1.H.264 介绍
H264是属于视频的编码层的标准格式，视频编码显然是为了压缩大小
2.H.264流格式
在大多数H.264流中，您将收到一组初始的SPS和PPS参数，后跟一个i帧（又名IDR帧或关键帧）NALU。然后您将收到几个P帧的NALU（可能几十个左右），然后是另一组参数（可能与初始参数相同）和一个i帧，更多的P帧等。i帧比P帧。从概念上讲，您可以将i帧视为视频的整个图像，而P帧只是对该i帧所做的更改，直到收到下一个i帧为止。
图解：
image
步骤：
1.从您的H.264流中生成单个NALU。
2.使用CMVideoFormatDescriptionCreateFromH264ParameterSets（）从SPS和PPS NALU创建CMVideoFormatDescriptionRef。
3.根据“ AVCC”格式重新包装IDR和非IDR帧NALU。
4.将IDR和非IDR NALU帧打包到CMBlockBuffer中。
5.将CMBlockBuffer打包到CMSampleBuffer中。
6.创建一个VTDecompressionSessionRef并将示例缓冲区馈入VTDecompressionSessionDecodeFrame（）。
其他说明：
H.264流的变化很大。据我了解，NALU起始代码标头有时为3个字节（0x00 00 01），有时为4（0x00 00 00 01）。我的代码适用于4个字节；如果您正在使用3，则需要进行一些更改。
代码部分：
1.引入VideoToolbox框架
#import
@property (nonatomic, assign) CMVideoFormatDescriptionRef formatDesc;
@property (nonatomic, assign) VTDecompressionSessionRef decompressionSession;
@property (nonatomic, retain) AVSampleBufferDisplayLayer *videoLayer;
@property (nonatomic, assign) int spsSize;
@property (nonatomic, assign) int ppsSize;
2.解码部分
-(void) receivedRawVideoFrame:(uint8_t *)frame withSize:(uint32_t)frameSize isIFrame:(int)isIFrame
{
OSStatus status;
uint8_t *data = NULL;
uint8_t *pps = NULL;
uint8_t *sps = NULL;
// I know what my H.264 data source's NALUs look like so I know start code index is always 0.
// if you don't know where it starts, you can use a for loop similar to how i find the 2nd and 3rd start codes
int startCodeIndex = 0;
int secondStartCodeIndex = 0;
int thirdStartCodeIndex = 0;
long blockLength = 0;
CMSampleBufferRef sampleBuffer = NULL;
CMBlockBufferRef blockBuffer = NULL;
int nalu_type = (frame[startCodeIndex + 4] & 0x1F);
NSLog(@"~~~~~~~ Received NALU Type \"%@\" ~~~~~~~~", naluTypesStrings[nalu_type]);
// if we havent already set up our format description with our SPS PPS parameters, we
// can't process any frames except type 7 that has our parameters
if (nalu_type != 7 && _formatDesc == NULL)
{
NSLog(@"Video error: Frame is not an I Frame and format description is null");
return;
}
// NALU type 7 is the SPS parameter NALU
if (nalu_type == 7)
{
// find where the second PPS start code begins, (the 0x00 00 00 01 code)
// from which we also get the length of the first SPS code
for (int i = startCodeIndex + 4; i < startCodeIndex + 40; i++)
{
if (frame[i] == 0x00 && frame[i+1] == 0x00 && frame[i+2] == 0x00 && frame[i+3] == 0x01)
{
secondStartCodeIndex = i;
_spsSize = secondStartCodeIndex; // includes the header in the size
break;
}
}
// find what the second NALU type is
nalu_type = (frame[secondStartCodeIndex + 4] & 0x1F);
NSLog(@"~~~~~~~ Received NALU Type \"%@\" ~~~~~~~~", naluTypesStrings[nalu_type]);
}
// type 8 is the PPS parameter NALU
if(nalu_type == 8)
{
// find where the NALU after this one starts so we know how long the PPS parameter is
for (int i = _spsSize + 4; i < _spsSize + 30; i++)
{
if (frame[i] == 0x00 && frame[i+1] == 0x00 && frame[i+2] == 0x00 && frame[i+3] == 0x01)
{
thirdStartCodeIndex = i;
_ppsSize = thirdStartCodeIndex - _spsSize;
break;
}
}
// allocate enough data to fit the SPS and PPS parameters into our data objects.
