0x00. 光流

光流是进行视频中运动对象轨迹标记的一种很常用的方法，在OpenCV中实现光流也很容易。

CalcOpticalFlowPyrLK 函数计算一个稀疏特征集的光流，使用金字塔中的迭代 Lucas-Kanade 方法。

简单的实现流程：

1. 加载一段视频。

2. 调用GoodFeaturesToTrack函数寻找兴趣点。

3. 调用CalcOpticalFlowPyrLK函数计算出两帧图像中兴趣点的移动情况。

4. 删除未移动的兴趣点。

5. 在两次移动的点之间绘制一条线段。

代码示例：

import cv2.cv as cv

capture = cv.CaptureFromFile(\'img/myvideo.avi\')

nbFrames = int(cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FRAME_COUNT))
fps = cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FPS)
wait = int(1/fps * 1000/1)
width = int(cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FRAME_WIDTH))
height = int(cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FRAME_HEIGHT))

prev_gray = cv.CreateImage((width,height), 8, 1) #Will hold the frame at t-1
gray = cv.CreateImage((width,height), 8, 1) # Will hold the current frame

prevPyr = cv.CreateImage((height / 3, width + 8), 8, cv.CV_8UC1) #Will hold the pyr frame at t-1
currPyr = cv.CreateImage((height / 3, width + 8), 8, cv.CV_8UC1) # idem at t

max_count = 500
qLevel= 0.01
minDist = 10
prev_points = [] #Points at t-1
curr_points = [] #Points at t
lines=[] #To keep all the lines overtime

for f in xrange( nbFrames ):

    frame = cv.QueryFrame(capture) #Take a frame of the video

    cv.CvtColor(frame, gray, cv.CV_BGR2GRAY) #Convert to gray
    output = cv.CloneImage(frame)

    prev_points = cv.GoodFeaturesToTrack(gray, None, None, max_count, qLevel, minDist) #Find points on the image

    #Calculate the movement using the previous and the current frame using the previous points
    curr_points, status, err = cv.CalcOpticalFlowPyrLK(prev_gray, gray, prevPyr, currPyr, prev_points, (10, 10), 3, (cv.CV_TERMCRIT_ITER|cv.CV_TERMCRIT_EPS,20, 0.03), 0)


    #If points status are ok and distance not negligible keep the point
    k = 0
    for i in range(len(curr_points)):
        nb =  abs( int(prev_points[i][0])-int(curr_points[i][0]) ) + abs( int(prev_points[i][1])-int(curr_points[i][1]) )
        if status[i] and  nb > 2 :
            prev_points[k] = prev_points[i]
            curr_points[k] = curr_points[i]
            k += 1

    prev_points = prev_points[:k]
    curr_points = curr_points[:k]
    #At the end only interesting points are kept

    #Draw all the previously kept lines otherwise they would be lost the next frame
    for (pt1, pt2) in lines:
        cv.Line(frame, pt1, pt2, (255,255,255))

    #Draw the lines between each points at t-1 and t
    for prevpoint, point in zip(prev_points,curr_points):
        prevpoint = (int(prevpoint[0]),int(prevpoint[1]))
        cv.Circle(frame, prevpoint, 15, 0)
        point = (int(point[0]),int(point[1]))
        cv.Circle(frame, point, 3, 255)
        cv.Line(frame, prevpoint, point, (255,255,255))
        lines.append((prevpoint,point)) #Append current lines to the lines list


    cv.Copy(gray, prev_gray) #Put the current frame prev_gray
    prev_points = curr_points

    cv.ShowImage(\"The Video\", frame)
    #cv.WriteFrame(writer, frame)
    cv.WaitKey(wait)

