前言

• 這里我們重點(diǎn)介紹硬編碼的使用方式孔祸，也就是VideoToolBox框架的使用
• 編碼的流程：采集--> 獲取到視頻幀--> 對(duì)視頻幀進(jìn)行編碼 --> 獲取到視頻幀信息 --> 將編碼后的數(shù)據(jù)以NALU方式寫入到文件

視頻采集

• 視頻采集我們已經(jīng)在前面進(jìn)行了介紹和學(xué)習(xí)，所有這里就直接貼代碼发皿，只是我對(duì)采集過程進(jìn)行了一些簡單的封裝

  
  
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視頻硬件編碼

• 初始化壓縮編碼會(huì)話（VTCompressionSessionRef）
  • 在VideoToolbox框架的使用過程中崔慧，基本都是C語言函數(shù)
• 初始化后通過VTSessionSetProperty設(shè)置對(duì)象屬性
  • 編碼方式：H.264編碼
  • 幀率：每秒鐘多少幀畫面
  • 碼率：單位時(shí)間內(nèi)保存的數(shù)據(jù)量
  • 關(guān)鍵幀（GOPsize)間隔：多少幀為一個(gè)GOP
• 準(zhǔn)備編碼
• 代碼如下：

- (void)setupVideoSession {
    // 1.用于記錄當(dāng)前是第幾幀數(shù)據(jù)(畫面幀數(shù)非常多)
    self.frameID = 0;

    // 2.錄制視頻的寬度&高度
    int width = [UIScreen mainScreen].bounds.size.width;
    int height = [UIScreen mainScreen].bounds.size.height;

    // 3.創(chuàng)建CompressionSession對(duì)象,該對(duì)象用于對(duì)畫面進(jìn)行編碼
    // kCMVideoCodecType_H264 : 表示使用h.264進(jìn)行編碼
    // didCompressH264 : 當(dāng)一次編碼結(jié)束會(huì)在該函數(shù)進(jìn)行回調(diào),可以在該函數(shù)中將數(shù)據(jù),寫入文件中
    VTCompressionSessionCreate(NULL, width, height, kCMVideoCodecType_H264, NULL, NULL, NULL, didCompressH264, (__bridge void *)(self),  &_compressionSession);

    // 4.設(shè)置實(shí)時(shí)編碼輸出（直播必然是實(shí)時(shí)輸出,否則會(huì)有延遲）
    VTSessionSetProperty(self.compressionSession, kVTCompressionPropertyKey_RealTime, kCFBooleanTrue);

    // 5.設(shè)置期望幀率(每秒多少幀,如果幀率過低,會(huì)造成畫面卡頓)
    int fps = 30;
    CFNumberRef  fpsRef = CFNumberCreate(kCFAllocatorDefault, kCFNumberIntType, &fps);
    VTSessionSetProperty(self.compressionSession, kVTCompressionPropertyKey_ExpectedFrameRate, fpsRef);


    // 6.設(shè)置碼率(碼率: 編碼效率, 碼率越高,則畫面越清晰, 如果碼率較低會(huì)引起馬賽克 --> 碼率高有利于還原原始畫面,但是也不利于傳輸)
    int bitRate = 800*1024;
    CFNumberRef bitRateRef = CFNumberCreate(kCFAllocatorDefault, kCFNumberSInt32Type, &bitRate);
    VTSessionSetProperty(self.compressionSession, kVTCompressionPropertyKey_AverageBitRate, bitRateRef);
    NSArray *limit = @[@(bitRate * 1.5/8), @(1)];
    VTSessionSetProperty(self.compressionSession, kVTCompressionPropertyKey_DataRateLimits, (__bridge CFArrayRef)limit);

    // 7.設(shè)置關(guān)鍵幀（GOPsize)間隔
    int frameInterval = 30;
    CFNumberRef  frameIntervalRef = CFNumberCreate(kCFAllocatorDefault, kCFNumberIntType, &frameInterval);
    VTSessionSetProperty(self.compressionSession, kVTCompressionPropertyKey_MaxKeyFrameInterval, frameIntervalRef);

    // 8.基本設(shè)置結(jié)束, 準(zhǔn)備進(jìn)行編碼
    VTCompressionSessionPrepareToEncodeFrames(self.compressionSession);
}

• 將輸入的幀進(jìn)行編碼
  • 將CMSampleBufferRef轉(zhuǎn)成CVImageBufferRef
  • 開始對(duì)CVImageBufferRef進(jìn)行編碼

- (void)encodeSampleBuffer:(CMSampleBufferRef)sampleBuffer {
    // 1.將sampleBuffer轉(zhuǎn)成imageBuffer
    CVImageBufferRef imageBuffer = (CVImageBufferRef)CMSampleBufferGetImageBuffer(sampleBuffer);

    // 2.根據(jù)當(dāng)前的幀數(shù),創(chuàng)建CMTime的時(shí)間
    CMTime presentationTimeStamp = CMTimeMake(self.frameID++, 1000);
    VTEncodeInfoFlags flags;

    // 3.開始編碼該幀數(shù)據(jù)
    OSStatus statusCode = VTCompressionSessionEncodeFrame(self.compressionSession,
                                                          imageBuffer,
                                                          presentationTimeStamp,
                                                          kCMTimeInvalid,
                                                          NULL, (__bridge void * _Nullable)(self), &flags);
    if (statusCode == noErr) {
        NSLog(@"H264: VTCompressionSessionEncodeFrame Success");
    }
}

