下面我们结合代码重点介绍硬解码：
-(void)viewDidLoad {
[super viewDidLoad];
//1.创建NSInputStream对象
NSString *filePath = [[NSBundle mainBundle] pathForResource:@"abc.h264" ofType:nil];
self.inputStream = [NSInputStream inputStreamWithFileAtPath:filePath];
//2.创建定时器
CADisplayLink *displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(readFrame)];
//2.1.设置执行频率(自我理解,2毫秒执行一次,即一秒钟执行30次)
displayLink.frameInterval = 2;
[displayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSRunLoopCommonModes];
[displayLink setPaused:YES];
self.displayLink = displayLink;
//3.创建一个线程, 用于读取被编码后的数据, 解码数据
self.queue = dispatch_get_global_queue(0, 0);
//4.创建展示的layer
AAPLEAGLLayer *layer = [[AAPLEAGLLayer alloc] initWithFrame:self.view.bounds];
[self.view.layer addSublayer:layer];
self.previewLayer = layer;
}
//读取被编码的数据
- (void)readFrame{
dispatch_sync(self.queue, ^{
//1.读取一个NALU数据
[self readPacket];
//2.判断读取的数据是否为空,如果数据未NULL,就直接结束
if (packetBuffer == NULL || packetSize == 0){
[self.displayLink setPaused:YES];
[self.displayLink invalidate];
[self.inputStream close];
//不是直接return就行了吗,然后继续向内存中读取信息
return;
}
//3.根据NALU的不同类型做出不同处理
//sps pps I帧 其他帧
//3.1.内存地址由系统端模式转化为大端模式
uint32_t nalSize = (uint32_t)(packetSize - 4);
uint32_t *pNalSize = (uint32_t *)packetBuffer;
*pNalSize = CFSwapInt32HostToBig(nalSize);
//3.2.取得NALU单元的第五个字节, 然后只取后五位
int nalType = packetBuffer[4] & 0x1F;
CVImageBufferRef imageBuffer = NULL;
switch (nalType) {
case 0x07:
//开辟地址记录sps
mSPSSize = packetSize - 4;
pSPS = malloc(mSPSSize);
memcpy(pSPS, packetBuffer + 4, mSPSSize);
break;
case 0x08:
//开辟地址记录pps
mPPSSize = packetSize - 4;
pPPS = malloc(mPPSSize);
memcpy(pPPS, packetBuffer + 4, mPPSSize);
break;
case 0x05:
//I帧 根据SPS和PPS直接初始化Decompressionsession, 每一个 Decompressionsession都是不同的, 然后解码
[self initDecompressionsession];
imageBuffer = [self decodeFrame];
break;
//其他帧 直接解码
default:
imageBuffer = [self decodeFrame];
break;
}
//4.将解码数据进行展示
if (imageBuffer != NULL){
dispatch_async(dispatch_get_main_queue(), ^{
self.previewLayer.pixelBuffer = imageBuffer;
CFRelease(imageBuffer);
});
}
});
}
//读取一个NALU数据
- (void)readPacket{
//1.把之前的packetBuffer清空
if (packetSize != 0 || packetBuffer != NULL){
packetSize = 0;
free(packetBuffer);
packetBuffer = NULL;
}
//2.开始读取一定长度的数据（如果长度没有超过最大长度,并且输入流里面有信息）
if (leftLength < maxReadLength && self.inputStream.hasBytesAvailable){
leftLength += [self.inputStream read:dataPointer + leftLength maxLength:maxReadLength - leftLength];
}
//3.从dataPointer内存中取出一个NALU单元到packetBuffer
uint8_t *pStart = dataPointer + 4;
uint8_t *pEnd = dataPointer + leftLength;
//是NALU单元的前提是,头部和startCode吻合,并且除了头标志外是有内容的
if (memcmp(startCode, dataPointer, 4) == 0 && leftLength > 4){
//当起始和末尾相差不等于4的时候，就继续向后寻找NALU的末尾
while (pStart != pEnd - 3){
//获取NALU的长度（通过对比数据是否为0x00 00 00 01）
if (memcmp(pStart, startCode, 4) == 0){
//如果和NALU开头标志相同了,代表找到一个完整的NALU了,此时的pStart为此NALU的结束地址的后一个地址
packetSize = pStart - dataPointer;
packetBuffer = malloc(packetSize);
memcpy(packetBuffer, dataPointer, packetSize);
//把dataPointer中信息向前移动
memmove(dataPointer, dataPointer + packetSize, leftLength - packetSize);
leftLength -= packetSize;
break;
}else{
pStart++;
}
}
}
}
//根据SPS和PPS直接初始化VTDecompressionsession, 每一个VTDecompressionsession都是不同的
- (void)initDecompressionsession{
//1.创建CMVideoFormatDescriptionRef
const uint8_t *parameterSetPointers[2] = {pSPS, pPPS};
const size_t parameterSetSizes[2] = {mSPSSize, mPPSSize};
CMVideoFormatDescriptionCreateFromH264ParameterSets(NULL, 2, parameterSetPointers, parameterSetSizes, 4, &_format);
//2.
