iOS h264 硬解

记录。

http://www.voidcn.com/blog/dongtinghong/article/p-5047279.html
首先要把 VideoToolbox.framework 添加到工程里，并且包含以下头文件。
#include <VideoToolbox/VideoToolbox.h>

解码主要需要以下三个函数
VTDecompressionSessionCreate 创建解码 session
VTDecompressionSessionDecodeFrame 解码一个frame
VTDecompressionSessionInvalidate 销毁解码 session

首先要创建 decode session，方法如下：
OSStatus status = VTDecompressionSessionCreate(kCFAllocatorDefault,
decoderFormatDescription,
NULL, attrs,
&callBackRecord,
&deocderSession);

其中 decoderFormatDescription 是 CMVideoFormatDescriptionRef 类型的视频格式描述，这个需要用H.264的 sps 和 pps数据来创建，调用以下函数创建 decoderFormatDescription
CMVideoFormatDescriptionCreateFromH264ParameterSets
需要注意的是，这里用的 sps和pps数据是不包含“00 00 00 01”的start code的。

attr是传递给decode session的属性词典
CFDictionaryRef attrs = NULL;
const void *keys[] = { kCVPixelBufferPixelFormatTypeKey };
// kCVPixelFormatType_420YpCbCr8Planar is YUV420
// kCVPixelFormatType_420YpCbCr8BiPlanarFullRange is NV12
uint32_t v = kCVPixelFormatType_420YpCbCr8BiPlanarFullRange;
const void *values[] = { CFNumberCreate(NULL, kCFNumberSInt32Type, &v) };
attrs = CFDictionaryCreate(NULL, keys, values, 1, NULL, NULL);
其中重要的属性就一个，kCVPixelBufferPixelFormatTypeKey，指定解码后的图像格式，必须指定成NV12，苹果的硬解码器只支持NV12。

callBackRecord 是用来指定回调函数的，解码器支持异步模式，解码后会调用这里的回调函数。

如果 decoderSession创建成功就可以开始解码了。
VTDecodeFrameFlags flags = 0;
//kVTDecodeFrame_EnableTemporalProcessing | kVTDecodeFrame_EnableAsynchronousDecompression;
VTDecodeInfoFlags flagOut = 0;
CVPixelBufferRef outputPixelBuffer = NULL;
OSStatus decodeStatus = VTDecompressionSessionDecodeFrame(deocderSession,
sampleBuffer,
flags,
&outputPixelBuffer,
&flagOut);
其中 flags 用0 表示使用同步解码，这样比较简单。
其中 sampleBuffer是输入的H.264视频数据，每次输入一个frame。
先用CMBlockBufferCreateWithMemoryBlock 从H.264数据创建一个CMBlockBufferRef实例。
然后用 CMSampleBufferCreateReady创建CMSampleBufferRef实例。
这里要注意的是，传入的H.264数据需要Mp4风格的，就是开始的四个字节是数据的长度而不是“00 00 00 01”的start code，四个字节的长度是big-endian的。
一般来说从视频里读出的数据都是 “00 00 00 01”开头的，这里需要自己转换下。

解码成功之后，outputPixelBuffer里就是一帧 NV12格式的YUV图像了。
如果想获取YUV的数据可以通过
CVPixelBufferLockBaseAddress(outputPixelBuffer, 0);
void *baseAddress = CVPixelBufferGetBaseAddress(outputPixelBuffer);
获得图像数据的指针，需要说明baseAddress并不是指向YUV数据，而是指向一个CVPlanarPixelBufferInfo_YCbCrBiPlanar结构体，结构体里记录了两个plane的offset和pitch。

但是如果想把视频播放出来是不需要去读取YUV数据的，因为CVPixelBufferRef是可以直接转换成OpenGL的Texture或者UIImage的。
调用CVOpenGLESTextureCacheCreateTextureFromImage，可以直接创建OpenGL Texture

从 CVPixelBufferRef 创建 UIImage
CIImage *ciImage = [CIImage imageWithCVPixelBuffer:pixelBuffer];
UIImage *uiImage = [UIImage imageWithCIImage:ciImage];

解码完成后销毁 decoder session
VTDecompressionSessionInvalidate(deocderSession)

硬解码的基本流程就是这样了，如果需要成功解码播放视频还需要一些H.264视频格式，YUV图像格式，OpenGL等基础知识。

代码https://github.com/stevenyao/iOSHardwareDecoder/blob/master/H264DecodeDemo/ViewController.m

#import "ViewController.h"
#import "VideoFileParser.h"
#import "AAPLEAGLLayer.h"
#import <VideoToolbox/VideoToolbox.h>

@interface ViewController ()
{
uint8_t *_sps;
NSInteger _spsSize;
uint8_t *_pps;
NSInteger _ppsSize;
VTDecompressionSessionRef _deocderSession;
CMVideoFormatDescriptionRef _decoderFormatDescription;
AAPLEAGLLayer *_glLayer;
}

@end

static void didDecompress( void *decompressionOutputRefCon, void *sourceFrameRefCon, OSStatus status, VTDecodeInfoFlags infoFlags, CVImageBufferRef pixelBuffer, CMTime presentationTimeStamp, CMTime presentationDuration ){

CVPixelBufferRef *outputPixelBuffer = (CVPixelBufferRef *)sourceFrameRefCon;
*outputPixelBuffer = CVPixelBufferRetain(pixelBuffer);

