一、前言

前一段時間一直在做新的項目，已經很久沒更新文章了，今天終于閑下來了，筆者很高興吶，趁這時間，分享一下（直播）拉流端視頻編碼。iOS8.0以后，可以直接用系統的VideoToolbox框架進行編解碼，CPU占用率相比較ffmpeg來說會低很多，在做拉流時，我就是用的這個框架。下面廢話也不多說了，直接講解步驟上代碼。

二、視頻硬編碼

1.編碼器類DDHardwareVideoEncoder.h中，該類繼承自DDVideoEncoding（編碼器抽象接口類）其中，DDLiveVideoConfiguration是視頻配置文件，里面是視頻幀率、輸入方向、分辨率等視頻相關屬性，具體文件實現如下：

#import "DDVideoEncoding.h"

@interface DDHardwareVideoEncoder : NSObject<DDVideoEncoding>

#pragma mark - Initializer
///=============================================================================
/// @name Initializer
///=============================================================================
- (nullable instancetype)init UNAVAILABLE_ATTRIBUTE;
+ (nullable instancetype)new UNAVAILABLE_ATTRIBUTE;

@property (nonatomic, strong, nonnull) DDLiveVideoConfiguration *configuration;

// 是否插入關鍵幀
@property (assign, nonatomic) BOOL isInsertKeyFrame;

@end

2.編碼器抽象接口類DDVideoEncoding.h文件實現如下：
其中DDVideoFrame類是編碼成功后數據處理類，里面有每幀編碼成功后的data（h264裸流）、videoFrameRate（幀率）、frameCount（幀數）、timestamp（時間戳）等屬性。

#import <Foundation/Foundation.h>
#import "DDVideoFrame.h"
#import "DDLiveVideoConfiguration.h"

@protocol DDVideoEncoding;
/// 編碼器編碼后回調
@protocol DDVideoEncodingDelegate <NSObject>
@required
- (void)videoEncoder:(nullable id<DDVideoEncoding>)encoder videoFrame:(nullable DDVideoFrame*)frame;
@end

/// 編碼器抽象的接口
@protocol DDVideoEncoding <NSObject>
@required
- (void)encodeVideoData:(nullable CVImageBufferRef)pixelBuffer timeStamp:(uint64_t)timeStamp;

- (void)stopEncoder;
@optional
@property (nonatomic, assign) NSInteger videoBitRate;
- (nullable instancetype)initWithVideoStreamConfiguration:(nullable DDLiveVideoConfiguration*)configuration;
- (void)setDelegate:(nullable id<DDVideoEncodingDelegate>)delegate;

3.下面是具體實現的DDHardwareVideoEncoder.m類文件

#import "DDHardwareVideoEncoder.h"
#import <VideoToolbox/VideoToolbox.h>

@interface DDHardwareVideoEncoder (){
    VTCompressionSessionRef compressionSession;
    NSInteger frameCount;
    NSData *sps;
    NSData *pps;
}


@property (nonatomic,weak) id<DDVideoEncodingDelegate> h264Delegate;
@property (nonatomic) BOOL isBackGround;
@property (nonatomic) NSInteger currentVideoBitRate;
@property (assign, nonatomic) uint64_t lastTimestamp;

@end

@implementation DDHardwareVideoEncoder

#pragma mark -- LifeCycle
- (instancetype)initWithVideoStreamConfiguration:(DDLiveVideoConfiguration *)configuration{
    if(self = [super init]){
        _configuration = configuration;
        [self initCompressionSession];
        
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(willEnterBackground:) name:UIApplicationWillResignActiveNotification object:nil];
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(willEnterForeground:) name:UIApplicationDidBecomeActiveNotification object:nil];
    }
    return self;
}

- (void)updateConfiguration {
    [self initCompressionSession];
    self->sps = NULL;
}

- (void)clearCompressionSession {
    if(compressionSession){
        VTCompressionSessionCompleteFrames(compressionSession, kCMTimeInvalid);
        
        VTCompressionSessionInvalidate(compressionSession);
        CFRelease(compressionSession);
        compressionSession = NULL;
    }
}

- (void)initCompressionSession{
    [self clearCompressionSession];

    [self configCompressionSession];
}

- (void)configCompressionSession {
    // VideoCompressonOutputCallback回調函數：視頻圖像編碼成功后調用
    OSStatus status = VTCompressionSessionCreate(NULL, _configuration.videoSize.width, _configuration.videoSize.height, kCMVideoCodecType_H264, NULL, NULL, NULL, VideoCompressonOutputCallback, (__bridge void *)self, &compressionSession);
    if(status != noErr){
        return;
    }
    