直接调用摄像头使用该方法：

import cv2.cv as cv

capture = cv.CaptureFromCAM(0)

width = int(cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FRAME_WIDTH))
height = int(cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FRAME_HEIGHT))

prev_gray = cv.CreateImage((width,height), 8, 1)
gray = cv.CreateImage((width,height), 8, 1)

prevPyr = cv.CreateImage((height / 3, width + 8), 8, cv.CV_8UC1) #Will hold the pyr frame at t-1
currPyr = cv.CreateImage((height / 3, width + 8), 8, cv.CV_8UC1) # idem at t

max_count = 500
qLevel= 0.01
minDist = 10
prev_points = [] #Points at t-1
curr_points = [] #Points at t
lines=[] #To keep all the lines overtime

while True:
    frame = cv.QueryFrame(capture)
    cv.CvtColor(frame, gray, cv.CV_BGR2GRAY) #Convert to gray
    output = cv.CloneImage(frame)

    prev_points = cv.GoodFeaturesToTrack(gray, None, None, max_count, qLevel, minDist)
    curr_points, status, err = cv.CalcOpticalFlowPyrLK(prev_gray, gray, prevPyr, currPyr, prev_points, (10, 10), 3, (cv.CV_TERMCRIT_ITER|cv.CV_TERMCRIT_EPS,20, 0.03), 0)

    #If points status are ok and distance not negligible keep the point
    k = 0
    for i in range(len(curr_points)):
        nb =  abs( int(prev_points[i][0])-int(curr_points[i][0]) ) + abs( int(prev_points[i][1])-int(curr_points[i][1]) )
        if status[i] and  nb > 2 :
            prev_points[k] = prev_points[i]
            curr_points[k] = curr_points[i]
            k += 1

    prev_points = prev_points[:k]
    curr_points = curr_points[:k]
    #At the end only interesting points are kept

    #Draw all the previously kept lines otherwise they would be lost the next frame
    for (pt1, pt2) in lines:
        cv.Line(frame, pt1, pt2, (255,255,255))

    #Draw the lines between each points at t-1 and t
    for prevpoint, point in zip(prev_points,curr_points):
        prevpoint = (int(prevpoint[0]),int(prevpoint[1]))
        cv.Circle(frame, prevpoint, 15, 0)
        point = (int(point[0]),int(point[1]))
        cv.Circle(frame, point, 3, 255)
        cv.Line(frame, prevpoint, point, (255,255,255))
        lines.append((prevpoint,point)) #Append current lines to the lines list


    cv.Copy(gray, prev_gray) #Put the current frame prev_gray
    prev_points = curr_points

    cv.ShowImage(\"The Video\", frame)
    #cv.WriteFrame(writer, frame)
    c = cv.WaitKey(1)
    if c == 27: #Esc on Windows
        break

0x01. 寻找最大特征值的角点

cv.GoodFeaturesToTrack 函数可以检测出图像中最大特征值的角点，使用这个函数可以对图像中的特征点进行跟踪，从而绘制出运动轨迹。

直接加载视频：

import cv2.cv as cv

capture = cv.CaptureFromFile(\'img/myvideo.avi\')

#-- Informations about the video --
nbFrames = int(cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FRAME_COUNT))
fps = cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FPS)
wait = int(1/fps * 1000/1)
width = int(cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FRAME_WIDTH))
height = int(cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FRAME_HEIGHT))
#For recording
#codec = cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FOURCC)
#writer=cv.CreateVideoWriter(\"img/output.avi\", int(codec), int(fps), (width,height), 1) #Create writer with same parameters
#----------------------------------

prev_gray = cv.CreateImage((width,height), 8, 1) #Will hold the frame at t-1
gray = cv.CreateImage((width,height), 8, 1) # Will hold the current frame

output = cv.CreateImage((width,height), 8, 3)

prevPyr = cv.CreateImage((height / 3, width + 8), 8, cv.CV_8UC1)
currPyr = cv.CreateImage((height / 3, width + 8), 8, cv.CV_8UC1)

max_count = 500
qLevel= 0.01
minDist = 10

begin = True

initial = []
features = []
prev_points = []
curr_points = []

for f in xrange( nbFrames ):

    frame = cv.QueryFrame(capture)

    cv.CvtColor(frame, gray, cv.CV_BGR2GRAY) #Convert to gray
    cv.Copy(frame, output)


    if (len(prev_points) <= 10): #Try to get more points
        #Detect points on the image
        features = cv.GoodFeaturesToTrack(gray, None, None, max_count, qLevel, minDist)
        prev_points.extend(features) #Add the new points to list
        initial.extend(features) #Idem

    if begin:
        cv.Copy(gray, prev_gray) #Now we have two frames to compare
        begin = False

