[Libav-user] decoding H264 frames

[srikanta mondal]

The encoded raw data (nal[] units) can decode using ffmpeg library. You
have to initialize the AVCodecContext there and pass each raw encoded-data
to avcodec_decode_video2() method.

On Mon, Feb 4, 2013 at 2:18 PM, Imran Khan <imrank at cdac.in> wrote:

> Hello All
>
> I am trying to decode raw H264 frames using libavcodec. After getting
> frames from webcam I am encoding it using H264 codec. After that I am
> passing these frames one by one to a H264 decoder. In decoder
> AVCodecContext I am setting width,height and  pixel format only. Problem
> is decoder not decompressing the frames. the size of the frame are same as
> the encoded frame size. Can anyone tell me what may be the problem.
>
> Here is my decoder code...
>
> #include "video_decoder.h"
>
> using namespace std;
>
> uint8_t* rgb_bytes;
>
> AVFrame *alloc_dec_picture(enum PixelFormat pix_fmt, int width, int
> height){
>
>         AVFrame *picture;
>
>     uint8_t *picture_buf;
>
>     int size;
>     picture = avcodec_alloc_frame();
>
>     if (!picture)
>         return NULL;
>
>     size = avpicture_get_size(pix_fmt, width, height);
>
>     picture_buf = (uint8_t*)av_malloc(size);
>
>     if (!picture_buf) {
>
>         av_free(picture);
>
>         return NULL;
>
>     }
>
>     avpicture_fill((AVPicture *)picture, picture_buf,pix_fmt, width,
> height);
>
>     return picture;
>
> }
>
> VideoDecoder::VideoDecoder(){
>
>         avcodec_register_all();
>
>         avctx = NULL;
>
>         picture = NULL;
>
>         frame = NULL;
>
> }
>
>
> char* VideoDecoder::decoder_init(AVCodecID codec_id){
>
>         int error;
>
>         AVCodec* codec;
>
>         av_init_packet(&avpkt);
>
>         codec = avcodec_find_decoder(codec_id);
>
>         if(!codec){
>
>         return "codec not found";
>
>
>         }
>
>         avctx = avcodec_alloc_context3(codec);
>
>         if(!avctx){
>
>                 return "can not allocate avcontext ::
> avocdec_alloc_context";
>
>         }
>
>         avctx->width=640;
>
>         avctx->height=480;
>
>         avctx->pix_fmt=AV_PIX_FMT_YUV420P;
>
>         if(codec->capabilities&CODEC_CAP_TRUNCATED)
>         avctx->flags|= CODEC_FLAG_TRUNCATED;
>
>         error = avcodec_open2(avctx ,codec ,NULL);
>
>         if(error < 0 ){
>
>                 return "can not open codec :: avcodec_open2";
>
>         }
>
>         frame = avcodec_alloc_frame();
>
>         picture = alloc_dec_picture(PIX_FMT_RGB24, 640, 480);
>
>         return "decoder init completed\n";
> }
>
> char* VideoDecoder::decode_frame(uint8_t *frame_buffer, size_t
> data_length){
>
>         int w = 640;
>
>     int h = 480;
>
>         rgb_bytes =NULL;
>
>         int memBytes = avpicture_get_size(PIX_FMT_RGB24 , avctx->width ,
> avctx->height );
>
>         int len ,got_frame;
>
>         avpkt.size = data_length;
>
>         avpkt.data = frame_buffer;
>
>         if(!frame_buffer){
>
>         return "frame buffer empty\n";
>
>         }
>
>         len = avcodec_decode_video2(avctx ,frame ,&got_frame ,&avpkt);
>
>         if( len < 0){
>
>                 return "error while decoding\n";
>
>         }
>
>         if( got_frame ){
>
>         static struct SwsContext *img_convert_ctx;
>
>      if(img_convert_ctx == NULL) {
>
>       img_convert_ctx = sws_getContext(w, h,
>                   PIX_FMT_YUV420P, avctx->width,
>                   avctx->height, PIX_FMT_BGR24,
>                   SWS_BICUBIC, NULL, NULL, NULL);
>
>        if(img_convert_ctx == NULL) {
>
>             return  "Cannot initialize the conversion context!\n";
>
>       }
>
>     }
>
>         sws_scale(img_convert_ctx,
>                 frame->data , frame->linesize ,
>                 0, h ,picture->data,
>                 picture->linesize );
>
>         rgb_bytes = new uint8_t[memBytes];
>
>         memcpy(rgb_bytes ,picture->data ,memBytes);
>
>         }
>
>         rgb_bytes = new uint8_t[memBytes];
>
>         memcpy(rgb_bytes ,frame->data ,memBytes);
>
>
>         av_free_packet(&avpkt);
>
>         return "completed decoding\n";
> }
>
>
> --
> Thanks and Regards
> Imran Khan
>
>
>
>
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