前言
VideoToolBox是iOS8之后，苹果开发的用于硬解码编码H264/H265(iOS11以后支持)的API。
对于H264还不了解的童鞋一定要先看下这边的H264的简介。
编码流程
我们实现一个简单的Demo，从摄像头获取到视频数据，然后再编码成H264裸数据保存在沙盒中。
1. 创建初始化VideoToolBox
- (void)initVideoToolBox { dispatch_sync(encodeQueue , ^{ frameNO = 0; int width = 480, height = 640; OSStatus status = VTCompressionSessionCreate(NULL, width, height, kCMVideoCodecType_H264, NULL, NULL, NULL, didCompressH264, (__bridge void *)(self), &encodingSession); NSLog(@"H264: VTCompressionSessionCreate %d", (int)status); if (status != 0) { NSLog(@"H264: Unable to create a H264 session"); return ; } // 设置实时编码输出（避免延迟） VTSessionSetProperty(encodingSession, kVTCompressionPropertyKey_RealTime, kCFBooleanTrue); VTSessionSetProperty(encodingSession, kVTCompressionPropertyKey_ProfileLevel, kVTProfileLevel_H264_Baseline_AutoLevel); // 设置关键帧（GOPsize)间隔 int frameInterval = 24; CFNumberRef frameIntervalRef = CFNumberCreate(kCFAllocatorDefault, kCFNumberIntType, &frameInterval); VTSessionSetProperty(encodingSession, kVTCompressionPropertyKey_MaxKeyFrameInterval, frameIntervalRef); //设置期望帧率 int fps = 24; CFNumberRef fpsRef = CFNumberCreate(kCFAllocatorDefault, kCFNumberIntType, &fps); VTSessionSetProperty(encodingSession, kVTCompressionPropertyKey_ExpectedFrameRate, fpsRef); //设置码率，均值，单位是byte int bitRate = width * height * 3 * 4 * 8; CFNumberRef bitRateRef = CFNumberCreate(kCFAllocatorDefault, kCFNumberSInt32Type, &bitRate); VTSessionSetProperty(encodingSession, kVTCompressionPropertyKey_AverageBitRate, bitRateRef); //设置码率，上限，单位是bps int bitRateLimit = width * height * 3 * 4; CFNumberRef bitRateLimitRef = CFNumberCreate(kCFAllocatorDefault, kCFNumberSInt32Type, &bitRateLimit); VTSessionSetProperty(encodingSession, kVTCompressionPropertyKey_DataRateLimits, bitRateLimitRef); //开始编码 VTCompressionSessionPrepareToEncodeFrames(encodingSession); });}复制代码 初始化这里设置了编码类型kCMVideoCodecType_H264, 分辨率640 * 480，fps，GOP，码率。
2. 从摄像头获取视频数据丢给VideoToolBox编码成H264
初始化视频采集端核心代码如下
//初始化摄像头采集端- (void)initCapture{ self.captureSession = [[AVCaptureSession alloc]init]; //设置录制640 * 480 self.captureSession.sessionPreset = AVCaptureSessionPreset640x480; AVCaptureDevice *inputCamera = [self cameraWithPostion:AVCaptureDevicePositionBack]; self.captureDeviceInput = [[AVCaptureDeviceInput alloc] initWithDevice:inputCamera error:nil]; if ([self.captureSession canAddInput:self.captureDeviceInput]) { [self.captureSession addInput:self.captureDeviceInput]; } self.captureDeviceOutput = [[AVCaptureVideoDataOutput alloc] init]; [self.captureDeviceOutput setAlwaysDiscardsLateVideoFrames:NO]; //设置YUV420p输出 [self.captureDeviceOutput setVideoSettings:[NSDictionary dictionaryWithObject:[NSNumber numberWithInt:kCVPixelFormatType_420YpCbCr8BiPlanarFullRange] forKey:(id)kCVPixelBufferPixelFormatTypeKey]]; [self.captureDeviceOutput