageUnlock(img));
        throw;
    }
 
    add(cvImage, cvMask, cvImage);
 
    // Create the output file name
    std::string fname = baseFileName;
    int ext           = fname.rfind('.');
 
    char buffer[512] = {};
    snprintf(buffer, sizeof(buffer) - 1, "%s_%04d%s", fname.substr(0, ext).c_str(), frameCounter,
             fname.substr(ext).c_str());
 
    // Finally, write frame to disk
    if (!imwrite(buffer, cvImage, {cv::IMWRITE_JPEG_QUALITY, 70}))
    {
        throw std::runtime_error("Can't write to " + std::string(buffer));
    }
}
 
// Sort keypoints by decreasing score, and retain only the first 'max'
static void SortKeypoints(VPIArray keypoints, VPIArray scores, int max)
{
    VPIArrayData ptsData, scoresData;
    CHECK_STATUS(vpiArrayLock(keypoints, VPI_LOCK_READ_WRITE, &ptsData));
    CHECK_STATUS(vpiArrayLock(scores, VPI_LOCK_READ_WRITE, &scoresData));
 
    std::vector<int> indices(*ptsData.sizePointer);
    std::iota(indices.begin(), indices.end(), 0);
 
    stable_sort(indices.begin(), indices.end(), [&scoresData](int a, int b) {
        uint32_t *score = reinterpret_cast<uint32_t *>(scoresData.data);
        return score[a] >= score[b]; // decreasing score order
    });
 
    // keep the only 'max' indexes.
    indices.resize(std::min<size_t>(indices.size(), max));
 
    VPIKeypoint *kptData = reinterpret_cast<VPIKeypoint *>(ptsData.data);
 
    // reorder the keypoints to keep the first 'max' with highest scores.
    std::vector<VPIKeypoint> kpt;
    std::transform(indices.begin(), indices.end(), std::back_inserter(kpt),
                   [kptData](int idx) { return kptData[idx]; });
    std::copy(kpt.begin(), kpt.end(), kptData);
 
    // update keypoint array size.
    *ptsData.sizePointer = kpt.size();
 
    vpiArrayUnlock(scores);
    vpiArrayUnlock(keypoints);
}
 
static int UpdateMask(cv::Mat &cvMask, const std::vector<cv::Scalar> &trackColors, VPIArray prevFeatures,
                      VPIArray curFeatures, VPIArray status)
{
    // Now that optical flow is completed, there are usually two approaches to take:
    // 1. Add new feature points from current frame using a feature detector such as
    //    \ref algo_harris_corners "Harris Corner Detector"
    // 2. Keep using the points that are being tracked.
    //
    // The sample app uses the valid feature point and continue to do the tracking.
 
    // Lock the input and output arrays to draw the tracks to the output mask.
    VPIArrayData curFeaturesData, statusData;
    CHECK_STATUS(vpiArrayLock(curFeatures, VPI_LOCK_READ_WRITE, &curFeaturesData));
    CHECK_STATUS(vpiArrayLock(status, VPI_LOCK_READ, &statusData));
 
    const VPIKeypoint *pCurFeatures = (VPIKeypoint *)curFeaturesData.data;
    const uint8_t *pStatus          = (uint8_t *)statusData.data;
 
    const VPIKeypoint *pPrevFeatures;
    if (prevFeatures)
    {
        VPIArrayData prevFeaturesData;
        CHECK_STATUS(vpiArrayLock(prevFeatures, VPI_LOCK_READ, &prevFeaturesData));
        pPrevFeatures = (VPIKeypoint *)prevFeaturesData.data;
    }
    else
    {
        pPrevFeatures = NULL;
    }
 
    int numTrackedKeypoints = 0;
    int totKeypoints        = *curFeaturesData.sizePointer;
 
    for (int i = 0; i < totKeypoints; i++)
    {
        // keypoint is being tracked?
        if (pStatus[i] == 0)
        {
            // draw the tracks
            cv::Point curPoint{(int)round(pCurFeatures[i].x), (int)round(pCurFeatures[i].y)};
            if (pPrevFeatures != NULL)
            {
                cv::Point2f prevPoint{pPrevFeatures[i].x, pPrevFeatures[i].y};
                line(cvMask, prevPoint, curPoint, trackColors[i], 2);
            }
 
            circle(cvMask, curPoint, 5, trackColors[i], -1);
 
            numTrackedKeypoints++;
        }
    }
 
    // We're finished working with the arrays.
    if (prevFeatures)
    {
        CHECK_STATUS(vpiArrayUnlock(prevFeatures));
    }
    CHECK_STATUS(vpiArrayUnlock(curFeatures));
    CHECK_STATUS(vpiArrayUnlock(status));
 
    return numTrackedKeypoints;
}
 
int main(int argc, char *argv[])
{
    // OpenCV image that will be wrapped by a VPIImage.
    // Define it here so that it's destroyed *after* wrapper is destroyed
    cv::Mat cvFrame;
 