• 當(dāng)編碼成功后，將編碼后的碼流寫入文件
  • 編碼成功后會(huì)回調(diào)之前輸入的函數(shù)
  • 1> 先判斷是否是關(guān)鍵幀：
    • 如果是關(guān)鍵幀穴墅，則需要在寫入關(guān)鍵幀之前惶室，先寫入PPS、SPS的NALU
    • 取出PPS玄货、SPS數(shù)據(jù)皇钞，并且封裝成NALU單元，寫入文件
  • 2> 將I幀誉结、P幀鹅士、B幀分別封裝成NALU單元寫入文件

  • 寫入后，數(shù)據(jù)存儲(chǔ)方式：

    
    
    image.png
• 代碼如下：

// 編碼完成回調(diào)
void didCompressH264(void *outputCallbackRefCon, void *sourceFrameRefCon, OSStatus status, VTEncodeInfoFlags infoFlags, CMSampleBufferRef sampleBuffer) {

    // 1.判斷狀態(tài)是否等于沒有錯(cuò)誤
    if (status != noErr) {
        return;
    }

    // 2.根據(jù)傳入的參數(shù)獲取對(duì)象
    VideoEncoder* encoder = (__bridge VideoEncoder*)outputCallbackRefCon;

    // 3.判斷是否是關(guān)鍵幀
    bool isKeyframe = !CFDictionaryContainsKey( (CFArrayGetValueAtIndex(CMSampleBufferGetSampleAttachmentsArray(sampleBuffer, true), 0)), kCMSampleAttachmentKey_NotSync);
    // 判斷當(dāng)前幀是否為關(guān)鍵幀
    // 獲取sps & pps數(shù)據(jù)
    if (isKeyframe)
    {
        // 獲取編碼后的信息（存儲(chǔ)于CMFormatDescriptionRef中）
        CMFormatDescriptionRef format = CMSampleBufferGetFormatDescription(sampleBuffer);

        // 獲取SPS信息
        size_t sparameterSetSize, sparameterSetCount;
        const uint8_t *sparameterSet;
        CMVideoFormatDescriptionGetH264ParameterSetAtIndex(format, 0, &sparameterSet, &sparameterSetSize, &sparameterSetCount, 0 );

        // 獲取PPS信息
        size_t pparameterSetSize, pparameterSetCount;
        const uint8_t *pparameterSet;
        CMVideoFormatDescriptionGetH264ParameterSetAtIndex(format, 1, &pparameterSet, &pparameterSetSize, &pparameterSetCount, 0 );

        // 裝sps/pps轉(zhuǎn)成NSData惩坑，以方便寫入文件
        NSData *sps = [NSData dataWithBytes:sparameterSet length:sparameterSetSize];
        NSData *pps = [NSData dataWithBytes:pparameterSet length:pparameterSetSize];

        // 寫入文件
        [encoder gotSpsPps:sps pps:pps];
    }

    // 獲取數(shù)據(jù)塊
    CMBlockBufferRef dataBuffer = CMSampleBufferGetDataBuffer(sampleBuffer);
    size_t length, totalLength;
    char *dataPointer;
    OSStatus statusCodeRet = CMBlockBufferGetDataPointer(dataBuffer, 0, &length, &totalLength, &dataPointer);
    if (statusCodeRet == noErr) {
        size_t bufferOffset = 0;
        static const int AVCCHeaderLength = 4; // 返回的nalu數(shù)據(jù)前四個(gè)字節(jié)不是0001的startcode掉盅，而是大端模式的幀長度length

        // 循環(huán)獲取nalu數(shù)據(jù)
        while (bufferOffset < totalLength - AVCCHeaderLength) {
            uint32_t NALUnitLength = 0;
            // Read the NAL unit length
            memcpy(&NALUnitLength, dataPointer + bufferOffset, AVCCHeaderLength);

            // 從大端轉(zhuǎn)系統(tǒng)端
            NALUnitLength = CFSwapInt32BigToHost(NALUnitLength);

            NSData* data = [[NSData alloc] initWithBytes:(dataPointer + bufferOffset + AVCCHeaderLength) length:NALUnitLength];
            [encoder gotEncodedData:data isKeyFrame:isKeyframe];

            // 移動(dòng)到寫一個(gè)塊，轉(zhuǎn)成NALU單元
            // Move to the next NAL unit in the block buffer
            bufferOffset += AVCCHeaderLength + NALUnitLength;
        }
    }
}

- (void)gotSpsPps:(NSData*)sps pps:(NSData*)pps
{
    // 1.拼接NALU的header
    const char bytes[] = "\x00\x00\x00\x01";
    size_t length = (sizeof bytes) - 1;
    NSData *ByteHeader = [NSData dataWithBytes:bytes length:length];

    // 2.將NALU的頭&NALU的體寫入文件
    [self.fileHandle writeData:ByteHeader];
    [self.fileHandle writeData:sps];
    [self.fileHandle writeData:ByteHeader];
    [self.fileHandle writeData:pps];

}
- (void)gotEncodedData:(NSData*)data isKeyFrame:(BOOL)isKeyFrame
{
    NSLog(@"gotEncodedData %d", (int)[data length]);
    if (self.fileHandle != NULL)
    {
        const char bytes[] = "\x00\x00\x00\x01";
        size_t length = (sizeof bytes) - 1; //string literals have implicit trailing '\0'
        NSData *ByteHeader = [NSData dataWithBytes:bytes length:length];
        [self.fileHandle writeData:ByteHeader];
        [self.fileHandle writeData:data];
    }
}