输出视频文件为空，因为clip.shape与out的分辨率不匹配。
out的分辨率在out = cv2.VideoWriter('merged_video.mp4', fourcc, 30, (video_width, video_height))中定义为1920x1080。
执行out.write(clip)时，clip的形状必须为(1080, 1920, 3)。
clip = np.concatenate((clip, frame), axis=1)沿着水平轴连接子帧，因此结果帧比1920像素宽得多。
我们最好打开12个cv2.VideoCapture对象，而不是尝试将所有帧放在一个列表中，然后连接所有帧。

• 从每个视频文件读取一帧-一帧来自CAM010000000_compressed.mp4，一帧来自CAM020000000_compressed.mp4...一个来自CAM120000000_compressed.mp4
• 将子帧并排排列，形成4x 3 clip镶嵌帧。
• 将镶嵌clip帧写入输出视频文件。

假设12个文件可能具有不同的持续时间，解决方案可能有点复杂-我们必须使用前一帧的情况下一个文件结束，而其他文件没有。
注意主循环不能迭代行和列-我们必须迭代输出视频文件的帧（我们不能返回并更新已经写入的视频帧）。
更新的代码示例：

import cv2
import numpy as np
#import math
#from tqdm import tqdm

# Set the dimensions of the merged video
video_width = 1920
video_height = 1080

# Set the dimensions of each video
sub_video_width = 480
sub_video_height = 360

# Set the margin between each video
margin = 20

# Create a black background frame
clip = np.zeros((video_height, video_width, 3), dtype=np.uint8)

file_name_list = ['CAM010000000_compressed.mp4', 'CAM020000000_compressed.mp4', 'CAM030000000_compressed.mp4', 'CAM040000000_compressed.mp4', 'CAM050000000_compressed.mp4', 'CAM060000000_compressed.mp4', 'CAM070000000_compressed.mp4', 'CAM080000000_compressed.mp4', 'CAM090000000_compressed.mp4', 'CAM100000000_compressed.mp4', 'CAM110000000_compressed.mp4', 'CAM120000000_compressed.mp4']

# Calculate the number of rows and columns
num_videos = len(file_name_list)
num_rows = 3
num_cols = 4
num_total = num_rows * num_cols

# Determine the size of each sub-clip
sub_video_x = sub_video_width + margin
sub_video_y = sub_video_height + margin

# Build a list of 12 cv2.VideoCapture objects 
cap_list = []
for filename in file_name_list:
    cap = cv2.VideoCapture(filename)
    cap_list.append(cap)

# Initialize the final video
#fourcc = cv2.VideoWriter_fourcc(*'mp4v')
#out = cv2.VideoWriter('merged_video.mp4', fourcc, 1, (video_width, video_height))

fourcc = cv2.VideoWriter_fourcc(*'avc1')  # In Windows, requires openh264-1.8.0-win64.dll
out = cv2.VideoWriter('merged_video.mp4', fourcc, 1, (video_width, video_height))  # Use 1 fps for testing

# Iterate over each row and column
sub_frames = [None]*num_total  # Initialize with Nones
while True:
    any_ret = False
    for i in range(num_rows):
        for j in range(num_cols):
            # Calculate the index of the video to be inserted
            video_index = i * num_cols + j

            ret, frame = cap_list[video_index].read()  # Read frame from "cap object" that matches video_index

            if ret:
                frame = cv2.resize(frame, (sub_video_width, sub_video_height))  # Resize the "sub-frame" to the desired resolution.
                sub_frames[video_index] = frame  # Store frame in the list.

            any_ret |= ret  # Going to be False only when ret of from all videos is false. 

    if not any_ret:
        break  # Break the main loop when all video files are ended.

    for i in range(num_rows):
        for j in range(num_cols):
            # Calculate the index of the video to be inserted
            video_index = i * num_cols + j
        
            # Place the sub_frame in the correct location in "clip 4x3 mosaic frame"
            clip[i*sub_video_height:i*sub_video_height+sub_video_height, j*sub_video_width:j*sub_video_width+sub_video_width, :] = sub_frames[video_index]

            # Calculate the position of the sub-clip
            x = sub_video_width * j + margin//4  # sub_video_x * j + margin
            y = sub_video_height * i + margin  # sub_video_y * i + margin

            # Add the filename to the sub-clip
            font = cv2.FONT_HERSHEY_SIMPLEX
            text = file_name_list[video_index]
            textsize = cv2.getTextSize(text, font, 1, 2)[0]
            text_x = x #+ sub_video_width - textsize[0] - margin ???
            text_y = y + textsize[1] #+ sub_video_height - textsize[1] - margin ???
            cv2.putText(clip, text, (text_x, text_y), font, 1, (255, 255, 255), 2, cv2.LINE_AA)

    # Add the sub-clip to the final video
    out.write(clip)  # write the clip 4x3 mosaic frame to the output video

# Release the final video
out.release()

for cap in cap_list:
    cap.release()

为了测试，我们可以使用FFmpeg CLI来构建合成视频文件。
如果代码示例不起作用，请使用合成视频文件-它们使解决方案可重复（不依赖于只有您才能访问的视频文件）。
创建12个示例视频文件：
ffmpeg -y -f lavfi -i testsrc=size=480x360:rate=1 -t 12 -vcodec libx264 CAM010000000_compressed.mp4
ffmpeg -y -f lavfi -i mandelbrot=size=480x360:rate=1 -t 10 -vcodec libx264 CAM020000000_compressed.mp4
ffmpeg -y -f lavfi -i color=blue:size=480x360:rate=1 -t 10 -vcodec libx264 CAM030000000_compressed.mp4
ffmpeg -y -f lavfi -i color=black:size=480x360:rate=1 -t 11 -vcodec libx264 CAM040000000_compressed.mp4
ffmpeg -y -f lavfi -i mandelbrot=size=480x360:rate=1 -t 9 -vcodec libx264 CAM050000000_compressed.mp4
ffmpeg -y -f lavfi -i mandelbrot=size=480x360:rate=1 -t 8 -vcodec libx264 CAM060000000_compressed.mp4
ffmpeg -y -f lavfi -i mandelbrot=size=480x360:rate=1 -t 10 -vcodec libx264 CAM070000000_compressed.mp4
ffmpeg -y -f lavfi -i mandelbrot=size=480x360:rate=1 -t 12 -vcodec libx264 CAM080000000_compressed.mp4
ffmpeg -y -f lavfi -i testsrc=size=480x360:rate=1 -t 10 -vcodec libx264 CAM090000000_compressed.mp4
ffmpeg -y -f lavfi -i testsrc=size=480x360:rate=1 -t 10 -vcodec libx264 CAM100000000_compressed.mp4
ffmpeg -y -f lavfi -i testsrc=size=480x360:rate=1 -t 10 -vcodec libx264 CAM110000000_compressed.mp4
ffmpeg -y -f lavfi -i testsrc=size=480x360:rate=1 -t 10 -vcodec libx264 CAM120000000_compressed.mp4
示例输出帧：