How to decode a H.264 frame on iOS by hardware decoding?

Question

I have been used ffmpeg to decode every single frame that I received from my ip cam. The brief code looks like this:

-(void) decodeFrame:(unsigned char *)frameData frameSize:(int)frameSize{
   AVFrame frame;
   AVPicture picture;
   AVPacket pkt;
   AVCodecContext *context;
   pkt.data = frameData;
   pat.size = frameSize;
   avcodec_get_frame_defaults(&frame);
   avpicture_alloc(&picture, PIX_FMT_RGB24, targetWidth, targetHeight);
   avcodec_decode_video2(&context, &frame, &got_picture, &pkt);
}

The code woks fine, but it‘s software decoding. I want to enhance the decoding performance by hardware decoding. After lots of research, I know it may be achieved by AVFoundation framework. The AVAssetReader class may help, but I can‘t figure out what‘s the next.Could anyone points out the following steps for me? Any help would be appreciated.

rpj · Accepted Answer · 2015-09-23 01:33:53Z

iOS does not provide any public access directly to the hardware decode engine, because hardware is always used to decode H.264 video on iOS.

Therefore, session 513 gives you all the information you need to allow frame-by-frame decoding on iOS. In short, per that session:

Generate individual network abstraction layer units (NALUs) from your H.264 elementary stream. There is much information on how this is done online. VCL NALUs (IDR and non-IDR) contain your video data and are to be fed into the decoder.
Re-package those NALUs according to the "AVCC" format, removing NALU start codes and replacing them with a 4-byte NALU length header.
Create a CMVideoFormatDescriptionRef from your SPS and PPS NALUs via CMVideoFormatDescriptionCreateFromH264ParameterSets()
Package NALU frames as CMSampleBuffers per session 513.
Create a VTDecompressionSessionRef, and feed VTDecompressionSessionDecodeFrame() with the sample buffers
- Alternatively, use AVSampleBufferDisplayLayer, whose -enqueueSampleBuffer: method obviates the need to create your own decoder.

This works as of iOS 8. Note that the 4-byte NALU length header is in big-endian format, so if you have a UInt32 value it must be byte-swapped before copying to the CMBlockBuffer (use CFSwapInt32). – 12on Dec 12 ‘14 at 16:17 — 12on, Dec 12 ‘14 at 16:17
Thank you 12on, I was banging my head against decode errors for a long time until I tried swapping the bytes like you said. – Greg Feb 27 at 23:30 — Greg, Feb 27 at 23:30
@rpj - can you explain the 3rd (packaging) step? How many NALU frames should I pack (for example all with the same frame number)? – Tomasz Wójcik May 7 at 8:57 — Tomasz Wójcik, May 7 at 8:57
This link provide more detail explanation on how to decode h.264 step by step: stackoverflow.com/a/29525001/3156169 – ChihHao May 22 at 2:40 — ChihHao, May 22 at 2:40

How to decode a H.264 frame on iOS by hardware decoding?

How to decode a H.264 frame on iOS by hardware decoding?

2 Answers