“OpenCV保存视频失败，只有6KB”原因分析和解决方法
1. 问题描述：
学习OpenCV tutorials 时，根据用例中的内容，保存视频文件，只获取到一个6KB大小的文件。
代码如下：
# from camera
import cv2 as cv
cap = cv.VideoCapture(0)
# define the codec and create VideoWriter object
fourcc = cv.VideoWriter_fourcc(*'XVID')
out = cv.VideoWriter('output.avi', fourcc, 20.0, (640,480))
while(1):
ret, frame = cap.read()
if ret ==True:
out.write(frame.astype('uint8'))
cv.namedWindow('frame', cv.WINDOW_NORMAL)
cv.imshow('frame', frame)
if cv.waitKey(1) & 0xFF == ord('q'):
break
else:
break
cap.release()
out.release()
cv.destroyAllWindows()
2. 问题分析
调用VideoWriter函数时，保存的视频文件大小修改了，和原大小不一致，导致保存时评失败。
3. 解决方法
保持原视频大小，即可以解决该问题。 原视频文件的尺寸可以通过cap.get函数进行获取。
解决方法为：将上面的cv.VideoWriter函数中最后一个参数由(640,480)修改为(int(cap.get(3)),int(cap.get(4)))。即：out = cv.VideoWriter('output.avi', fourcc, 20.0, (int(cap.get(3)),int(cap.get(4))))
更改后，完整代码为：
# from camera
import cv2 as cv
cap = cv.VideoCapture(0)
# define the codec and create VideoWriter object
fourcc = cv.VideoWriter_fourcc(*'XVID')
out = cv.VideoWriter('output.avi', fourcc, 20.0, (int(cap.get(3)),int(cap.get(4))))
while(1):
ret, frame = cap.read()
if ret ==True:
out.write(frame.astype('uint8'))
cv.namedWindow('frame', cv.WINDOW_NORMAL)
cv.imshow('frame', frame)
if cv.waitKey(1) & 0xFF == ord('q'):
break
else:
break
cap.release()
out.release()
cv.destroyAllWindows()
上面代码中，get(3),get(4)分别代表原视频流文件的Width和Height，详细可以见下面的cap.get函数。
4. other
4.1 cap.get函数
函数原型为：retval = cv.VideoCapture.get( propId )，其中propId参数是枚举值cv::VideoCaptureProperties中的一个
其中各枚举值的含义为：
enum cv::VideoCaptureProperties {
cv::CAP_PROP_POS_MSEC =0,
cv::CAP_PROP_POS_FRAMES =1,
cv::CAP_PROP_POS_AVI_RATIO =2,
cv::CAP_PROP_FRAME_WIDTH =3,
cv::CAP_PROP_FRAME_HEIGHT =4,
cv::CAP_PROP_FPS =5,
cv::CAP_PROP_FOURCC =6,
cv::CAP_PROP_FRAME_COUNT =7,
cv::CAP_PROP_FORMAT =8,
cv::CAP_PROP_MODE =9,
cv::CAP_PROP_BRIGHTNESS =10,
cv::CAP_PROP_CONTRAST =11,
cv::CAP_PROP_SATURATION =12,
cv::CAP_PROP_HUE =13,
cv::CAP_PROP_GAIN =14,
cv::CAP_PROP_EXPOSURE =15,
cv::CAP_PROP_CONVERT_RGB =16,
cv::CAP_PROP_WHITE_BALANCE_BLUE_U =17,
cv::CAP_PROP_RECTIFICATION =18,
cv::CAP_PROP_MONOCHROME =19,
cv::CAP_PROP_SHARPNESS =20,
cv::CAP_PROP_AUTO_EXPOSURE =21,
cv::CAP_PROP_GAMMA =22,
cv::CAP_PROP_TEMPERATURE =23,
cv::CAP_PROP_TRIGGER =24,
cv::CAP_PROP_TRIGGER_DELAY =25,
cv::CAP_PROP_WHITE_BALANCE_RED_V =26,
cv::CAP_PROP_ZOOM =27,
cv::CAP_PROP_FOCUS =28,
cv::CAP_PROP_GUID =29,
cv::CAP_PROP_ISO_SPEED =30,
cv::CAP_PROP_BACKLIGHT =32,
cv::CAP_PROP_PAN =33,
cv::CAP_PROP_TILT =34,
cv::CAP_PROP_ROLL =35,
cv::CAP_PROP_IRIS =36,
cv::CAP_PROP_SETTINGS =37,
cv::CAP_PROP_BUFFERSIZE =38,
cv::CAP_PROP_AUTOFOCUS =39,
cv::CAP_PROP_SAR_NUM =40,
cv::CAP_PROP_SAR_DEN =41,
cv::CAP_PROP_BACKEND =42,
cv::CAP_PROP_CHANNEL =43,
cv::CAP_PROP_AUTO_WB =44,
cv::CAP_PROP_WB_TEMPERATURE =45
}
其中，3、4的含义为：
3 - Width of the frames in the video stream.
4 - Height of the frames in the video stream.
4.2 其他问题
使用cv.imshow('frame', frame)显示视频流中的一帧帧图像，有如下错误提示：
cv2.error: OpenCV(4.0.0) /Users/travis/build/skvark/opencv-python/opencv/modules/highgui/src/window.cpp:350: error: (-215:Assertion failed) size.width>0 && size.height>0 in function ‘imshow’
找到了一些原因：路径不全、路径中含有中文，等都进行尝试，未能解决问题