    // VPI objects that will be used
    VPIStream stream        = NULL;
    VPIImage imgTempFrame   = NULL;
    VPIImage imgFrame       = NULL;
    VPIPyramid pyrPrevFrame = NULL, pyrCurFrame = NULL;
    VPIArray prevFeatures = NULL, curFeatures = NULL, status = NULL;
    VPIPayload optflow = NULL;
    VPIArray scores    = NULL;
    VPIPayload harris  = NULL;
 
    int retval = 0;
 
    try
    {
        // ============================
        // Parse command line arguments
 
        if (argc != 5)
        {
            throw std::runtime_error(std::string("Usage: ") + argv[0] +
                                     " <cpu|cuda> <input_video> <pyramid_levels> <output>");
        }
 
        std::string strBackend     = argv[1];
        std::string strInputVideo  = argv[2];
        int32_t pyrLevel           = std::stoi(argv[3]);
        std::string strOutputFiles = argv[4];
 
        // Now parse the backend
        VPIBackend backend;
 
        if (strBackend == "cpu")
        {
            backend = VPI_BACKEND_CPU;
        }
        else if (strBackend == "cuda")
        {
            backend = VPI_BACKEND_CUDA;
        }
        else
        {
            throw std::runtime_error("Backend '" + strBackend + "' not recognized, it must be either cpu or cuda.");
        }
 
        {
            int ext        = strOutputFiles.rfind('.');
            strOutputFiles = strOutputFiles.substr(0, ext) + "_" + strBackend + strOutputFiles.substr(ext);
        }
 
        // ====================
        // Load the input video
        cv::VideoCapture invid;
        if (!invid.open(strInputVideo))
        {
            throw std::runtime_error("Can't open '" + strInputVideo + "'");
        }
 
        // Fetch the first frame and wrap it into a VPIImage.
        // The points to be tracked will be gathered from this frame later on.
        if (!invid.read(cvFrame))
        {
            throw std::runtime_error("Can't retrieve first frame from '" + strInputVideo + "'");
        }
 
        // =================================================
        // Allocate VPI resources and do some pre-processing
 
        // Create the stream where processing will happen.
        CHECK_STATUS(vpiStreamCreate(0, &stream));
 
        CHECK_STATUS(vpiImageCreateOpenCVMatWrapper(cvFrame, 0, &imgTempFrame));
 
        // Create grayscale image representation of input.
        CHECK_STATUS(vpiImageCreate(cvFrame.cols, cvFrame.rows, VPI_IMAGE_FORMAT_U8, 0, &imgFrame));
 
        // Create the image pyramids used by the algorithm
        CHECK_STATUS(
            vpiPyramidCreate(cvFrame.cols, cvFrame.rows, VPI_IMAGE_FORMAT_U8, pyrLevel, 0.5, 0, &pyrPrevFrame));
        CHECK_STATUS(vpiPyramidCreate(cvFrame.cols, cvFrame.rows, VPI_IMAGE_FORMAT_U8, pyrLevel, 0.5, 0, &pyrCurFrame));
 
        // Create input and output arrays
        CHECK_STATUS(vpiArrayCreate(MAX_HARRIS_CORNERS, VPI_ARRAY_TYPE_KEYPOINT, 0, &prevFeatures));
        CHECK_STATUS(vpiArrayCreate(MAX_HARRIS_CORNERS, VPI_ARRAY_TYPE_KEYPOINT, 0, &curFeatures));
        CHECK_STATUS(vpiArrayCreate(MAX_HARRIS_CORNERS, VPI_ARRAY_TYPE_U8, 0, &status));
 
        // Create Optical Flow payload
        CHECK_STATUS(vpiCreateOpticalFlowPyrLK(backend, cvFrame.cols, cvFrame.rows, VPI_IMAGE_FORMAT_U8, pyrLevel, 0.5,
                                               &optflow));
 
        // Parameters we'll use. No need to change them on the fly, so just define them here.
        // We're using the default parameters.
        VPIOpticalFlowPyrLKParams lkParams;
        CHECK_STATUS(vpiInitOpticalFlowPyrLKParams(&lkParams));
 
        // Create a mask image for drawing purposes
        cv::Mat cvMask = cv::Mat::zeros(cvFrame.size(), CV_8UC3);
 
        // Gather feature points from first frame using Harris Corners on CPU.
        {
            CHECK_STATUS(vpiArrayCreate(MAX_HARRIS_CORNERS, VPI_ARRAY_TYPE_U32, 0, &scores));
 
            VPIHarrisCornerDetectorParams harrisParams;
            CHECK_STATUS(vpiInitHarrisCornerDetectorParams(&harrisParams));
            harrisParams.strengthThresh = 0;
            harrisParams.sensitivity    = 0.01;
 
