OpenCV光流示例代码错误

Question

import numpy as np
import cv2

cap = cv2.VideoCapture("slow.avi")
while not cap.isOpened():
    cap = cv2.VideoCapture("slow.avi")
    cv2.waitKey(1000)
    print "Wait for the header"

# params for ShiTomasi corner detection
feature_params = dict( maxCorners = 100,
                       qualityLevel = 0.3,
                       minDistance = 7,
                       blockSize = 7 )

# Parameters for lucas kanade optical flow
lk_params = dict( winSize  = (15,15),
                  maxLevel = 2,
                  criteria = (cv2.TERM_CRITERIA_EPS | cv2.TERM_CRITERIA_COUNT, 10, 0.03))

# Create some random colors
color = np.random.randint(0,255,(100,3))

# Take first frame and find corners in it
ret, old_frame = cap.read()
old_gray = cv2.cvtColor(old_frame, cv2.COLOR_BGR2GRAY)
p0 = cv2.goodFeaturesToTrack(old_gray, mask = None, **feature_params)

#print old_frame

# Create a mask image for drawing purposes
mask = np.zeros_like(old_frame)

while(1):    
    ret,frame = cap.read()

    frame_gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    # calculate optical flow
    p1, st, err = cv2.calcOpticalFlowPyrLK(old_gray, frame_gray, p0, None, **lk_params)

    # Select good points
    good_new = p1[st==1]
    good_old = p0[st==1]

    # draw the tracks
    for i,(new,old) in enumerate(zip(good_new,good_old)):
        a,b = new.ravel()
        c,d = old.ravel()
        mask = cv2.line(mask, (a,b),(c,d), color[i].tolist(), 2)
        frame = cv2.circle(frame,(a,b),5,color[i].tolist(),-1)
    img = cv2.add(frame,mask)

    cv2.imshow('frame',img)
    k = cv2.waitKey(30) & 0xff
    if k == 27:
        break

    # Now update the previous frame and previous points
    old_gray = frame_gray.copy()
    p0 = good_new.reshape(-1,1,2)

cv2.destroyAllWindows()
cap.release()

我从这个链接 http://docs.opencv.org/trunk/doc/py_tutorials/py_video/py_lucas_kanade/py_lucas_kanade.html 复制了光流的示例代码，但它没有正常工作，并且出现了这个错误信息。我尝试修复它，但没有成功。

line 56, in <module>
cv2.imshow('frame',img)
error: ..\..\..\opencv-2.4.8.1\modules\highgui\src\window.cpp:269: 
error: (-215) size.width>0 && size.height>0 in function cv::imshow

Answer 1

错误信息很明确：你的图片无法显示（在第56行，cv2.imshow('frame', img)），因为它基本上是空的——宽度和高度都是零。

要确认这一点，可以在显示图片之前，先用 print img.shape 打印一下图片的形状。然后你可能需要调试你的代码，逐行检查，找出结果和你预期的不一样的地方。

Answer 2

这里出现了错误：

mask = cv2.line(mask, (a,b),(c,d), color[i].tolist(), 2)
frame = cv2.circle(frame,(a,b),5,color[i].tolist(),-1)

改成：

#drawing is inplace replacement, line() and circle() will return None!
cv2.line(mask, (a,b),(c,d), color[i].tolist(), 2)
cv2.circle(frame,(a,b),5,color[i].tolist(),-1)