如何使用cv2.VideoWriter输出经过x265压缩的视频

Question

如何使用cv2.VideoWriter输出经过x265压缩的视频

13

我正在对一段45分钟1.2GB的视频进行渲染，该视频有800,000个大小为1344x756的帧，并且视频格式为mp4。我试图输出一个使用x265压缩的视频，但问题是当我使用cv2.VideoWriter时，10分钟视频的输出大小超过了2GB，这不是我的意图，所以我在Mac OSX和ubuntu 18上尝试了以下方法：

codec = cv2.VideoWriter_fourcc(*'HEVC')
out = cv2.VideoWriter('output.mp4', 'HEVC', fps, (width, height))

我得到的只是运行时警告：

OpenCV: FFMPEG: tag 0x43564548/'HEVC' is not found (format 'mp4 / MP4 (MPEG-4 Part 14)')'

我想要实现的并不是最高质量的输出，而是尽可能达到良好的质量并且文件大小最小化。

- user12690225

不要使用ffmpeg作为子进程，并通过管道传输视频数据，而应该使用PyAV。它是ffmpeg的适当封装器。将numpy矩阵放入视频文件的示例：https://github.com/PyAV-Org/PyAV/blob/main/examples/numpy/generate_video.py - Christoph Rackwitz

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- Rotem · Accepted Answer

据我所知，OpenCV VideoWriter目前尚不支持HEVC编码。

我建议您使用FFmpeg作为子进程，并将渲染的帧管道传输到ffmpeg的stdin输入流中。

您可以使用像ffmpeg-python这样的Python绑定，或者使用Python subprocess来执行ffmpeg。

与cv2.VideoWriter相比，使用ffmpeg可以更好地控制视频编码参数（cv2.VideoWriter设计了简单易用的灵活性）。

下面是一段示例代码，它渲染50个帧并将帧流传输到ffmpeg，以HEVC视频编解码器编码MP4视频文件：

import cv2
import numpy as np
import subprocess as sp
import shlex

width, height, n_frames, fps = 1344, 756, 50, 25  # 50 frames, resolution 1344x756, and 25 fps

output_filename = 'output.mp4'

# Open ffmpeg application as sub-process
# FFmpeg input PIPE: RAW images in BGR color format
# FFmpeg output MP4 file encoded with HEVC codec.
# Arguments list:
# -y                   Overwrite output file without asking
# -s {width}x{height}  Input resolution width x height (1344x756)
# -pixel_format bgr24  Input frame color format is BGR with 8 bits per color component
# -f rawvideo          Input format: raw video
# -r {fps}             Frame rate: fps (25fps)
# -i pipe:             ffmpeg input is a PIPE
# -vcodec libx265      Video codec: H.265 (HEVC)
# -pix_fmt yuv420p     Output video color space YUV420 (saving space compared to YUV444)
# -crf 24              Constant quality encoding (lower value for higher quality and larger output file).
# {output_filename}    Output file name: output_filename (output.mp4)
process = sp.Popen(shlex.split(f'ffmpeg -y -s {width}x{height} -pixel_format bgr24 -f rawvideo -r {fps} -i pipe: -vcodec libx265 -pix_fmt yuv420p -crf 24 {output_filename}'), stdin=sp.PIPE)

# Build synthetic video frames and write them to ffmpeg input stream.
for i in range(n_frames):
    # Build synthetic image for testing ("render" a video frame).
    img = np.full((height, width, 3), 60, np.uint8)
    cv2.putText(img, str(i+1), (width//2-100*len(str(i+1)), height//2+100), cv2.FONT_HERSHEY_DUPLEX, 10, (255, 30, 30), 20)  # Blue number

    # Write raw video frame to input stream of ffmpeg sub-process.
    process.stdin.write(img.tobytes())

# Close and flush stdin
process.stdin.close()

# Wait for sub-process to finish
process.wait()

# Terminate the sub-process
process.terminate()  # Note: We don't have to terminate the sub-process (after process.wait(), the sub-process is supposed to be closed).

