// avcodec_sample.cpp
// A small sample program that shows how to use libavformat and libavcodec to
// read video from a file.
//
// Use
//
// g++ -o avcodec_sample avcodec_sample.cpp -lavformat -lavcodec -lz
//
// to build (assuming libavformat and libavcodec are correctly installed on
// your system).
//
// Run using
//
// avcodec_sample myvideofile.mpg
//
// to write the first five frames from “myvideofile.mpg” to disk in PPM
// format.
#include <avcodec.h>
#include <avformat.h>
#include <stdio.h>
bool GetNextFrame(AVFormatContext *pFormatCtx, AVCodecContext *pCodecCtx,
int videoStream, AVFrame *pFrame)
{
static AVPacket packet;
static int bytesRemaining=0;
static uint8_t *rawData;
static bool fFirstTime=true;
int bytesDecoded;
int frameFinished;
// First time were called, set packet.data to NULL to indicate it
// doesnt have to be freed
if(fFirstTime)
{
fFirstTime=false;
packet.data=NULL;
}
// Decode packets until we have decoded a complete frame
while(true)
{
// Work on the current packet until we have decoded all of it
while(bytesRemaining > 0)
{
// Decode the next chunk of data
bytesDecoded=avcodec_decode_video(pCodecCtx, pFrame,
&frameFinished, rawData, bytesRemaining);
// Was there an error?
if(bytesDecoded < 0)
{
fprintf(stderr, “Error while decoding frame\n”);
return false;
}
bytesRemaining-=bytesDecoded;
rawData+=bytesDecoded;
// Did we finish the current frame? Then we can return
if(frameFinished)
return true;
}
// Read the next packet, skipping all packets that arent for this
// stream
do
{
// Free old packet
if(packet.data!=NULL)
av_free_packet(&packet);
// Read new packet
if(av_read_packet(pFormatCtx, &packet)<0)
goto loop_exit;
} while(packet.stream_index!=videoStream);
bytesRemaining=packet.size;
rawData=packet.data;
}
loop_exit:
// Decode the rest of the last frame
bytesDecoded=avcodec_decode_video(pCodecCtx, pFrame, &frameFinished,
rawData, bytesRemaining);
// Free last packet
if(packet.data!=NULL)
av_free_packet(&packet);
return frameFinished!=0;
}
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;
int numBytes;
uint8_t *buffer;
// 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; // Couldnt open file
// Retrieve stream information
if(av_find_stream_info(pFormatCtx)<0)
return -1; // Couldnt find stream information
// Dump information about file onto standard error
dump_format(pFormatCtx, 0, argv[1], false);
// 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; // Didnt find a video stream
// Get a pointer to the codec context for the video stream
pCodecCtx=&pFormatCtx->streams[videoStream]->codec;
// Find the decoder for the video stream
pCodec=avcodec_find_decoder(pCodecCtx->codec_id);
if(pCodec==NULL)
return -1; // Codec not found
// Inform the codec that we can handle truncated bitstreams — i.e.,
// bitstreams where frame boundaries can fall in the middle of packets
if(pCodec->capabilities & CODEC_CAP_TRUNCATED)
pCodecCtx->flags|=CODEC_FLAG_TRUNCATED;
// Open codec
if(avcodec_open(pCodecCtx, pCodec)<0)
return -1; // Could not open codec
// Hack to correct wrong frame rates that seem to be generated by some
// codecs
if(pCodecCtx->frame_rate>1000 && pCodecCtx->frame_rate_base==1)
pCodecCtx->frame_rate_base=1000;
// 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=new uint8_t[numBytes];
// Assign appropriate parts of buffer to image planes in pFrameRGB
avpicture_fill((AVPicture *)pFrameRGB, buffer, PIX_FMT_RGB24,
pCodecCtx->width, pCodecCtx->height);
// Read frames and save first five frames to disk
i=0;
while(GetNextFrame(pFormatCtx, pCodecCtx, videoStream, pFrame))
{
img_convert((AVPicture *)pFrameRGB, PIX_FMT_RGB24, (AVPicture*)pFrame,
pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height);
// Save the frame to disk
if(++i<=5)
SaveFrame(pFrameRGB, pCodecCtx->width, pCodecCtx->height, i);
}
// Free the RGB image
delete [] 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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