// VTD doesn't want you to include the start code header (4 bytes long) so we add the - 4 here
sps = malloc(_spsSize - 4);
pps = malloc(_ppsSize - 4);
// copy in the actual sps and pps values, again ignoring the 4 byte header
memcpy (sps, &frame[4], _spsSize-4);
memcpy (pps, &frame[_spsSize+4], _ppsSize-4);
// now we set our H264 parameters
uint8_t* parameterSetPointers[2] = {sps, pps};
size_t parameterSetSizes[2] = {_spsSize-4, _ppsSize-4};
// suggestion from @Kris Dude's answer below
if (_formatDesc)
{
CFRelease(_formatDesc);
_formatDesc = NULL;
}
status = CMVideoFormatDescriptionCreateFromH264ParameterSets(kCFAllocatorDefault, 2,
(const uint8_t *const*)parameterSetPointers,
parameterSetSizes, 4,
&_formatDesc);
NSLog(@"\t\t Creation of CMVideoFormatDescription: %@", (status == noErr) ? @"successful!" : @"failed...");
if(status != noErr) NSLog(@"\t\t Format Description ERROR type: %d", (int)status);
// See if decomp session can convert from previous format description
// to the new one, if not we need to remake the decomp session.
// This snippet was not necessary for my applications but it could be for yours
/*BOOL needNewDecompSession = (VTDecompressionSessionCanAcceptFormatDescription(_decompressionSession, _formatDesc) == NO);
if(needNewDecompSession)
{
[self createDecompSession];
}*/
// now lets handle the IDR frame that (should) come after the parameter sets
// I say "should" because that's how I expect my H264 stream to work, YMMV
nalu_type = (frame[thirdStartCodeIndex + 4] & 0x1F);
NSLog(@"~~~~~~~ Received NALU Type \"%@\" ~~~~~~~~", naluTypesStrings[nalu_type]);
}
// create our VTDecompressionSession. This isnt neccessary if you choose to use AVSampleBufferDisplayLayer
if((status == noErr) && (_decompressionSession == NULL))
{
[self createDecompSession];
}
// type 5 is an IDR frame NALU. The SPS and PPS NALUs should always be followed by an IDR (or IFrame) NALU, as far as I know
if(nalu_type == 5)
{
// find the offset, or where the SPS and PPS NALUs end and the IDR frame NALU begins
int offset = _spsSize + _ppsSize;
blockLength = frameSize - offset;
data = malloc(blockLength);
data = memcpy(data, &frame[offset], blockLength);
// replace the start code header on this NALU with its size.
// AVCC format requires that you do this.
// htonl converts the unsigned int from host to network byte order
uint32_t dataLength32 = htonl (blockLength - 4);
memcpy (data, &dataLength32, sizeof (uint32_t));
// create a block buffer from the IDR NALU
status = CMBlockBufferCreateWithMemoryBlock(NULL, data, // memoryBlock to hold buffered data
blockLength, // block length of the mem block in bytes.
kCFAllocatorNull, NULL,
0, // offsetToData
blockLength, // dataLength of relevant bytes, starting at offsetToData
0, &blockBuffer);
NSLog(@"\t\t BlockBufferCreation: \t %@", (status == kCMBlockBufferNoErr) ? @"successful!" : @"failed...");
}
// NALU type 1 is non-IDR (or PFrame) picture
if (nalu_type == 1)
{
// non-IDR frames do not have an offset due to SPS and PSS, so the approach
// is similar to the IDR frames just without the offset
blockLength = frameSize;
data = malloc(blockLength);
data = memcpy(data, &frame[0], blockLength);
// again, replace the start header with the size of the NALU
uint32_t dataLength32 = htonl (blockLength - 4);
memcpy (data, &dataLength32, sizeof (uint32_t));
status = CMBlockBufferCreateWithMemoryBlock(NULL, data, // memoryBlock to hold data. If NULL, block will be alloc when needed
blockLength, // overall length of the mem block in bytes
kCFAllocatorNull, NULL,
0, // offsetToData
blockLength, // dataLength of relevant data bytes, starting at offsetToData
0, &blockBuffer);
NSLog(@"\t\t BlockBufferCreation: \t %@", (status == kCMBlockBufferNoErr) ? @"successful!" : @"failed...");
}
// now create our sample buffer from the block buffer,
if(status == noErr)
{
// here I'm not bothering with any timing specifics since in my case we displayed all frames immediately
const size_t sampleSize = blockLength;
status = CMSampleBufferCreate(kCFAllocatorDefault,
blockBuffer, true, NULL, NULL,
_formatDesc, 1, 0, NULL, 1,
&sampleSize, &sampleBuffer);
NSLog(@"\t\t SampleBufferCreate: \t %@", (status == noErr) ? @"successful!" : @"failed...");
}
if(status == noErr)
{
// set some values of the sample buffer's attachments
CFArrayRef attachments = CMSampleBufferGetSampleAttachmentsArray(sampleBuffer, YES);
CFMutableDictionaryRef dict = (CFMutableDictionaryRef)CFArrayGetValueAtIndex(attachments, 0);
CFDictionarySetValue(dict, kCMSampleAttachmentKey_DisplayImmediately, kCFBooleanTrue);
// either send the samplebuffer to a VTDecompressionSession or to an AVSampleBufferDisplayLayer
[self render:sampleBuffer];
}
// free memory to avoid a memory leak, do the same for sps, pps and blockbuffer
if (NULL != data)
{
free (data);
data = NULL;
}
}
3.第一次收到sps和pps时需要创建VTD会话。
-(void) createDecompSession
{
// make sure to destroy the old VTD session
_decompressionSession = NULL;
VTDecompressionOutputCallbackRecord callBackRecord;
callBackRecord.decompressionOutputCallback = decompressionSessionDecodeFrameCallback;
// this is necessary if you need to make calls to Objective C "self" from within in the callback method.