            CHECK_STATUS(vpiCreateHarrisCornerDetector(VPI_BACKEND_CPU, cvFrame.cols, cvFrame.rows, &harris));
 
            // Convert input to grayscale to conform with harris corner detector restrictions
            CHECK_STATUS(vpiSubmitConvertImageFormat(stream, VPI_BACKEND_CUDA, imgTempFrame, imgFrame, NULL));
 
            CHECK_STATUS(vpiSubmitHarrisCornerDetector(stream, VPI_BACKEND_CPU, harris, imgFrame, curFeatures, scores,
                                                       &harrisParams));
 
            CHECK_STATUS(vpiStreamSync(stream));
 
            SortKeypoints(curFeatures, scores, MAX_KEYPOINTS);
        }
 
        // Create some random colors
        std::vector<cv::Scalar> trackColors;
        {
            std::vector<cv::Vec3b> tmpTrackColors;
 
            VPIArrayData ptsData;
            CHECK_STATUS(vpiArrayLock(curFeatures, VPI_LOCK_READ, &ptsData));
 
            const VPIKeypoint *pts = (VPIKeypoint *)ptsData.data;
 
            for (int i = 0; i < *ptsData.sizePointer; i++)
            {
                // Track hue depends on its initial position
                int hue = ((int)pts[i].x ^ (int)pts[i].y) % 180;
 
                tmpTrackColors.push_back(cv::Vec3b(hue, 255, 255));
            }
            CHECK_STATUS(vpiArrayUnlock(curFeatures));
 
            cvtColor(tmpTrackColors, tmpTrackColors, cv::COLOR_HSV2BGR);
 
            for (size_t i = 0; i < tmpTrackColors.size(); i++)
            {
                trackColors.push_back(cv::Scalar(tmpTrackColors[i]));
            }
        }
 
        // Update the mask with info from first frame.
        int numTrackedKeypoints = UpdateMask(cvMask, trackColors, NULL, curFeatures, status);
 
        // =================================================
        // Main processing stage
 
        // Generate pyramid for first frame.
        CHECK_STATUS(vpiSubmitGaussianPyramidGenerator(stream, backend, imgFrame, pyrCurFrame));
 
        // Counter for the frames
        int idxFrame = 0;
 
        while (true)
        {
            // Save frame to disk
            SaveFileToDisk(imgFrame, cvMask, strOutputFiles, idxFrame);
 
            printf("Frame id=%d: %d points tracked. \n", idxFrame, numTrackedKeypoints);
 
            // Last iteration's current frame/features become this iteration's prev frame/features.
            // The former will contain information gathered in this iteration.
            std::swap(prevFeatures, curFeatures);
            std::swap(pyrPrevFrame, pyrCurFrame);
 
            // Fetch a new frame
            if (!invid.read(cvFrame))
            {
                printf("Video ended.\n");
                break;
            }
 
            ++idxFrame;
 
            // Wrap frame into a VPIImage, reusing the existing imgFrame.
            CHECK_STATUS(vpiImageSetWrappedOpenCVMat(imgTempFrame, cvFrame));
 
            // Convert it to grayscale
            CHECK_STATUS(vpiSubmitConvertImageFormat(stream, backend, imgTempFrame, imgFrame, NULL))
 
            // Generate a pyramid out of it
            CHECK_STATUS(vpiSubmitGaussianPyramidGenerator(stream, backend, imgFrame, pyrCurFrame));
 
            // Estimate the features' position in current frame given their position in previous frame
            CHECK_STATUS(vpiSubmitOpticalFlowPyrLK(stream, 0, optflow, pyrPrevFrame, pyrCurFrame, prevFeatures,
                                                   curFeatures, status, &lkParams));
 
            // Wait for processing to finish.
            CHECK_STATUS(vpiStreamSync(stream));
 
            // Update the output mask
            numTrackedKeypoints = UpdateMask(cvMask, trackColors, prevFeatures, curFeatures, status);
 
            // No more keypoints being tracked?
            if (numTrackedKeypoints == 0)
            {
                printf("No keypoints to track.\n");
                break; // we can finish procesing.
            }
        }
    }
    catch (std::exception &e)
    {
        std::cerr << e.what() << std::endl;
        retval = 1;
    }
 
    vpiStreamDestroy(stream);
    vpiPayloadDestroy(harris);
    vpiPayloadDestroy(optflow);
 
    vpiPyramidDestroy(pyrPrevFrame);
    vpiImageDestroy(imgTempFrame);
    vpiImageDestroy(imgFrame);
    vpiArrayDestroy(prevFeatures);
    vpiArrayDestroy(curFeatures);
    vpiArrayDestroy(status);
    vpiArrayDestroy(scores);
 
    return retval;
}
 
// vim: ts=8:sw=4:sts=4:et:ai