NSDictionary *attr = @{(__bridge NSString *)kCVPixelBufferPixelFormatTypeKey : @(kCVPixelFormatType_420YpCbCr8BiPlanarFullRange)};
//3.解码之后的回调函数
VTDecompressionOutputCallbackRecord callbackRecord;
callbackRecord.decompressionOutputCallback = decompressionCallBack;
//4.创建VTDecompressionsession
VTDecompressionSessionCreate(NULL, self.format, NULL, (__bridge CFDictionaryRef _Nullable)(attr), &callbackRecord, &_session);
}
//解码：VTDecompressionSessionDecodeFrame
- (CVPixelBufferRef)decodeFrame{
//1.创建CMBlockBufferRef
CMBlockBufferRef blockBuffer = NULL;
CMBlockBufferCreateWithMemoryBlock(kCFAllocatorDefault, packetBuffer, packetSize, kCFAllocatorNull, NULL, 0, packetSize, 0, &blockBuffer);
//2.创建CMSampleBufferRef
CMSampleBufferRef sampleBuffer = NULL;
const size_t sampleSizeArray[] = {packetSize};
CMSampleBufferCreateReady(NULL, blockBuffer, self.format, 0, 0, NULL, 0, sampleSizeArray, &sampleBuffer);
//3.开始解码
CVPixelBufferRef outputPixelBuffer = NULL;
VTDecompressionSessionDecodeFrame(self.session, sampleBuffer, 0, &outputPixelBuffer, NULL);
//4.释放资源
CFRelease(sampleBuffer);
CFRelease(blockBuffer);
return outputPixelBuffer;
}
- (IBAction)play:(id)sender {
//1.对于读取信息常量进行赋值
maxReadLength = 1280 * 720;
leftLength = 0;
dataPointer = malloc(maxReadLength);
//2.打开输入流
[self.inputStream open];
//3.开启定时器来读取信息
[self.displayLink setPaused:NO];
}
//解码之后的回调函数
void decompressionCallBack(
void * CM_NULLABLE decompressionOutputRefCon,
void * CM_NULLABLE sourceFrameRefCon,
OSStatus status,
VTDecodeInfoFlags infoFlags,
CM_NULLABLE CVImageBufferRef imageBuffer,
CMTime presentationTimeStamp,
CMTime presentationDuration ){
CVImageBufferRef *pointer = sourceFrameRefCon;
*pointer = CVBufferRetain(imageBuffer);
}
解码要点：
1.创建定时器CADisplayLink
2.创建NSInputStream对象实时读取NALU单元
3.根据NALU单元的不同类型做出不同处理
3.1.如果是sps或pps，记录
3.2.如果是I帧，就根据SPS和PPS初始化VTDecompressionsession，然后解码
3.3.如果是其它帧，直接解码
4.展示解码后的帧
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