}

@implementation ViewController

-(BOOL)initH264Decoder {

if(_deocderSession) {
return YES;
}

const uint8_t* const parameterSetPointers[2] = { _sps, _pps };
const size_t parameterSetSizes[2] = { _spsSize, _ppsSize };
OSStatus status = CMVideoFormatDescriptionCreateFromH264ParameterSets(kCFAllocatorDefault,
2, //param count
parameterSetPointers,
parameterSetSizes,
4, //nal start code size
&_decoderFormatDescription);

if(status == noErr) {
CFDictionaryRef attrs = NULL;
const void *keys[] = { kCVPixelBufferPixelFormatTypeKey };
// kCVPixelFormatType_420YpCbCr8Planar is YUV420
// kCVPixelFormatType_420YpCbCr8BiPlanarFullRange is NV12
uint32_t v = kCVPixelFormatType_420YpCbCr8BiPlanarFullRange;
const void *values[] = { CFNumberCreate(NULL, kCFNumberSInt32Type, &v) };
attrs = CFDictionaryCreate(NULL, keys, values, 1, NULL, NULL);
VTDecompressionOutputCallbackRecord callBackRecord;
callBackRecord.decompressionOutputCallback = didDecompress;
callBackRecord.decompressionOutputRefCon = NULL;
status = VTDecompressionSessionCreate(kCFAllocatorDefault,
_decoderFormatDescription,
NULL, attrs,
&callBackRecord,
&_deocderSession);
CFRelease(attrs);

} else {

NSLog(@"IOS8VT: reset decoder session failed status=%d", status);

}

return YES;

}

-(void)clearH264Deocder {

if(_deocderSession) {
VTDecompressionSessionInvalidate(_deocderSession);
CFRelease(_deocderSession);
_deocderSession = NULL;

}

if(_decoderFormatDescription) {

CFRelease(_decoderFormatDescription);
_decoderFormatDescription = NULL;
}

free(_sps);
free(_pps);
_spsSize = _ppsSize = 0;
}

-(CVPixelBufferRef)decode:(VideoPacket*)vp {

CVPixelBufferRef outputPixelBuffer = NULL;
CMBlockBufferRef blockBuffer = NULL;

OSStatus status = CMBlockBufferCreateWithMemoryBlock(kCFAllocatorDefault,

(void*)vp.buffer, vp.size,
kCFAllocatorNull,

NULL, 0, vp.size,

0, &blockBuffer);

if(status == kCMBlockBufferNoErr) {

CMSampleBufferRef sampleBuffer = NULL;

const size_t sampleSizeArray[] = {vp.size};

status = CMSampleBufferCreateReady(kCFAllocatorDefault,

blockBuffer,

_decoderFormatDescription ,

1, 0, NULL, 1, sampleSizeArray,

&sampleBuffer);

if (status == kCMBlockBufferNoErr && sampleBuffer) {

VTDecodeFrameFlags flags = 0;

VTDecodeInfoFlags flagOut = 0;

OSStatus decodeStatus = VTDecompressionSessionDecodeFrame(_deocderSession,

sampleBuffer,

flags,

&outputPixelBuffer,

&flagOut);

if(decodeStatus == kVTInvalidSessionErr) {

NSLog(@"IOS8VT: Invalid session, reset decoder session");

} else if(decodeStatus == kVTVideoDecoderBadDataErr) {

NSLog(@"IOS8VT: decode failed status=%d(Bad data)", decodeStatus);

} else if(decodeStatus != noErr) {

NSLog(@"IOS8VT: decode failed status=%d", decodeStatus);

}

CFRelease(sampleBuffer);

}

CFRelease(blockBuffer);

}

return outputPixelBuffer;

}

-(void)decodeFile:(NSString*)fileName fileExt:(NSString*)fileExt {

NSString *path = [[NSBundle mainBundle] pathForResource:fileName ofType:fileExt];
VideoFileParser *parser = [VideoFileParser alloc];
[parser open:path];
VideoPacket *vp = nil;
while(true) {
vp = [parser nextPacket];
if(vp == nil) {
break;
}
uint32_t nalSize = (uint32_t)(vp.size - 4);
uint8_t *pNalSize = (uint8_t*)(&nalSize);
vp.buffer[0] = *(pNalSize + 3);
vp.buffer[1] = *(pNalSize + 2);
vp.buffer[2] = *(pNalSize + 1);
vp.buffer[3] = *(pNalSize);

CVPixelBufferRef pixelBuffer = NULL;
int nalType = vp.buffer[4] & 0x1F;

switch (nalType) {

case 0x05:

NSLog(@"Nal type is IDR frame");

if([self initH264Decoder]) {

pixelBuffer = [self decode:vp];

}

break;

case 0x07:

NSLog(@"Nal type is SPS");

_spsSize = vp.size - 4;

_sps = malloc(_spsSize);

memcpy(_sps, vp.buffer + 4, _spsSize);

break;

case 0x08:

NSLog(@"Nal type is PPS");

_ppsSize = vp.size - 4;

_pps = malloc(_ppsSize);

memcpy(_pps, vp.buffer + 4, _ppsSize);

break;

default:

NSLog(@"Nal type is B/P frame");

pixelBuffer = [self decode:vp];

break;

}

if(pixelBuffer) {

dispatch_sync(dispatch_get_main_queue(), ^{
_glLayer.pixelBuffer = pixelBuffer;

});

CVPixelBufferRelease(pixelBuffer);
}

NSLog(@"Read Nalu size %ld", vp.size);

}

[parser close];

}

-(IBAction)on_playButton_clicked:(id)sender {

dispatch_async(dispatch_get_global_queue(0, 0), ^{

[self decodeFile:@"mtv" fileExt:@"h264"];

});

}

- (void)viewDidLoad {

[super viewDidLoad];

// Do any additional setup after loading the view, typically from a nib.

_glLayer = [[AAPLEAGLLayer alloc] initWithFrame:self.view.bounds];

[self.view.layer addSublayer:_glLayer];

}

- (void)didReceiveMemoryWarning {

[super didReceiveMemoryWarning];

// Dispose of any resources that can be recreated.

}

@end

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