libavfilter trying to apply some filters

#include <stdio.h>
#include <stdlib.h>
#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#include <libavdevice/avdevice.h>
#include <libswscale/swscale.h>

// filters
#include <libavfilter/avfilter.h>
#include <libavfilter/avfiltergraph.h>
#include <libavfilter/buffersink.h>
#include <libavfilter/avcodec.h>

int main(int argc, char **argv)
{
  if (argc < 3) {
    fprintf(stderr,"usage: %s input-file format\n",argv[0]);
    exit(1);
  }

  // Prepare ffmpeg library
  av_register_all();
  avdevice_register_all();
  avfilter_register_all();
  avformat_network_init();

  // Open video stream
  AVInputFormat *pIFormat = av_find_input_format(argv[2]);

  AVDictionary *params = NULL;

  if (strcmp(argv[2],"video4linux") == 0) {
    av_dict_set(&params,"video_size","640x240",0);
    av_dict_set(&params,"standard","ntsc",0);
  }

  AVFormatContext *pIFormatCtx = NULL;

  int bRet = avformat_open_input(&pIFormatCtx, argv[1], pIFormat, &params);

  if (bRet != 0) {
      fprintf(stderr, "Could not open file\n");
      return 1;
  }

  /* Retrieve stream information */
  if (avformat_find_stream_info(pIFormatCtx,NULL) < 0) {
      fprintf(stderr, "No stream info\n");
      return 1;
  }

  /* Find the first video stream */
  int ixInputStream = -1;
  int ix;
  for (ix = 0; ix < pIFormatCtx->nb_streams; ix++) {
      if (pIFormatCtx->streams[ix]->codec->codec_type == AVMEDIA_TYPE_VIDEO) {
          ixInputStream = ix;
          break;
      }
  }
  if (ixInputStream == -1) {
      fprintf(stderr, "No video stream in file\n");
      return 1;
  }

  /* Get a pointer to the codec pICodecCtx for the video stream */
  AVCodecContext *pICodecCtx = pIFormatCtx->streams[ixInputStream]->codec;

  /* Find the decoder for the video stream */
  AVCodec *pICodec = avcodec_find_decoder(pICodecCtx->codec_id);
  if (!pICodec) {
      fprintf(stderr, "Codec not found\n");
      return 1;
  }

  /* Open input codec */
  if (avcodec_open2(pICodecCtx, pICodec,NULL) < 0) {
      fprintf(stderr, "Could not open codec\n");
      return 1;
  }

  AVPacket inputPacket;

  /* Get 1 frame from input */
  av_init_packet(&inputPacket);

  bRet = av_read_frame(pIFormatCtx, &inputPacket);

  if (bRet < 0) {
    fprintf(stderr, "Could not read frame\n");
    return 1;
  }

  if (inputPacket.stream_index != ixInputStream) {
    fprintf(stderr, "Packet is not for our stream\n");
    return 1;
  }

  /* Allocate video frame */
  AVFrame *pFrame = avcodec_alloc_frame();

  /* Determine required buffer size and allocate buffer */
  int szBuffer = avpicture_get_size(pICodecCtx->pix_fmt, pICodecCtx->width, pICodecCtx->height);
  uint8_t *pBuffer = av_mallocz(szBuffer);

  /* Decode video frame */
  int got_frame = 0;
  int e = avcodec_decode_video2(pICodecCtx, pFrame, &got_frame,&inputPacket);

  av_free_packet(&inputPacket);

  if (!got_frame) {
    fprintf(stderr, "Error reading frame\n");
    return 1;
  }

  pFrame->pts = 1;

  /* ------------------------------------------------------------------------------------ */

  // description of the filter
  // TODO: write here the filter description
  const char *filter_desc = "drawbox=20:20:20:20:red,format=rgb24";

  /* ---------------------------------------------------------------------------- */
  /* Init the filter */
  AVFilterGraph *filter_graph = avfilter_graph_alloc();
  AVFilterContext *buffersink_ctx = NULL;
  AVFilterContext *buffersrc_ctx = NULL;
  {
    char args[512];

    AVFilterInOut *outputs = avfilter_inout_alloc();
    AVFilterInOut *inputs  = avfilter_inout_alloc();

    enum PixelFormat pix_fmts[] = { pICodecCtx->pix_fmt, PIX_FMT_NONE };

    /* buffer video source: the decoded frames from the decoder will be inserted here. */
    snprintf(args, sizeof(args), "%d:%d:%d:%d:%d:%d:%d:flags=%d",
              pICodecCtx->width, pICodecCtx->height, pICodecCtx->pix_fmt,
              pICodecCtx->time_base.num, pICodecCtx->time_base.den,
              pICodecCtx->sample_aspect_ratio.num, pICodecCtx->sample_aspect_ratio.den,
              SWS_BILINEAR
            );

    int ret = avfilter_graph_create_filter(&buffersrc_ctx, avfilter_get_by_name("buffer"), "in",
                                        args, NULL, filter_graph);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot create buffer source\n");
        return ret;
    }