setSampleBufferDelegate:self queue:captureQueue]; if ([self.captureSession canAddOutput:self.captureDeviceOutput]) { [self.captureSession addOutput:self.captureDeviceOutput]; } //建立连接 AVCaptureConnection *connection = [self.captureDeviceOutput connectionWithMediaType:AVMediaTypeVideo]; [connection setVideoOrientation:AVCaptureVideoOrientationPortrait];}复制代码 这里需要注意设置的视频分辨率和编码器一致640 * 480. AVCaptureVideoDataOutput类型选用YUV420p。
摄像头数据回调部分
- (void)captureOutput:(AVCaptureOutput *)output didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection{ dispatch_sync(encodeQueue, ^{ [self encode:sampleBuffer]; });}//编码sampleBuffer- (void) encode:(CMSampleBufferRef )sampleBuffer{ CVImageBufferRef imageBuffer = (CVImageBufferRef)CMSampleBufferGetImageBuffer(sampleBuffer); // 帧时间，如果不设置会导致时间轴过长。 CMTime presentationTimeStamp = CMTimeMake(frameNO++, 1000); VTEncodeInfoFlags flags; OSStatus statusCode = VTCompressionSessionEncodeFrame(encodingSession, imageBuffer, presentationTimeStamp, kCMTimeInvalid, NULL, NULL, &flags); if (statusCode != noErr) { NSLog(@"H264: VTCompressionSessionEncodeFrame failed with %d", (int)statusCode); VTCompressionSessionInvalidate(encodingSession); CFRelease(encodingSession); encodingSession = NULL; return; } NSLog(@"H264: VTCompressionSessionEncodeFrame Success");}复制代码 3.框架中出现的数据结构 CMSampleBufferRef 存放一个或者多个压缩或未压缩的媒体数据； 下图列举了两种CMSampleBuffer。
CMTime 64位的value，32位的scale，media的时间格式；
CMBlockBuffer 这里可以叫裸数据；
CVPixelBuffer 包含未压缩的像素数据，图像宽度、高度等；
pixelBufferAttributes CFDictionary包括宽高、像素格式（RGBA、YUV）、使用场景（OpenGL ES、Core Animation）
CVPixelBufferPool CVPixelBuffer的缓冲池，因为CVPixelBuffer的创建和销毁开销很大
CMVideoFormatDescription video格式，包括宽高、颜色空间、编码格式信息等；对于H264，还包含sps和pps数据；
4. 编码完成后的数据写入H264
这里编码完成我们先判断的是否为I帧，如果是需要读取sps和pps参数集，为什么要这样呢？
我们先看一下一个裸数据H264（Elementary Stream）的NALU构成
H.264裸流中，不存在单独的SPS、PPS包或帧，而是附加在I帧前面，存储的一般形式为
00 00 00 01 SPS 00 00 00 01 PPS 00 00 00 01 I帧
前面的这些00 00数据称为起始码（Start Code），它们不属于SPS、PPS的内容。
SPS（Sequence Parameter Sets）和PPS（Picture Parameter Set）：H.264的SPS和PPS包含了初始化H.264解码器所需要的信息参数，包括编码所用的profile，level，图像的宽和高，deblock滤波器等。
上面介绍了sps和pps是封装在CMFormatDescriptionRef中，所以我们得先CMFormatDescriptionRef中取出sps和pps写入h264裸流中。
这就不难理解写入H264的流程了。
代码如下
// 编码完成回调void didCompressH264(void *outputCallbackRefCon, void *sourceFrameRefCon, OSStatus status, VTEncodeInfoFlags infoFlags, CMSampleBufferRef sampleBuffer) { NSLog(@"didCompressH264 called with status %d infoFlags %d", (int)status, (int)infoFlags); if (status != 0) { return; } if (!CMSampleBufferDataIsReady(sampleBuffer)) { NSLog(@"didCompressH264 data is not ready "); return; } ViewController* encoder = (__bridge ViewController*)outputCallbackRefCon; bool keyframe = !CFDictionaryContainsKey( (CFArrayGetValueAtIndex(CMSampleBufferGetSampleAttachmentsArray(sampleBuffer, true), 