exif.h

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#pragma once
// Functions in this module are taken from OpenCV
// https://github.com/opencv/opencv/blob/097891e311fae1d8354eb092a0fd0171e630d78c/modules/imgcodecs/src/exif.cpp

#include <jpeglib.h>
#include <torch/types.h>
#include <vector>

namespace vision {
namespace image {
namespace exif_private {

constexpr uint16_t APP1 = 0xe1;
constexpr uint16_t ENDIANNESS_INTEL = 0x49;
constexpr uint16_t ENDIANNESS_MOTO = 0x4d;
constexpr uint16_t REQ_EXIF_TAG_MARK = 0x2a;
constexpr uint16_t ORIENTATION_EXIF_TAG = 0x0112;
constexpr uint16_t INCORRECT_TAG = -1;

inline uint16_t get_endianness(const std::vector<unsigned char>& exif_data) {
  if ((exif_data.size() < 1) ||
      (exif_data.size() > 1 && exif_data[0] != exif_data[1])) {
    return 0;
  }
  if (exif_data[0] == 'I') {
    return ENDIANNESS_INTEL;
  }
  if (exif_data[0] == 'M') {
    return ENDIANNESS_MOTO;
  }
  return 0;
}

inline uint16_t get_uint16(
    const std::vector<unsigned char>& exif_data,
    uint16_t endianness,
    const size_t offset) {
  if (offset + 1 >= exif_data.size()) {
    return INCORRECT_TAG;
  }

  if (endianness == ENDIANNESS_INTEL) {
    return exif_data[offset] + (exif_data[offset + 1] << 8);
  }
  return (exif_data[offset] << 8) + exif_data[offset + 1];
}

inline uint32_t get_uint32(
    const std::vector<unsigned char>& exif_data,
    uint16_t endianness,
    const size_t offset) {
  if (offset + 3 >= exif_data.size()) {
    return INCORRECT_TAG;
  }

  if (endianness == ENDIANNESS_INTEL) {
    return exif_data[offset] + (exif_data[offset + 1] << 8) +
        (exif_data[offset + 2] << 16) + (exif_data[offset + 3] << 24);
  }
  return (exif_data[offset] << 24) + (exif_data[offset + 1] << 16) +
      (exif_data[offset + 2] << 8) + exif_data[offset + 3];
}

inline int fetch_exif_orientation(j_decompress_ptr cinfo) {
  int exif_orientation = -1;
  // Check for Exif marker APP1
  jpeg_saved_marker_ptr exif_marker = 0;
  jpeg_saved_marker_ptr cmarker = cinfo->marker_list;
  while (cmarker && exif_marker == 0) {
    if (cmarker->marker == APP1) {
      exif_marker = cmarker;
    }
    cmarker = cmarker->next;
  }

  if (exif_marker) {
    // Exif binary structure looks like this
    // First 6 bytes: [E, x, i, f, 0, 0]
    // Endianness, 2 bytes : [M, M] or [I, I]
    // Tag mark, 2 bytes: [0, 0x2a]
    // Offset, 4 bytes
    // Num entries, 2 bytes
    // Tag entries and data, tag has 2 bytes and its data has 10 bytes
    // For more details:
    // http://www.media.mit.edu/pia/Research/deepview/exif.html

    // Bytes from Exif size field to the first TIFF header
    constexpr size_t start_offset = 6;
    if (exif_marker->data_length > start_offset) {
      auto* exif_data_ptr = exif_marker->data + start_offset;
      auto size = exif_marker->data_length - start_offset;
      // Here we copy the data into the vector structure
      // TODO: we can avoid copying the data and read directly from the pointer
      std::vector<unsigned char> exif_data_vec(
          exif_data_ptr, exif_data_ptr + size);

      auto endianness = get_endianness(exif_data_vec);

      // Checking whether Tag Mark (0x002A) correspond to one contained in the
      // Jpeg file
      uint16_t tag_mark = get_uint16(exif_data_vec, endianness, 2);
      if (tag_mark == REQ_EXIF_TAG_MARK) {
        auto offset = get_uint32(exif_data_vec, endianness, 4);
        size_t num_entry = get_uint16(exif_data_vec, endianness, offset);
        offset += 2; // go to start of tag fields
        constexpr size_t tiff_field_size = 12;
        for (size_t entry = 0; entry < num_entry; entry++) {
          // Here we just search for orientation tag and parse it
          auto tag_num = get_uint16(exif_data_vec, endianness, offset);
          if (tag_num == INCORRECT_TAG) {
            break;
          }
          if (tag_num == ORIENTATION_EXIF_TAG) {
            exif_orientation =
                get_uint16(exif_data_vec, endianness, offset + 8);
            break;
          }
          offset += tiff_field_size;
        }
      }
    }
  }
  return exif_orientation;
}

constexpr uint16_t IMAGE_ORIENTATION_TL = 1; // normal orientation
constexpr uint16_t IMAGE_ORIENTATION_TR = 2; // needs horizontal flip
constexpr uint16_t IMAGE_ORIENTATION_BR = 3; // needs 180 rotation
constexpr uint16_t IMAGE_ORIENTATION_BL = 4; // needs vertical flip
constexpr uint16_t IMAGE_ORIENTATION_LT =
    5; // mirrored horizontal & rotate 270 CW
constexpr uint16_t IMAGE_ORIENTATION_RT = 6; // rotate 90 CW
constexpr uint16_t IMAGE_ORIENTATION_RB =
    7; // mirrored horizontal & rotate 90 CW
constexpr uint16_t IMAGE_ORIENTATION_LB = 8; // needs 270 CW rotation

inline torch::Tensor exif_orientation_transform(
    const torch::Tensor& image,
    int orientation) {
  if (orientation == IMAGE_ORIENTATION_TL) {
    return image;
  } else if (orientation == IMAGE_ORIENTATION_TR) {
    return image.flip(-1);
  } else if (orientation == IMAGE_ORIENTATION_BR) {
    // needs 180 rotation equivalent to
    // flip both horizontally and vertically
    return image.flip({-2, -1});
  } else if (orientation == IMAGE_ORIENTATION_BL) {
    return image.flip(-2);
  } else if (orientation == IMAGE_ORIENTATION_LT) {
    return image.transpose(-1, -2);
  } else if (orientation == IMAGE_ORIENTATION_RT) {
    return image.transpose(-1, -2).flip(-1);
  } else if (orientation == IMAGE_ORIENTATION_RB) {
    return image.transpose(-1, -2).flip({-2, -1});
  } else if (orientation == IMAGE_ORIENTATION_LB) {
    return image.transpose(-1, -2).flip(-2);
  }
  return image;
}

} // namespace exif_private
} // namespace image
} // namespace vision