    #Compute movement
    curr_points, status, err = cv.CalcOpticalFlowPyrLK(prev_gray, gray, prevPyr, currPyr, prev_points, (10, 10), 3, (cv.CV_TERMCRIT_ITER|cv.CV_TERMCRIT_EPS,20, 0.03), 0)

    #If points status are ok and distance not negligible keep the point
    k = 0
    for i in range(len(curr_points)):
        nb =  abs( int(prev_points[i][0])-int(curr_points[i][0]) ) + abs( int(prev_points[i][1])-int(curr_points[i][1]) )
        if status[i] and  nb > 2 :
            initial[k] = initial[i]
            curr_points[k] = curr_points[i]
            k += 1

    curr_points = curr_points[:k]
    initial = initial[:k]
    #At the end only interesting points are kept

    #Draw the line between the first position of a point and the
    #last recorded position of the same point
    for i in range(len(curr_points)):
        cv.Line(output, (int(initial[i][0]),int(initial[i][1])), (int(curr_points[i][0]),int(curr_points[i][1])), (255,255,255))
        cv.Circle(output, (int(curr_points[i][0]),int(curr_points[i][1])), 3, (255,255,255))


    cv.Copy(gray, prev_gray)
    prev_points = curr_points


    cv.ShowImage(\"The Video\",  output)
    cv.WriteFrame(writer, output)
    cv.WaitKey(wait)

调用摄像头绘制：

import cv2.cv as cv

capture = cv.CaptureFromCAM(0)

width = int(cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FRAME_WIDTH))
height = int(cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FRAME_HEIGHT))

prev_gray = cv.CreateImage((width,height), 8, 1) #Will hold the frame at t-1
gray = cv.CreateImage((width,height), 8, 1) # Will hold the current frame

output = cv.CreateImage((width,height), 8, 3)

prevPyr = cv.CreateImage((height / 3, width + 8), 8, cv.CV_8UC1)
currPyr = cv.CreateImage((height / 3, width + 8), 8, cv.CV_8UC1)

max_count = 500
qLevel= 0.01
minDist = 10

begin = True

initial = []
features = []
prev_points = []
curr_points = []

while True:

    frame = cv.QueryFrame(capture)

    cv.CvtColor(frame, gray, cv.CV_BGR2GRAY) #Convert to gray
    cv.Copy(frame, output)


    if (len(prev_points) <= 10): #Try to get more points
        #Detect points on the image
        features = cv.GoodFeaturesToTrack(gray, None, None, max_count, qLevel, minDist)
        prev_points.extend(features) #Add the new points to list
        initial.extend(features) #Idem

    if begin:
        cv.Copy(gray, prev_gray) #Now we have two frames to compare
        begin = False

    #Compute movement
    curr_points, status, err = cv.CalcOpticalFlowPyrLK(prev_gray, gray, prevPyr, currPyr, prev_points, (10, 10), 3, (cv.CV_TERMCRIT_ITER|cv.CV_TERMCRIT_EPS,20, 0.03), 0)

    #If points status are ok and distance not negligible keep the point
    k = 0
    for i in range(len(curr_points)):
        nb =  abs( int(prev_points[i][0])-int(curr_points[i][0]) ) + abs( int(prev_points[i][1])-int(curr_points[i][1]) )
        if status[i] and  nb > 2 :
            initial[k] = initial[i]
            curr_points[k] = curr_points[i]
            k += 1

    curr_points = curr_points[:k]
    initial = initial[:k]
    for i in range(len(curr_points)):
        cv.Line(output, (int(initial[i][0]),int(initial[i][1])), (int(curr_points[i][0]),int(curr_points[i][1])), (255,255,255))
        cv.Circle(output, (int(curr_points[i][0]),int(curr_points[i][1])), 3, (255,255,255))


    cv.Copy(gray, prev_gray)
    prev_points = curr_points

    cv.ShowImage(\"The Video\", output)
    c = cv.WaitKey(1)
    if c == 27: #Esc on Windows
        break