callBackRecord.decompressionOutputRefCon = (__bridge void *)self;
// you can set some desired attributes for the destination pixel buffer. I didn't use this but you may
// if you need to set some attributes, be sure to uncomment the dictionary in VTDecompressionSessionCreate
NSDictionary *destinationImageBufferAttributes = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:YES],
(id)kCVPixelBufferOpenGLESCompatibilityKey,
nil];
OSStatus status = VTDecompressionSessionCreate(NULL, _formatDesc, NULL,
NULL, // (__bridge CFDictionaryRef)(destinationImageBufferAttributes)
&callBackRecord, &_decompressionSession);
NSLog(@"Video Decompression Session Create: \t %@", (status == noErr) ? @"successful!" : @"failed...");
if(status != noErr) NSLog(@"\t\t VTD ERROR type: %d", (int)status);
}
每当VTD完成解压缩发送给它的任何帧时，都会调用此方法。即使出现错误或丢帧，也会调用此方法。
void decompressionSessionDecodeFrameCallback(void *decompressionOutputRefCon,
void *sourceFrameRefCon,
OSStatus status,
VTDecodeInfoFlags infoFlags,
CVImageBufferRef imageBuffer,
CMTime presentationTimeStamp,
CMTime presentationDuration)
{
THISCLASSNAME *streamManager = (__bridge THISCLASSNAME *)decompressionOutputRefCon;
if (status != noErr)
{
NSError *error = [NSError errorWithDomain:NSOSStatusErrorDomain code:status userInfo:nil];
NSLog(@"Decompressed error: %@", error);
}
else
{
NSLog(@"Decompressed sucessfully");
// do something with your resulting CVImageBufferRef that is your decompressed frame
[streamManager displayDecodedFrame:imageBuffer];
}
}
我们实际上将sampleBuffer发送到VTD进行解码的地方。
- (void) render:(CMSampleBufferRef)sampleBuffer
{
VTDecodeFrameFlags flags = kVTDecodeFrame_EnableAsynchronousDecompression;
VTDecodeInfoFlags flagOut;
NSDate* currentTime = [NSDate date];
VTDecompressionSessionDecodeFrame(_decompressionSession, sampleBuffer, flags,
(void*)CFBridgingRetain(currentTime), &flagOut);
CFRelease(sampleBuffer);
// if you're using AVSampleBufferDisplayLayer, you only need to use this line of code
// [videoLayer enqueueSampleBuffer:sampleBuffer];
}
如果使用AVSampleBufferDisplayLayer，请确保在viewDidLoad或其他其他init方法中初始化这样的图层。
-(void) viewDidLoad
{
// create our AVSampleBufferDisplayLayer and add it to the view
videoLayer = [[AVSampleBufferDisplayLayer alloc] init];
videoLayer.frame = self.view.frame;
videoLayer.bounds = self.view.bounds;
videoLayer.videoGravity = AVLayerVideoGravityResizeAspect;
// set Timebase, you may need this if you need to display frames at specific times
// I didn't need it so I haven't verified that the timebase is working
CMTimebaseRef controlTimebase;
CMTimebaseCreateWithMasterClock(CFAllocatorGetDefault(), CMClockGetHostTimeClock(), &controlTimebase);
//videoLayer.controlTimebase = controlTimebase;
CMTimebaseSetTime(self.videoLayer.controlTimebase, kCMTimeZero);
CMTimebaseSetRate(self.videoLayer.controlTimebase, 1.0);
[[self.view layer] addSublayer:videoLayer];
}