ios - Extracting h264 from CMBlockBuffer

Question

I am using Apple VideoTool Box (iOS) to compress raw frames captured by the device camera.

My callback is being called with a CMSampleBufferRef object that contains CMBlockBuffer.

The CMBlockBuffer object contain the H264 elementary stream but I didn't find any way to get a pointer to the elementary stream.

When I printed into the console the CMSampleBufferRef object I got:

(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)

Answer 1

I've been struggling with this myself for quite some time now, and have finally figured everything out.

The function CMBlockBufferGetDataPointer gives you access to all the data you need, but there are a few not very obvious things you need to do to convert it to an elementary stream.

AVCC vs Annex B format

The data in the CMBlockBuffer is stored in AVCC format, while elementary streams are typically following the Annex B specification (here is an excellent overview of the two formats). In the AVCC format, the 4 first bytes contains the length of the NAL unit (another word for H264 packet). You need to replace this header with the 4 byte start code: 0x00 0x00 0x00 0x01, which functions as a separator between NAL units in an Annex B elementary stream (the 3 byte version 0x00 0x00 0x01 works fine too).

Multiple NAL units in a single CMBlockBuffer

The next not very obvious thing is that a single CMBlockBuffer will sometimes contain multiple NAL units. Apple seems to add an additional NAL unit (SEI) containing metadata to every I-Frame NAL unit (also called IDR). This is probably why you are seeing multiple buffers in a single CMBlockBuffer object. However, the CMBlockBufferGetDataPointer function gives you a single pointer with access to all the data. That being said, the presence of multiple NAL units complicates the conversion of the AVCC headers. Now you actually have to read the length value contained in the AVCC header to find the next NAL unit, and continue converting headers until you have reached the end of the buffer.

Big-Endian vs Little-Endian

The next not very obvious thing is that the AVCC header is stored in Big-Endian format, and iOS is Little-Endian natively. So when you are reading the length value contained in an AVCC header pass it to the CFSwapInt32BigToHost function first.

SPS and PPS NAL units

The final not very obvious thing is that the data inside the CMBlockBuffer does not contain the parameter NAL units SPS and PPS, which contains configuration parameters for the decoder such as profile, level, resolution, frame rate. These are stored as metadata in the sample buffer's format description and can be accessed via the function CMVideoFormatDescriptionGetH264ParameterSetAtIndex. Note that you have to add the start codes to these NAL units before sending. The SPS and PPS NAL units does not have to be sent with every new frame. A decoder only needs to read them once, but it is common to resend them periodically, for example before every new I-frame NAL unit.

Code Example

Below is a code example taking all of these things into account.

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;
    }
}