imdecode with crop

crop_imdecode.cpp

extern "C" {
#include <jerror.h>
#include <jpeglib.h>
#include <setjmp.h>
}

struct JpegErrorManager {
  struct jpeg_error_mgr pub;
  jmp_buf setjmp_buffer;
};

static void JpegErrorExit(j_common_ptr cinfo) {
  char error_message[JMSG_LENGTH_MAX] = {0};
  JpegErrorManager* myerr = (JpegErrorManager*) cinfo->err;
  (*(cinfo->err->format_message))(cinfo, error_message);
  LOG(ERROR) << error_message;
  longjmp(myerr->setjmp_buffer, 1);
}

//// 这里错误处理参考:
//// https://stackoverflow.com/questions/19857766/error-handling-in-libjpeg
cv::Mat CropDecodeJPEG(const std::vector<unsigned char>& content,
                       const int crop_width,
                       const int crop_height) {
  struct jpeg_decompress_struct cinfo;
  struct JpegErrorManager jerr;
  cinfo.err = jpeg_std_error(&jerr.pub);
  jerr.pub.error_exit = JpegErrorExit;
  if (setjmp(jerr.setjmp_buffer)) {
    jpeg_destroy_decompress(&cinfo);
    return cv::Mat();
  }

  jpeg_create_decompress(&cinfo);
  jpeg_mem_src(&cinfo, content.data(), content.size());
  jpeg_read_header(&cinfo, TRUE);
  cinfo.out_color_space = JCS_EXT_BGR;
  jpeg_start_decompress(&cinfo);

  const int width = cinfo.output_width;
  const int height = cinfo.output_height;
  const int channels = cinfo.out_color_components;
  CHECK_GE(width, crop_width);
  CHECK_GE(height, crop_height);
  CHECK_EQ(channels, 3);

  const int offset_x = (width - crop_width) / 2;
  const int offset_y = (height - crop_height) / 2;
  cv::Mat image(crop_height, crop_width, CV_8UC3);

  if (offset_x > 0) {
    unsigned int temp_offset_x = offset_x;
    unsigned int temp_width = crop_width;
    jpeg_crop_scanline(&cinfo, &temp_offset_x, &temp_width);
    CHECK_EQ(temp_offset_x, offset_x);
    CHECK_EQ(temp_width, crop_width);
  }

  uchar* temp = image.data;
  if (offset_y > 0) {
    jpeg_skip_scanlines(&cinfo, offset_y);
  }
  while(cinfo.output_scanline < height - offset_y) {
    if (jpeg_read_scanlines(&cinfo, &temp, 1) == 1) {
      temp += crop_width * channels;
    }
  }
  if (offset_y > 0) {
    jpeg_skip_scanlines(&cinfo, offset_y);
  }
  jpeg_finish_decompress(&cinfo);
  jpeg_destroy_decompress(&cinfo);
  return image;
}