Dear Friends,
Hi,
I already posted a request about error
concealment<http://lists.mplayerhq.hu/pipermail/libav-user/2008-December/001917.html>in
the mailing list.
Recently, I have another issue that your help is appreciated on:
1- Can anybody explain me the type of error concealment one will get by
setting the "error_concealment" flag in "AVCodecContext" structure of
"avcodec.h" to one?
I tried setting it to 1, and then in a code, I zeroed out every five
packets. But it cannot extract the frames from this corrupted video
(it could do before corruption)
2- I am trying to change FFMPEG's source code to force it decode the
corrupted video from the first place it makes sense.
I taken a look at the "av_decode_video"'s definition in "utils.c" and it
seems a member function of "AVCodec" structure called "decode" does
everything.
But I have never found this function's body.
Dose anybody know where it can be found?
Is the definition different for differnet codecs?
I am working with an MPEG-2 stream btw.
3- Is my approach to concealment correct?
I mean should I zero-out a packet's data (object of type AVPacket) to
corrupt the video and then do the concealment?
Thanks is in Advance.
Alireza Khoshgoftar Monfared
PS: I've attached my current code for reference.
PS: I appreciate any advice you may have for me to do this project, esp. if
you have some experience with concealment.
// concealment.c
// gcc -o concealment concealment.c -lavutil -lavformat -lavcodec -lz
//
// concealment videofile.mpg
//
#include "libavcodec/avcodec.h"
#include "libavformat/avformat.h"
#include <stdio.h>
void SaveFrame(AVFrame *pFrame, int width, int height, int iFrame) {
FILE *pFile;
char szFilename[32];
int y;
// Open file
sprintf(szFilename, "frame%d.ppm", iFrame);
pFile=fopen(szFilename, "wb");
if(pFile==NULL)
return;
// Write header
fprintf(pFile, "P6\n%d %d\n255\n", width, height);
// Write pixel data
for(y=0; y<height; y++)
fwrite(pFrame->data[0]+y*pFrame->linesize[0], 1, width*3, pFile);
// Close file
fclose(pFile);
}
int main(int argc, char *argv[]) {
AVFormatContext *pFormatCtx;
int i, videoStream;
AVCodecContext *pCodecCtx;
AVCodec *pCodec;
AVFrame *pFrame;
AVFrame *pFrameRGB;
AVPacket packet;
int frameFinished;
int numBytes;
uint8_t *buffer;
if(argc < 2) {
printf("Please provide a movie file\n");
return -1;
}
// Register all formats and codecs
av_register_all();
// Open video file
if(av_open_input_file(&pFormatCtx, argv[1], NULL, 0, NULL)!=0)
return -1; // Couldn't open file
// Retrieve stream information
if(av_find_stream_info(pFormatCtx)<0)
return -1; // Couldn't find stream information
// Dump information about file onto standard error
dump_format(pFormatCtx, 0, argv[1], 0);
// Find the first video stream
videoStream=-1;
for(i=0; i<pFormatCtx->nb_streams; i++)
if(pFormatCtx->streams[i]->codec->codec_type==CODEC_TYPE_VIDEO) {
videoStream=i;
break;
}
if(videoStream==-1)
return -1; // Didn't find a video stream
// Get a pointer to the codec context for the video stream
pCodecCtx=pFormatCtx->streams[videoStream]->codec;
pCodecCtx->error_concealment = 1;
// Find the decoder for the video stream
pCodec=avcodec_find_decoder(pCodecCtx->codec_id);
if(pCodec==NULL) {
fprintf(stderr, "Unsupported codec!\n");
return -1; // Codec not found
}
// Open codec
if(avcodec_open(pCodecCtx, pCodec)<0)
return -1; // Could not open codec
// Allocate video frame
pFrame=avcodec_alloc_frame();
// Allocate an AVFrame structure
pFrameRGB=avcodec_alloc_frame();
if(pFrameRGB==NULL)
return -1;
// Determine required buffer size and allocate buffer
numBytes=avpicture_get_size(PIX_FMT_RGB24, pCodecCtx->width,
pCodecCtx->height);
buffer=(uint8_t *)av_malloc(numBytes*sizeof(uint8_t));
// Assign appropriate parts of buffer to image planes in pFrameRGB
// Note that pFrameRGB is an AVFrame, but AVFrame is a superset
// of AVPicture
avpicture_fill((AVPicture *)pFrameRGB, buffer, PIX_FMT_RGB24,
pCodecCtx->width, pCodecCtx->height);
// Read frames and save first five frames to disk
i=0;
while(av_read_frame(pFormatCtx, &packet)>=0) {
if(fmod(i,5) == 0){
memset(packet.data, 0, packet.size);
//avcodec_decode_video(pCodecCtx, pFrame, &frameFinished,
// 0, packet.size);
}
else{
//Is this a packet from the video stream?
if(packet.stream_index==videoStream) {
// Decode video frame
avcodec_decode_video(pCodecCtx, pFrame, &frameFinished,
packet.data, packet.size);
// Did we get a video frame?
}
if(frameFinished) {
// Convert the image from its native format to RGB
img_convert((AVPicture *)pFrameRGB, PIX_FMT_RGB24,
(AVPicture*)pFrame, pCodecCtx->pix_fmt, pCodecCtx->width,
pCodecCtx->height);
// Save the frame to disk
//if(++i<=400)
SaveFrame(pFrameRGB, pCodecCtx->width, pCodecCtx->height,i);
i++;
}
}
// Free the packet that was allocated by av_read_frame
av_free_packet(&packet);
}
// Free the RGB image
av_free(buffer);
av_free(pFrameRGB);
// Free the YUV frame
av_free(pFrame);
// Close the codec
avcodec_close(pCodecCtx);
// Close the video file
av_close_input_file(pFormatCtx);
return 0;
}
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