如何使用 ffmpeg 库从视频中提取灰度图像？

Question

我编译并测试了这个 tutorial from here which works just fine. After I tried to edit the tutorial to read/convert frames into grayscale。我只是将 pFrameRGB 更改为 pFrameGray，将 PIX_FMT_RGB24 更改为 PIX_FMT_GRAY16 并仅保存第 200 帧。它编译并运行但图像未显示预期。怎么了？

图片：

修改后的代码：

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

#include <stdio.h>

// compatibility with newer API
#if LIBAVCODEC_VERSION_INT < AV_VERSION_INT(55,28,1)
#define av_frame_alloc avcodec_alloc_frame
#define av_frame_free avcodec_free_frame
#endif

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[]) {
  // Initalizing these to NULL prevents segfaults!
  AVFormatContext   *pFormatCtx = NULL;
  int               i, videoStream;
  AVCodecContext    *pCodecCtxOrig = NULL;
  AVCodecContext    *pCodecCtx = NULL;
  AVCodec           *pCodec = NULL;
  AVFrame           *pFrame = NULL;
  AVFrame           *pFrameGRAY = NULL;
  AVPacket          packet;
  int               frameFinished;
  int               numBytes;
  uint8_t           *buffer = NULL;
  struct SwsContext *sws_ctx = NULL;

  if(argc < 2) {
    printf("Please provide a movie file\n");
    return -1;
  }
  // Register all formats and codecs
  av_register_all();

  // Open video file
  if(avformat_open_input(&pFormatCtx, argv[1], NULL, NULL)!=0)
    return -1; // Couldn't open file

  // Retrieve stream information
  if(avformat_find_stream_info(pFormatCtx, NULL)<0)
    return -1; // Couldn't find stream information

  // Dump information about file onto standard error
  av_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==AVMEDIA_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
  pCodecCtxOrig=pFormatCtx->streams[videoStream]->codec;
  // Find the decoder for the video stream
  pCodec=avcodec_find_decoder(pCodecCtxOrig->codec_id);
  if(pCodec==NULL) {
    fprintf(stderr, "Unsupported codec!\n");
    return -1; // Codec not found
  }
  // Copy context
  pCodecCtx = avcodec_alloc_context3(pCodec);
  if(avcodec_copy_context(pCodecCtx, pCodecCtxOrig) != 0) {
    fprintf(stderr, "Couldn't copy codec context");
    return -1; // Error copying codec context
  }

  // Open codec
  if(avcodec_open2(pCodecCtx, pCodec, NULL)<0)
    return -1; // Could not open codec

  // Allocate video frame
  pFrame=av_frame_alloc();

  // Allocate an AVFrame structure
  pFrameGRAY=av_frame_alloc();
  if(pFrameGRAY==NULL)
    return -1;

  // Determine required buffer size and allocate buffer
  numBytes=avpicture_get_size(PIX_FMT_GRAY16, pCodecCtx->width,
                  pCodecCtx->height);
  buffer=(uint8_t *)av_malloc(numBytes*sizeof(uint8_t));

  // Assign appropriate parts of buffer to image planes in pFrameGRAY
  // Note that pFrameGRAY is an AVFrame, but AVFrame is a superset
  // of AVPicture
  avpicture_fill((AVPicture *)pFrameGRAY, buffer, PIX_FMT_GRAY16,
         pCodecCtx->width, pCodecCtx->height);

  // initialize SWS context for software scaling
  sws_ctx = sws_getContext(pCodecCtx->width,
               pCodecCtx->height,
               pCodecCtx->pix_fmt,
               pCodecCtx->width,
               pCodecCtx->height,
               PIX_FMT_GRAY16,
               SWS_BILINEAR,
               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_video2(pCodecCtx, pFrame, &frameFinished, &packet);

      // Did we get a video frame?
      if(frameFinished) {
    // Convert the image from its native format to GRAY
    sws_scale(sws_ctx, (uint8_t const * const *)pFrame->data,
          pFrame->linesize, 0, pCodecCtx->height,
          pFrameGRAY->data, pFrameGRAY->linesize);

    // Save the frame to disk
    if(++i==200)
      SaveFrame(pFrameGRAY, pCodecCtx->width, pCodecCtx->height, 
            i);
      }
    }

    // Free the packet that was allocated by av_read_frame
    av_free_packet(&packet);
  }

  // Free the GRAY image
  av_free(buffer);
  av_frame_free(&pFrameGRAY);

  // Free the YUV frame
  av_frame_free(&pFrame);

  // Close the codecs
  avcodec_close(pCodecCtx);
  avcodec_close(pCodecCtxOrig);

  // Close the video file
  avformat_close_input(&pFormatCtx);

  return 0;
}

Answer 1

你的主要问题在这里：

fprintf(pFile, "P6\n%d %d\n255\n", width, height);

根据关于 PPM 的一些信息，我发现：http://paulbourke.net/dataformats/ppm/ 您正在使用标识符 P6 编写 PPM header，这意味着 "rgb color"。如果您正在制作灰度图像，则需要使用 P5。

此外，您使用的是 ffmpeg 的像素格式 PIX_FMT_GRAY16，但您将其解释为字节。您可能打算改用 PIX_FMT_GRAY8 。如果您确实想要在输出中使用 16 位灰度，则应将 PPM header 中的第三个数字从 255 更改为 65535。

如何使用 ffmpeg 库从视频中提取灰度图像？

How to extract grayscale image from a video with ffmpeg-library?

c

video

ffmpeg

libav