从 CMBlockBuffer 中提取 h264

Extracting h264 from CMBlockBuffer

我正在使用 Apple VideoTool Box (iOS) 压缩设备相机捕获的原始帧。

正在使用包含 CMBlockBuffer 的 CMSampleBufferRef 对象调用我的回调。

CMBlockBuffer 对象包含 H264 基本流,但我没有找到任何方法来获取指向基本流的指针。

当我将 CMSampleBufferRef 对象打印到控制台时,我得到:

(lldb) po blockBufferRef
CMBlockBuffer 0x1701193e0 totalDataLength: 4264 retainCount: 1 allocator: 0x1957c2c80 subBlockCapacity: 2
 [0] 4264 bytes @ offset 128 Buffer Reference:
    CMBlockBuffer 0x170119350 totalDataLength: 4632 retainCount: 1 allocator: 0x1957c2c80 subBlockCapacity: 2
     [0] 4632 bytes @ offset 0 Memory Block 0x10295c000, 4632 bytes (custom V=0 A=0x0 F=0x18498bb44 R=0x0)

我设法获取指针的 CMBlockBuffer 对象似乎包含另一个不可访问的 CMBlockBuferRef(4632 字节)。

任何人都可以 post 如何访问 H264 elemantry 流?

谢谢!

我自己也为此苦苦挣扎了一段时间,终于弄明白了。

函数 CMBlockBufferGetDataPointer 可让您访问所需的所有数据,但要将其转换为基本流,您需要做一些不太明显的事情。

AVCC 与附件 B 格式

CMBlockBuffer 中的数据以 AVCC 格式存储,而基本流通常遵循 Annex B 规范(here 是对这两种格式的出色概述)。在 AVCC 格式中,前 4 个字节包含 NAL 单元的长度(H264 数据包的另一种说法)。您需要将此 header 替换为 4 字节起始代码:0x00 0x00 0x00 0x01,它在 Annex B 基本流中用作 NAL 单元之间的分隔符(3 字节版本 0x00 0x00 0x01 也可以正常工作)。

单个 CMBlockBuffer 中的多个 NAL 单元

下一个不是很明显的事情是单个 CMBlockBuffer 有时会包含多个 NAL 单元。 Apple 似乎向每个 I-Frame NAL 单元(也称为 IDR)添加了一个包含元数据的额外 NAL 单元 (SEI)。这可能就是您在单个 CMBlockBuffer object 中看到多个缓冲区的原因。但是,CMBlockBufferGetDataPointer 函数为您提供了一个可以访问所有数据的指针。也就是说,多个 NAL 单元的存在使 AVCC header 的转换复杂化。现在你实际上必须读取 AVCC header 中包含的长度值以找到下一个 NAL 单元,并继续转换 headers 直到到达缓冲区的末尾。

Big-Endian 对比 Little-Endian

接下来不是很明显的事情是AVCC header 以Big-Endian 格式存储,而iOS 是Little-Endian 本机。因此,当您读取 AVCC header 中包含的长度值时,首先将其传递给 CFSwapInt32BigToHost 函数。

SPS 和 PPS NAL 单元

最后不是很明显的是CMBlockBuffer里面的数据不包含参数NAL units SPS和PPS,里面包含解码器的配置参数,比如profile,level,resolution,frame rate。这些作为元数据存储在样本缓冲区的格式描述中,可以通过函数 CMVideoFormatDescriptionGetH264ParameterSetAtIndex 访问。请注意,您必须在发送前将起始代码添加到这些 NAL 单元。 SPS 和 PPS NAL 单元不必与每个新帧一起发送。解码器只需要读取一次,但通常会定期重新发送它们,例如在每个新的 I-frame NAL 单元之前。

代码示例

下面是一个考虑了所有这些因素的代码示例。

static void videoFrameFinishedEncoding(void *outputCallbackRefCon,
                                       void *sourceFrameRefCon,
                                       OSStatus status,
                                       VTEncodeInfoFlags infoFlags,
                                       CMSampleBufferRef sampleBuffer) {
    // Check if there were any errors encoding
    if (status != noErr) {
        NSLog(@"Error encoding video, err=%lld", (int64_t)status);
        return;
    }

    // In this example we will use a NSMutableData object to store the
    // elementary stream.
    NSMutableData *elementaryStream = [NSMutableData data];


    // Find out if the sample buffer contains an I-Frame.
    // If so we will write the SPS and PPS NAL units to the elementary stream.
    BOOL isIFrame = NO;
    CFArrayRef attachmentsArray = CMSampleBufferGetSampleAttachmentsArray(sampleBuffer, 0);
    if (CFArrayGetCount(attachmentsArray)) {
        CFBooleanRef notSync;
        CFDictionaryRef dict = CFArrayGetValueAtIndex(attachmentsArray, 0);
        BOOL keyExists = CFDictionaryGetValueIfPresent(dict,
                                                       kCMSampleAttachmentKey_NotSync,
                                                       (const void **)&notSync);
        // An I-Frame is a sync frame
        isIFrame = !keyExists || !CFBooleanGetValue(notSync);
    }

    // This is the start code that we will write to
    // the elementary stream before every NAL unit
    static const size_t startCodeLength = 4;
    static const uint8_t startCode[] = {0x00, 0x00, 0x00, 0x01};

    // Write the SPS and PPS NAL units to the elementary stream before every I-Frame
    if (isIFrame) {
        CMFormatDescriptionRef description = CMSampleBufferGetFormatDescription(sampleBuffer);

        // Find out how many parameter sets there are
        size_t numberOfParameterSets;
        CMVideoFormatDescriptionGetH264ParameterSetAtIndex(description,
                                                           0, NULL, NULL,
                                                           &numberOfParameterSets,
                                                           NULL);

