load gif to opencv mat, using giflib & opencv

giflib_to_opencv.cpp

//std headers
#include <fstream>
#include <iostream>
//lib headers
#include <gif_lib.h>
#define USING_NEW_GIF_LIB defined(GIFLIB_MAJOR) && GIFLIB_MAJOR >= 5

cv::Mat imread_gif(const std::string& filename, const int flags)
{
  std::ifstream stream(filename, std::ios::in | std::ios::binary);
  std::vector<unsigned char> contents((std::istreambuf_iterator<char>(stream)), std::istreambuf_iterator<char>());
  return imdecode_gif(&contents[0], contents.size(), flags);
}

struct DataDesc
{
  const unsigned char* buffer;
  int len;
  int pos;
};
static int gif_data_cb(GifFileType* f, GifByteType* buffer, int size)
{
  DataDesc* user_data = (DataDesc*)f->UserData;
  if (size > user_data->len)
  {
    size = user_data->len;
  }
  memcpy(buffer, user_data->buffer + user_data->pos, size);
  user_data->pos += size;
  return size;
}

#if USING_NEW_GIF_LIB
#else
static const char* GifErrorString(int error)
{
  return "unknown error.";
}
#endif
cv::Mat imdecode_gif(const unsigned char* buffer, const size_t len, const int flags)
{
  cv::Mat result;
  int ret = 0;
  int error = 0;

  DataDesc data_desc;
  data_desc.buffer = buffer;
  data_desc.len = len;	
  data_desc.pos = 0;
#if USING_NEW_GIF_LIB
  GifFileType* f = DGifOpen(&data_desc, gif_data_cb, &error);
#define CLOSE_GIF(f, error) DGifCloseFile(f, &error);
#else
  GifFileType* f = DGifOpen(&data_desc, gif_data_cb);
#define CLOSE_GIF(f, error) DGifCloseFile(f);
#endif //USING_NEW_GIF_LIB
  if (!f || error != 0)
  {
    std::cout << "gif decode error: " << GifErrorString(error) << std::endl;
    return result;
  }

  ret = DGifSlurp(f);
  if (ret != GIF_OK)
  {
    std::cout << "gif decode error: " << GifErrorString(error) << std::endl;
    CLOSE_GIF(f, &error);
    return result;
  }

  const int width = f->Image.Width;
  const int height = f->Image.Height;
  if (f->ImageCount <= 0 || width <= 0 || height<= 0)
  {
    std::cout << "gif had no any frames."<< std::endl;
    CLOSE_GIF(f, &error);
    return result;
  }

  result = cv::Mat(cv::Size(width, height), CV_8UC3);

  const ColorMapObject* ColorMap = (f->Image.ColorMap ? f->Image.ColorMap : f->SColorMap);
  const unsigned char* src = f->SavedImages[0].RasterBits;		

  for (int i = 0; i < height; i++)
  {
    unsigned char* dst = result.data + i * result.step[0];
    int loc = width * i;
    for (int j = 0; j < width; j++)
    {
      GifColorType* ColorMapEntry = &(ColorMap->Colors[*(src + loc + j)]);
      //BGR
      *dst++ = ColorMapEntry->Blue;
      *dst++ = ColorMapEntry->Green;
      *dst++ = ColorMapEntry->Red;
    }			
  }

  CLOSE_GIF(f, &error);
  return result;
}