如何使用Nodejs将视频帧直接读入内存？

Question

我想做的是拍摄一段视频并将其分解为帧，然后将这些帧传递给模型以检测每一帧中的对象，但问题是提取过程花费了太多时间，我不需要我磁盘上的帧。

Answer 1

fmpeg-stream 提供流功能。所以不需要写入文件。

也可以直接使用ffmpeg和spawn新的子进程。它的 .stdout 属性是一个可读流。在event数据上，可以读取chunk。

const fs = require("fs");
const tf = require("@tensorflow/tfjs-node")

const logStream = fs.createWriteStream('./logFile.log');


const spawnProcess = require('child_process').spawn,
    ffmpeg = spawnProcess('ffmpeg', [
        '-i', 'videfile.mp4',
        '-vcodec', 'png',
        '-f', 'rawvideo',
        '-s', 'h*w', // size of one frame
        'pipe:1'
    ]);

ffmpeg.stderr.pipe(logStream); // for debugging

let i = 0

ffmpeg.stdout.on('data', (data) => {
    try {
        console.log(tf.node.decodeImage(data).shape)
        console.log(`${++i} frames read`)
        // dispose all tensors
    } catch(e) {
        console.log(e)
    } 
})


ffmpeg.on('close', function (code) {
    console.log('child process exited with code ' + code);
});

解码图像在 try catch 块中，以防止当块与帧不匹配时引发错误。

防止解码与图像不对应的块的更健壮的代码如下：

const { Transform } = require("stream")

class ExtractFrames extends Transform {
    constructor(delimiter) {
        super({ readableObjectMode: true })
        this.delimiter = Buffer.from(delimiter, "hex")
        this.buffer = Buffer.alloc(0)
    }

    _transform(data, enc, cb) {
        // Add new data to buffer
        this.buffer = Buffer.concat([this.buffer, data])
        const start = this.buffer.indexOf(this.delimiter)
        if (start < 0) return // there's no frame data at all
        const end = this.buffer.indexOf(
            this.delimiter,
            start + this.delimiter.length,
        )
        if (end < 0) return // we haven't got the whole frame yet
        this.push(this.buffer.slice(start, end)) // emit a frame
        this.buffer = this.buffer.slice(end) // remove frame data from buffer

        if (start > 0) console.error(`Discarded ${start} bytes of invalid data`)
        cb()
    }
    _flush(callback) {
        // push remaining buffer to readable stream
        callback(null, this.buffer);
    }
}

const fs = require("fs");
const tf = require("@tensorflow/tfjs-node")

const logStream = fs.createWriteStream('./logFile.log');


const spawnProcess = require('child_process').spawn,
    ffmpeg = spawnProcess('ffmpeg', [
        '-i', 'generique.mp4',
        '-vcodec', 'mjpeg',
        '-f', 'rawvideo',
        '-s', '420x360', // size of one frame
        'pipe:1'
    ]);

ffmpeg.stderr.pipe(logStream); // for debugging

let i = 0

ffmpeg.stdout
.pipe(new ExtractFrames("FFD8FF")).on('data', (data) => {
    try {
        console.log(tf.node.decodeImage(data).shape)
        console.log(`${++i} frames read`)
        // dispose all tensors
    } catch(e) {
        console.log(e)
    } 
})


ffmpeg.on('close', function (code) {
    console.log('child process exited with code ' + code);
});

尽管上面的代码有效，它仍然会很快填满内存。将帧提取与数据处理本身分开会有所帮助。

async function* frames() {
        let resolve;
        let promise = new Promise(r => resolve = r);
        let bool = true;
        ls.stdout.pipe(new ExtractFrames("FFD8FF")).on('data', data => {
            resolve(data); 

            promise = new Promise(r => resolve = r); 
        });

        ls.on('close', function (code) {
            bool = false
            console.log('code')
        });

        while (bool) {
            const data = await promise; 
            yield data;
        }
}

(async() => {
     // data processing
     // possibly create tf.dataset for training
     for await (const data of stream()) {
         console.log(tf.node.decodeImage(data).shape)
         console.log(data);
    }
})()

如何使用Nodejs将视频帧直接读入内存？

how to read video Frames directly into memory with Nodejs?

node.js

tensorflow.js