        // Write each parameter set to the elementary stream
        for (int i = 0; i < numberOfParameterSets; i++) {
            const uint8_t *parameterSetPointer;
            size_t parameterSetLength;
            CMVideoFormatDescriptionGetH264ParameterSetAtIndex(description,
                                                               i,
                                                               &parameterSetPointer,
                                                               &parameterSetLength,
                                                               NULL, NULL);

            // Write the parameter set to the elementary stream
            [elementaryStream appendBytes:startCode length:startCodeLength];
            [elementaryStream appendBytes:parameterSetPointer length:parameterSetLength];
        }
    }

    // Get a pointer to the raw AVCC NAL unit data in the sample buffer
    size_t blockBufferLength;
    uint8_t *bufferDataPointer = NULL;
    CMBlockBufferGetDataPointer(CMSampleBufferGetDataBuffer(sampleBuffer),
                                0,
                                NULL,
                                &blockBufferLength,
                                (char **)&bufferDataPointer);

    // Loop through all the NAL units in the block buffer
    // and write them to the elementary stream with
    // start codes instead of AVCC length headers
    size_t bufferOffset = 0;
    static const int AVCCHeaderLength = 4;
    while (bufferOffset < blockBufferLength - AVCCHeaderLength) {
        // Read the NAL unit length
        uint32_t NALUnitLength = 0;
        memcpy(&NALUnitLength, bufferDataPointer + bufferOffset, AVCCHeaderLength);
        // Convert the length value from Big-endian to Little-endian
        NALUnitLength = CFSwapInt32BigToHost(NALUnitLength);
        // Write start code to the elementary stream
        [elementaryStream appendBytes:startCode length:startCodeLength];
        // Write the NAL unit without the AVCC length header to the elementary stream
        [elementaryStream appendBytes:bufferDataPointer + bufferOffset + AVCCHeaderLength
                               length:NALUnitLength];
        // Move to the next NAL unit in the block buffer
        bufferOffset += AVCCHeaderLength + NALUnitLength;
    }
}   

感谢安东的出色回答!我正在为有兴趣直接在基于 Swift 的项目中使用此处讨论的概念的人们提供您的解决方案的简单 Swift 端口。

public func didEncodeFrame(frame: CMSampleBuffer)
{
    print ("Received encoded frame in delegate...")

    //----AVCC to Elem stream-----//
    var elementaryStream = NSMutableData()

    //1. check if CMBuffer had I-frame
    var isIFrame:Bool = false
    let attachmentsArray:CFArray = CMSampleBufferGetSampleAttachmentsArray(frame, false)!
    //check how many attachments
    if ( CFArrayGetCount(attachmentsArray) > 0 ) {
        let dict = CFArrayGetValueAtIndex(attachmentsArray, 0)
        let dictRef:CFDictionaryRef = unsafeBitCast(dict, CFDictionaryRef.self)
        //get value
        let value = CFDictionaryGetValue(dictRef, unsafeBitCast(kCMSampleAttachmentKey_NotSync, UnsafePointer<Void>.self))
        if ( value != nil ){
            print ("IFrame found...")
            isIFrame = true
        }
    }

    //2. define the start code
    let nStartCodeLength:size_t = 4
    let nStartCode:[UInt8] = [0x00, 0x00, 0x00, 0x01]

    //3. write the SPS and PPS before I-frame
    if ( isIFrame == true ){
        let description:CMFormatDescriptionRef = CMSampleBufferGetFormatDescription(frame)!
        //how many params
        var numParams:size_t = 0
        CMVideoFormatDescriptionGetH264ParameterSetAtIndex(description, 0, nil, nil, &numParams, nil)

        //write each param-set to elementary stream
        print("Write param to elementaryStream ", numParams)
        for i in 0..<numParams {
            var parameterSetPointer:UnsafePointer<UInt8> = nil
            var parameterSetLength:size_t = 0
            CMVideoFormatDescriptionGetH264ParameterSetAtIndex(description, i, &parameterSetPointer, &parameterSetLength, nil, nil)
            elementaryStream.appendBytes(nStartCode, length: nStartCodeLength)
            elementaryStream.appendBytes(parameterSetPointer, length: unsafeBitCast(parameterSetLength, Int.self))
        }
    }

    //4. Get a pointer to the raw AVCC NAL unit data in the sample buffer
    var blockBufferLength:size_t = 0
    var bufferDataPointer: UnsafeMutablePointer<Int8> = nil
    CMBlockBufferGetDataPointer(CMSampleBufferGetDataBuffer(frame)!, 0, nil, &blockBufferLength, &bufferDataPointer)
    print ("Block length = ", blockBufferLength)

    //5. Loop through all the NAL units in the block buffer
    var bufferOffset:size_t = 0
    let AVCCHeaderLength:Int = 4
    while (bufferOffset < (blockBufferLength - AVCCHeaderLength) ) {
        // Read the NAL unit length
        var NALUnitLength:UInt32 =  0
        memcpy(&NALUnitLength, bufferDataPointer + bufferOffset, AVCCHeaderLength)
        //Big-Endian to Little-Endian
        NALUnitLength = CFSwapInt32(NALUnitLength)
        if ( NALUnitLength > 0 ){
            print ( "NALUnitLen = ", NALUnitLength)
            // Write start code to the elementary stream
            elementaryStream.appendBytes(nStartCode, length: nStartCodeLength)
            // Write the NAL unit without the AVCC length header to the elementary stream
            elementaryStream.appendBytes(bufferDataPointer + bufferOffset + AVCCHeaderLength, length: Int(NALUnitLength))
            // Move to the next NAL unit in the block buffer
            bufferOffset += AVCCHeaderLength + size_t(NALUnitLength);
            print("Moving to next NALU...")
        }
    }
    print("Read completed...")
}