关于用FFMPEG截取视频图像保存为BMP图像颠倒及颜色不正常的问题

ffmpeg-tutorial01就是将一个视频中的图像截取并保存为PMG的程序，稍作修改可将图像保存为BMP格式，程序代码如下：

// tutorial01.c
// Code based on a tutorial by Martin Bohme ([email protected])
// Tested on Gentoo, CVS version 5/01/07 compiled with GCC 4.1.1

// A small sample program that shows how to use libavformat and libavcodec to
// read video from a file.
//
// Use
//
// gcc -o tutorial01 tutorial01.c -lavformat -lavcodec -lz
//
// to build (assuming libavformat and libavcodec are correctly installed
// your system).
//
// Run using
//
// tutorial01 myvideofile.mpg
//
// to write the first five frames from "myvideofile.mpg" to disk in PPM
// format.

#include "libavcodec/avcodec.h"
#include "libavformat/avformat.h"
#include "libswscale/swscale.h"
#include <windows.h>
#include <stdio.h>

static int av_create_bmp(AVFrame *pRGBBuffer,int width,int height,int bpp)
{
   BITMAPFILEHEADER bmpheader;
   BITMAPINFO bmpinfo;
   FILE *fp;

  FILE *pFile;
  char szFilename[32];
  int  y;
 
  // Open file
  sprintf(szFilename, "frame%d.bmp", bpp);
   fp = fopen(szFilename, "wb");
   if(!fp)return -1;

   bmpheader.bfType = ('M'<<8)|'B';
   bmpheader.bfReserved1 = 0;
   bmpheader.bfReserved2 = 0;
   bmpheader.bfOffBits = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);
   bmpheader.bfSize = bmpheader.bfOffBits + width*height*bpp/8;
   /*----注意，这里的bmpinfo.bmiHeader.biHeight变量的正负决定bmp文件的存储方式，如果
   为负值，表示像素是倒过来的*/
   bmpinfo.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
   bmpinfo.bmiHeader.biWidth = width;
   bmpinfo.bmiHeader.biHeight = -height;
   bmpinfo.bmiHeader.biPlanes = 1;
   bmpinfo.bmiHeader.biBitCount = 24;
   bmpinfo.bmiHeader.biCompression = BI_RGB;
   bmpinfo.bmiHeader.biSizeImage = 0;
   bmpinfo.bmiHeader.biXPelsPerMeter = 100;
   bmpinfo.bmiHeader.biYPelsPerMeter = 100;
   bmpinfo.bmiHeader.biClrUsed = 0;
   bmpinfo.bmiHeader.biClrImportant = 0;

   fwrite(&bmpheader,sizeof(BITMAPFILEHEADER),1,fp);
   fwrite(&bmpinfo.bmiHeader,sizeof(BITMAPINFOHEADER),1,fp);
//   fwrite(pRGBBuffer,width*height*bpp/8,1,fp);
     for(y=0; y<height; y++)
    fwrite(pRGBBuffer->data[0]+y*pRGBBuffer->linesize[0], 1, width*3, fp);
   fclose(fp);

   return 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;
  AVPacket        packet;
  int             frameFinished;
  int             numBytes;
  uint8_t         *buffer;
  struct SwsContext      *ctx;
 
  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;
 
  // 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;
  /*这一段中原来图像格式是PIX_FMT_RGB24,它的问题就是保存为BMP图像颜色不正确，将其换为BGR就正确了*/
  // Determine required buffer size and allocate buffer
  numBytes=avpicture_get_size(PIX_FMT_BGR24, 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_BGR24,
   pCodecCtx->width, pCodecCtx->height);
 
  ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height,
   pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height,
   PIX_FMT_BGR24, SWS_BICUBIC, NULL, NULL, NULL);
 
  // Read frames and save first five frames to disk
  i=0;
  while(av_read_frame(pFormatCtx, &packet)>=0) {
    // 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);
   
    sws_scale(ctx, pFrame->data, pFrame->linesize,
     0, pCodecCtx->height,(AVPicture *)pFrameRGB->data,
     (AVPicture *)pFrameRGB->linesize);
    //    sws_scale(ctx, pFrame->data, pFrame->linesize,
    //     0, pCodecCtx->height,pict.data, pict.linesize);
   
    // Save the frame to disk
    if(++i<=5)
//    SaveFrame(pFrameRGB, pCodecCtx->width, pCodecCtx->height, i);
     av_create_bmp(pFrameRGB, pCodecCtx->width, pCodecCtx->height, 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;
}