注：

ffmpeg executable must be in the execution path of the Python script.

For Linux, in case ffmpeg is not in the execution path, you may use the full path:

 process = sp.Popen(shlex.split(f'/usr/bin/ffmpeg -y -s {width}x{height} -pixel_format bgr24 -f rawvideo -r {fps} -i pipe: -vcodec libx265 -pix_fmt yuv420p -crf 24 {output_filename}'), stdin=sp.PIPE)

(Assuming ffmpeg executable is in /usr/bin/).

Python 3's f-Strings syntax requires Python version 3.6 or above.

C++示例:

在Python中，有多个FFmpeg绑定可以实现H.265视频编码。
在C++中，可选项要少得多...

我们可以使用类似的解决方案来使用C++（使用FFmpeg子进程）。
为了执行FFmpeg子进程并打开stdin管道，我们可以在Windows中使用_popen，在Linux中使用popen。

注意：

我注意到_popen不如CreateProcess可靠，并且我们需要等待（例如在结尾处等待一秒钟）输出文件关闭。
我不确定在Linux中是否存在类似的问题与popen。

C++代码示例：

#include <stdio.h>
#include <chrono>
#include <thread>
#include "opencv2/opencv.hpp"
#include <string>

int main()
{
    // 50 frames, resolution 1344x756, and 25 fps
    int width = 1344;
    int height = 756;
    int n_frames = 50;
    int fps = 25;

    const std::string output_filename = "output.mp4"; //Example for file name with spaces: "\"output with spaces.mp4\""

    //Open ffmpeg application as sub - process
    //FFmpeg input PIPE : RAW images in BGR color format
    //FFmpeg output MP4 file encoded with HEVC codec (using libx265 encoder).
    std::string ffmpeg_cmd = std::string("ffmpeg -y -f rawvideo -r ") + std::to_string(fps) +
        " -video_size " + std::to_string(width) + "x" + std::to_string(height) +
        " -pixel_format bgr24 -i pipe: -vcodec libx265 -crf 24 -pix_fmt yuv420p " + output_filename;

    //Execute FFmpeg as sub-process, open stdin pipe (of FFmpeg sub-process) for writing.
    //In Windows we need to use _popen and in Linux popen
#ifdef _MSC_VER
    FILE* pipeout = _popen(ffmpeg_cmd.c_str(), "wb");   //Windows (ffmpeg.exe must be in the execution path)
#else
    //https://batchloaf.wordpress.com/2017/02/12/a-simple-way-to-read-and-write-audio-and-video-files-in-c-using-ffmpeg-part-2-video/
    FILE* pipeout = popen(ffmpeg_cmd.c_str(), "w");     //Linux (assume ffmpeg exist in /usr/bin/ffmpeg (and in path).
#endif

    for (int i = 0; i < n_frames; i++)
    {
        //Build synthetic image for testing ("render" a video frame):
        cv::Mat frame = cv::Mat(height, width, CV_8UC3);
        frame = cv::Scalar(60, 60, 60); //Fill background with dark gray
        cv::putText(frame, std::to_string(i+1), cv::Point(width/2 - 100*(int)(std::to_string(i+1).length()), height/2+100), cv::FONT_HERSHEY_DUPLEX, 10, cv::Scalar(255, 30, 30), 20);  // Draw a blue number
        //cv::imshow("frame", frame); cv::waitKey(1); //Show the frame for testing

        //Write width*height*3 bytes to stdin pipe of FFmpeg sub-process (assume frame data is continuous in the RAM).
        fwrite(frame.data, 1, (size_t)width*height*3, pipeout);
    }

    //Flush and close input and output pipes
    fflush(pipeout);

#ifdef _MSC_VER
    _pclose(pipeout);   //Windows
#else
    pclose(pipeout);    //Linux
#endif

    //It looks like we need to wait one more second at the end. //https://dev59.com/-2855IYBdhLWcg3wuGwM#62804585
    std::this_thread::sleep_for(std::chrono::milliseconds(1000)); // sleep for 1 second

    return 0;
}