    /* buffer video sink: to terminate the filter chain. */
    ret = avfilter_graph_create_filter(&buffersink_ctx, avfilter_get_by_name("buffersink"), "out",
                                        NULL, pix_fmts, filter_graph);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot create buffer sink\n");
        return ret;
    }

    /* Endpoints for the filter graph. */
    outputs->name       = av_strdup("in");
    outputs->filter_ctx = buffersrc_ctx;
    outputs->pad_idx    = 0;
    outputs->next       = NULL;

    inputs->name       = av_strdup("out");
    inputs->filter_ctx = buffersink_ctx;
    inputs->pad_idx    = 0;
    inputs->next       = NULL;

    if (avfilter_graph_parse(filter_graph, filter_desc,&inputs, &outputs, NULL) < 0) {
     fprintf(stderr, "Could init filters\n");
     return 1;
    }


    if (avfilter_graph_config(filter_graph, NULL) < 0) {
      fprintf(stderr, "Could init filters\n");
      return 1;
    }
  }

  /* ---------------------------------------------------------------------------- */

  // Apply the filter in one frame
  if (av_vsrc_buffer_add_frame(buffersrc_ctx,pFrame,0) < 0) {
    fprintf(stderr, "Error reading frame II\n");
    return 1;
  }

  AVFilterBufferRef *picref = NULL;

  if (avfilter_poll_frame(buffersink_ctx->inputs[0])<0) {
    fprintf(stderr, "Error applying filter frame\n");
    return 1;
  }

  if (av_buffersink_get_buffer_ref(buffersink_ctx,&picref,0) > 0 || !picref) {
    fprintf(stderr, "Error applying filter frame II\n");
    return 1;
  }

  /* ----------------------------------------------------------------------------------- */

  // prepare the output
  AVCodec *pOCodec = avcodec_find_encoder(CODEC_ID_PNG);

  /* Allocate Output Codec */
  AVCodecContext *pOCodecCtx = avcodec_alloc_context3(pOCodec);
  if (!pOCodecCtx) {
      fprintf(stderr, "Could not allocate codec\n");
      return 1;
  }

  /* Initialize picture size and other format parameters */
  pOCodecCtx->width = pICodecCtx->width;
  pOCodecCtx->height = pICodecCtx->height;
  pOCodecCtx->pix_fmt = PIX_FMT_RGB24;
  pOCodecCtx->time_base.num = 1;
  pOCodecCtx->time_base.den = 1;
  pOCodecCtx->thread_count = 2;

  if (avcodec_open2(pOCodecCtx, pOCodec,NULL) < 0) {
      fprintf(stderr, "Could not open final codec\n");
      return 1;
  }

  /* -------------------------------------------------------------------------------------- */

  // frame in jpeg
  AVFrame *finalFrame = avcodec_alloc_frame();

  // copy the filtered picture to the final frame
  bRet = avfilter_fill_frame_from_video_buffer_ref(finalFrame,picref);

  if (bRet) {
    fprintf(stderr, "Error copyng filter to frame\n");
    return 1;
  }

  finalFrame->pts = 1;

  /* -------------------------------------------------------------------------------------- */

  // where the encoded frame will be stored
  AVPacket packet;
  av_init_packet(&packet);
  packet.size = szBuffer;
  packet.data = pBuffer;

  int gotPacket = 0;

  int szBufferActual = avcodec_encode_video2(pOCodecCtx, &packet,pFrame,&gotPacket);

  if (!szBufferActual && gotPacket && pOCodecCtx->coded_frame) {
      pOCodecCtx->coded_frame->pts       = packet.pts;
      pOCodecCtx->coded_frame->key_frame = !!(packet.flags & AV_PKT_FLAG_KEY);
  }

  /* Write frame to file */
  FILE *file = fopen("test.png", "wb");
  bRet = fwrite(pBuffer, sizeof(uint8_t), packet.size, file);
  fclose(file);
  if (bRet != packet.size) {
      fprintf(stderr, "Error writing jpeg file\n");
      return 1;
  }

  avfilter_unref_buffer(picref);

  /* Cleanup */
  if (pBuffer) {
      av_freep(&pBuffer);
      pBuffer = NULL;
      szBuffer = 0;
  }

  if (pFrame) {
      av_freep(&pFrame);
      pFrame = NULL;
  }

  if (pICodecCtx) {
      avcodec_close(pICodecCtx);
      pICodecCtx = NULL;
  }

  if (pOCodecCtx) {
      avcodec_close(pOCodecCtx);
      pOCodecCtx = NULL;
  }

  if (pIFormatCtx) {
      avformat_close_input(&pIFormatCtx);
      pIFormatCtx = NULL;
  }

  return 0;
}