Errors/Empty video after using write() from cv2.VideoWriter

Question

I am performing template matching and everything seems to be working fine in cv2.imshow(). I want to write a video with all frames resulting from the template matching process. Part of my code includes resizing the template image and the video frame dimensions, I don't know if the video writing problem comes from this step or not. The errors are coming mostly from the out.write(frame) part.

I get the following error after using the code below:

cv2.error: OpenCV(3.4.2) C:\Miniconda3\conda-bld\opencv-suite_1534379934306\work\modules\videoio\src\cap_ffmpeg.cpp:296: error: (-215:Assertion failed) image.depth() == 0 in function 'cv::`anonymous-namespace'::CvVideoWriter_FFMPEG_proxy::write'

Any ideas?

import cv2
import numpy as np

cap = cv2.VideoCapture(input_video_file)
fps = int(round(cap.get(5)))
frame_width = int(cap.get(3))
frame_height = int(cap.get(4))
fourcc = cv2.VideoWriter_fourcc(*'mp4v')   # saving output video as .mp4
out = cv2.VideoWriter(output_video_file, fourcc, fps, (frame_width, frame_height))

# while loop where the real work happens
    while cap.isOpened():
        ret, frame = cap.read()
        if ret:
            if cv2.waitKey(28) & 0xFF == ord('q'):
                break
            
            # Read template image from path
            template = cv2.imread('D:\KU_Leuven\Computer_Vision\Assignment_1\Videos\Raspberry_Template.jpg',0)
            h2,w2 = template.shape[:2]
                    
            #resize video frame dimensions
            scale_percent =60 # percentage of original video frame size
            width = int(frame.shape[1] * scale_percent / 100)
            height = int(frame.shape[0] * scale_percent / 100)
            dim = (width, height)
            frame = cv2.resize(frame , dsize = dim)

            #resize template dimensions
            scale_percent = 16# percentage of original template image size
            width = int(template.shape[1] * scale_percent / 100)
            height = int(template.shape[0] * scale_percent / 100)
            dim = (width, height)
            template = cv2.resize(template , dsize = dim)
            
            method = eval('cv2.TM_CCOEFF_NORMED')

            gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
            
            # template match
            res = cv2.matchTemplate(gray,template,method)
            min_val, max_val, min_loc, max_loc = cv2.minMaxLoc(res)
            
            # Draw box around the Max likelihood location of the template
            top_left = max_loc
            h,w =template.shape[:2]
            bottom_right = (top_left[0] + w, top_left[1] + h)
            cv2.rectangle(gray,top_left, bottom_right, (255,0,0), 2)
            
            res = cv2.putText(res,'Likelihood Map from Template Matching', (50,50),cv2.FONT_HERSHEY_SIMPLEX, 0.9, (255,255,255), 2, cv2.LINE_AA) 
            
            cv2.rectangle(frame,top_left, bottom_right, (51,255,51), 2)
            cv2.imshow("Color Video Feed - Box",frame)

            cv2.imshow("Grayscale Video Feed - Box",gray)
            
            cv2.imshow("Likelihood Map",res) # produces the correct result

            res = cv2.resize(res,(frame_width,frame_height))         
            out.write(res)
                                    
            # Press Q on keyboard to  exit
            if cv2.waitKey(25) & 0xFF == ord('q'):
                break

        # Break the loop
        else:
            break

    # When everything done, release the video capture and writing object
    cap.release()
    out.release()
    # Closes all the frames
    cv2.destroyAllWindows()

Answer 1

The following may help for using opencv VideoWriter in order to save H264 encoded video into mp4 file:

import cv2
#print(cv2.__version__)

# Uncommenting the following would allow checking if your opencv build has FFMPEG and GSTREAMER support
#print(cv2.getBuildInformation())

# Video source
cap = cv2.VideoCapture('videotestsrc ! videoconvert ! queue ! video/x-raw, format=BGR ! appsink drop=1', cv2.CAP_GSTREAMER)

# fps is a float value, width and height are integer values
fps = float(cap.get(cv2.CAP_PROP_FPS))
frame_width = int(cap.get(cv2.CAP_PROP_FRAME_WIDTH))
frame_height = int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))

# Using FFMPEG backend: 4CC specifies codec, container is found from filename extension...here using mp4
output_video_file='test_H264.mp4'
fourcc = cv2.VideoWriter_fourcc(*'X264')        
out = cv2.VideoWriter(output_video_file, cv2.CAP_FFMPEG, fourcc, fps, (frame_width, frame_height))

# Alternatively, using GSTREAMER backend:
#out = cv2.VideoWriter('appsrc ! queue ! video/x-raw,format=BGR ! videoconvert ! x264enc ! h264parse ! qtmux ! filesink location=test_H264.mp4', cv2.CAP_GSTREAMER, 0, fps, (frame_width, frame_height))
# With NVIDIA, you may use HW encoder:
#out = cv2.VideoWriter('appsrc ! queue ! video/x-raw,format=BGR ! videoconvert ! video/x-raw,format=BGRx ! nvvidconv ! video/x-raw(memory:NVMM),format=NV12 ! nvv4l2h264enc ! h264parse ! qtmux ! filesink location=test_H264.mp4', cv2.CAP_GSTREAMER, 0, fps, (frame_width, frame_height))


if  not cap.isOpened():
    print("Failed to open capture")
    exit()

if  not out.isOpened():
    print("Failed to open writer")
    exit()

while cap.isOpened():
    ret, frame = cap.read()
    out.write(frame)
    cv2.imshow('Test', frame)
    cv2.waitKey(1)