    _currentVideoBitRate = _configuration.videoBitRate;
    
    // 設置關鍵幀間隔
    status = VTSessionSetProperty(compressionSession, kVTCompressionPropertyKey_MaxKeyFrameInterval,(__bridge CFTypeRef)@(_configuration.videoMaxKeyframeInterval));
    status = VTSessionSetProperty(compressionSession, kVTCompressionPropertyKey_MaxKeyFrameIntervalDuration,(__bridge CFTypeRef)@(_configuration.videoMaxKeyframeInterval));
    
    // 設置期望幀率
    status = VTSessionSetProperty(compressionSession, kVTCompressionPropertyKey_ExpectedFrameRate, (__bridge CFTypeRef)@(_configuration.videoFrameRate));
    
    //設置碼率，均值，單位是byte
    status = VTSessionSetProperty(compressionSession, kVTCompressionPropertyKey_AverageBitRate, (__bridge CFTypeRef)@(_configuration.videoBitRate)); // bps
    status = VTSessionSetProperty(compressionSession, kVTCompressionPropertyKey_DataRateLimits, (__bridge CFArrayRef)@[@(_configuration.videoMaxBitRate), @1]); // Bps
    
    // 設置實時編碼輸出（避免延遲）
    status = VTSessionSetProperty(compressionSession, kVTCompressionPropertyKey_RealTime, kCFBooleanTrue);
    status = VTSessionSetProperty(compressionSession, kVTCompressionPropertyKey_ProfileLevel, kVTProfileLevel_H264_Baseline_AutoLevel);
    
//    status = VTSessionSetProperty(compressionSession, kVTCompressionPropertyKey_AllowFrameReordering, kCFBooleanFalse);
//    status = VTSessionSetProperty(compressionSession, kVTCompressionPropertyKey_H264EntropyMode, kVTH264EntropyMode_CAVLC);
    VTCompressionSessionPrepareToEncodeFrames(compressionSession);
}

- (void)setVideoBitRate:(NSInteger)videoBitRate{
    if(_isBackGround) return;
    VTSessionSetProperty(compressionSession, kVTCompressionPropertyKey_DataRateLimits, (__bridge CFArrayRef)@[@(_configuration.videoMaxBitRate), @1]);
    _currentVideoBitRate = videoBitRate;
}

-(NSInteger)videoBitRate{
    return _currentVideoBitRate;
}

- (void)dealloc{
    if(compressionSession != NULL)
    {
        VTCompressionSessionCompleteFrames(compressionSession, kCMTimeInvalid);
        VTCompressionSessionInvalidate(compressionSession);
        CFRelease(compressionSession);
        compressionSession = NULL;
    }
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

#pragma mark -- DDVideoEncoder
- (void)encodeVideoData:(CVImageBufferRef)pixelBuffer timeStamp:(uint64_t)timeStamp {
    if(_isBackGround) return;
    
    NSInteger timeCha = timeStamp - self.lastTimestamp;
    if (timeCha - 1000/(int32_t)_configuration.videoFrameRate < 0) {
        return;
    }
    self.lastTimestamp = timeStamp;
    
    frameCount ++;
    CMTime presentationTimeStamp = CMTimeMake(frameCount, 1000);
    VTEncodeInfoFlags flags;
    CMTime duration = CMTimeMake(1, (int32_t)_configuration.videoFrameRate);
    
    NSDictionary *properties = nil;
    if(frameCount % (int32_t)_configuration.videoMaxKeyframeInterval == 0 || self.isInsertKeyFrame == YES){
        properties = @{(__bridge NSString *)kVTEncodeFrameOptionKey_ForceKeyFrame: @YES};
    }
    NSNumber *timeNumber = @(timeStamp);
    
    // 編碼，編碼成功后調用回調函數
   OSStatus statusCode =  VTCompressionSessionEncodeFrame(compressionSession, pixelBuffer, presentationTimeStamp, duration, (__bridge CFDictionaryRef)properties, (__bridge_retained void *)timeNumber, &flags);

    if (frameCount > 262143) { // 該數值根據后臺要求定義
        frameCount = 0;
    }

    self.isInsertKeyFrame = NO; 
    
    if (statusCode != noErr) {
        VTCompressionSessionInvalidate(compressionSession);
        compressionSession = NULL;
        return;
    }
}

- (void)stopEncoder{
    VTCompressionSessionCompleteFrames(compressionSession, kCMTimeIndefinite);
}

- (void)setDelegate:(id<DDVideoEncodingDelegate>)delegate{
    _h264Delegate = delegate;
}