0)), kCMSampleAttachmentKey_NotSync); // 判断当前帧是否为关键帧 // 获取sps & pps数据 if (keyframe) { CMFormatDescriptionRef format = CMSampleBufferGetFormatDescription(sampleBuffer); size_t sparameterSetSize, sparameterSetCount; const uint8_t *sparameterSet; OSStatus statusCode = CMVideoFormatDescriptionGetH264ParameterSetAtIndex(format, 0, &sparameterSet, &sparameterSetSize, &sparameterSetCount, 0 ); if (statusCode == noErr) { // 获得了sps，再获取pps size_t pparameterSetSize, pparameterSetCount; const uint8_t *pparameterSet; OSStatus statusCode = CMVideoFormatDescriptionGetH264ParameterSetAtIndex(format, 1, &pparameterSet, &pparameterSetSize, &pparameterSetCount, 0 ); if (statusCode == noErr) { // 获取SPS和PPS data NSData *sps = [NSData dataWithBytes:sparameterSet length:sparameterSetSize]; NSData *pps = [NSData dataWithBytes:pparameterSet length:pparameterSetSize]; if (encoder) { [encoder gotSpsPps:sps pps:pps]; } } } } CMBlockBufferRef dataBuffer = CMSampleBufferGetDataBuffer(sampleBuffer); size_t length, totalLength; char *dataPointer; //这里获取了数据指针，和NALU的帧总长度，前四个字节里面保存的 OSStatus statusCodeRet = CMBlockBufferGetDataPointer(dataBuffer, 0, &length, &totalLength, &dataPointer); if (statusCodeRet == noErr) { size_t bufferOffset = 0; static const int AVCCHeaderLength = 4; // 返回的nalu数据前四个字节不是0001的startcode，而是大端模式的帧长度length // 循环获取nalu数据 while (bufferOffset < totalLength - AVCCHeaderLength) { uint32_t NALUnitLength = 0; // 读取NALU长度的数据 memcpy(&NALUnitLength, dataPointer + bufferOffset, AVCCHeaderLength); // 从大端转系统端 NALUnitLength = CFSwapInt32BigToHost(NALUnitLength); NSData* data = [[NSData alloc] initWithBytes:(dataPointer + bufferOffset + AVCCHeaderLength) length:NALUnitLength]; [encoder gotEncodedData:data]; // 移动到下一个NALU单元 bufferOffset += AVCCHeaderLength + NALUnitLength; } } }//填充SPS和PPS数据- (void)gotSpsPps:(NSData*)sps pps:(NSData*)pps{ NSLog(@"gotSpsPps %d %d", (int)[sps length], (int)[pps length]); 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]; //写入startcode [self.h264FileHandle writeData:ByteHeader]; [self.h264FileHandle writeData:sps]; //写入startcode [self.h264FileHandle writeData:ByteHeader]; [self.h264FileHandle writeData:pps]; }//填充NALU数据- (void)gotEncodedData:(NSData*)data{ NSLog(@"gotEncodedData %d", (int)[data length]); if (self.h264FileHandle != 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]; //写入startcode [self.h264FileHandle writeData:ByteHeader]; //写入NALU数据 [self.h264FileHandle writeData:data]; }}复制代码 结束编码后销毁session
- (void)EndVideoToolBox{ VTCompressionSessionCompleteFrames(encodingSession, kCMTimeInvalid); VTCompressionSessionInvalidate(encodingSession); CFRelease(encodingSession); encodingSession = NULL;}复制代码 这样就完成了使用VideoToolbox 的H264编码。编码好的H264文件可以从沙盒中取出。
总结
仅仅看流程不看代码肯定是学不会框架的，自己动手编码试试吧！ Demo下载地址：