#pragma mark -- NSNotification
- (void)willEnterBackground:(NSNotification*)notification{
    _isBackGround = YES;
}

- (void)willEnterForeground:(NSNotification*)notification{
    [self initCompressionSession];
    _isBackGround = NO;
}

#pragma mark -- VideoCallBack
// 視頻編碼成功后回調，將編碼成功的CMSampleBuffer轉換成H264碼流
static void VideoCompressonOutputCallback(void *VTref, void *VTFrameRef, OSStatus status, VTEncodeInfoFlags infoFlags, CMSampleBufferRef sampleBuffer) {
    if(!sampleBuffer) return;
    CFArrayRef array = CMSampleBufferGetSampleAttachmentsArray(sampleBuffer, true);
    if(!array) return;
    CFDictionaryRef dic = (CFDictionaryRef)CFArrayGetValueAtIndex(array, 0);
    if(!dic) return;
    
    BOOL keyframe = !CFDictionaryContainsKey(dic, kCMSampleAttachmentKey_NotSync);
    uint64_t timeStamp = [((__bridge_transfer NSNumber*)VTFrameRef) longLongValue];
    
    DDHardwareVideoEncoder *videoEncoder = (__bridge DDHardwareVideoEncoder *)VTref;
    if(status != noErr){
        return;
    }
    
    if (keyframe && !videoEncoder->sps)
    {
        // 描述信息
        CMFormatDescriptionRef format = CMSampleBufferGetFormatDescription(sampleBuffer);
        
        size_t sparameterSetSize, sparameterSetCount;
        const uint8_t *sparameterSet;
        OSStatus statusCode = CMVideoFormatDescriptionGetH264ParameterSetAtIndex(format, 0, &sparameterSet, &sparameterSetSize, &sparameterSetCount, 0 );
        if (statusCode == noErr)
        {
            size_t pparameterSetSize, pparameterSetCount;
            const uint8_t *pparameterSet;
            OSStatus statusCode = CMVideoFormatDescriptionGetH264ParameterSetAtIndex(format, 1, &pparameterSet, &pparameterSetSize, &pparameterSetCount, 0);
            if (statusCode == noErr)
            {
                videoEncoder->sps = [NSData dataWithBytes:sparameterSet length:sparameterSetSize];
                videoEncoder->pps = [NSData dataWithBytes:pparameterSet length:pparameterSetSize];
            }
        }
    }
    
    // 編碼后的數據結構
    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;
        while (bufferOffset < totalLength - AVCCHeaderLength) {
            // Read the NAL unit length
            uint32_t NALUnitLength = 0;
            memcpy(&NALUnitLength, dataPointer + bufferOffset, AVCCHeaderLength);
            
            NALUnitLength = CFSwapInt32BigToHost(NALUnitLength);

            DDVideoFrame *videoFrame = [[DDVideoFrame alloc] init];
            videoFrame.timestamp = timeStamp;
            videoFrame.isKeyFrame = keyframe;
            videoFrame.frameCount = videoEncoder->frameCount;
            videoFrame.videoFrameRate = videoEncoder.configuration.videoFrameRate;
            videoFrame.videoWidth = videoEncoder.configuration.videoSize.width;
            videoFrame.videoHeight = videoEncoder.configuration.videoSize.height;
            NSData *h264Data = [[NSData alloc] initWithBytes:(dataPointer + bufferOffset + AVCCHeaderLength) length:NALUnitLength];

            // 以后數據處理，根據各自后臺要求數據格式處理
            NSMutableData *mData = [NSMutableData data];
            const char bytes[] = "\x00\x00\x00\x01";
            size_t length = (sizeof bytes) - 1;
            NSData *ByteHeader = [NSData dataWithBytes:bytes length:length];
            if (keyframe) {
                [mData appendData:ByteHeader];
                [mData appendData:videoEncoder->sps];
                [mData appendData:ByteHeader];
                [mData appendData:videoEncoder->pps];
            }
                [mData appendData:ByteHeader];
                [mData appendData:h264Data];
                videoFrame.data = mData;
            if(videoEncoder.h264Delegate && [videoEncoder.h264Delegate respondsToSelector:@selector(videoEncoder:videoFrame:)]){
                [videoEncoder.h264Delegate videoEncoder:videoEncoder videoFrame:videoFrame]; // 數據傳出去之后，實現該代理方法，根據后臺數據格式進行數據封裝，然后發送
            }
            bufferOffset += AVCCHeaderLength + NALUnitLength;
        }
    }
}

@end