Metadata loss: EXIF, ICC profiles or Photoshop metadata is stripped

[tim-janik]

The current implementation only provides image pixel loading, watermark embedding and saving.
There is no functionality for preserving image formats (compression) or metadata implemented yet.
I.e. watermarked output images are stripped of EXIF, ICC profiles and Photoshop data.
It would be desirable to preserve this information (for comments, copyright, color accuracy, location, etc).

Here is an example image with lots of Exif data that is currently being stripped from: https://exiv2.org/sample.html

[tim-janik]

Gemini 3.1 Pro

To preserve metadata perfectly in the OpenCV version, the short answer is: You cannot do it using only OpenCV.

OpenCV is designed for computer vision and matrix math, not for image editing. By design, cv::imread strips almost all metadata (EXIF, XMP, IPTC, and ICC color profiles) to give you raw pixel data, and cv::imwrite writes a fresh file with no historical metadata.

To preserve metadata in an OpenCV pipeline, you must use an external tool or library to copy the metadata from the original file to the newly saved file after OpenCV has written it.

Here are the two standard ways to do this:

Option 1: Use Exiv2 (The robust C++ way)Exiv2 is the industry-standard C++ library for reading and writing image metadata. You can use it to inject the metadata from the source image into the output image right after cv::imwrite.

1. Install Exiv2:
You will need to link against libexiv2 in your build system (e.g., pkg-config --cflags --libs exiv2).

2. Add this function to your code:

#include <exiv2/exiv2.hpp>

static void
copy_metadata (const std::string &src_path, const std::string &dst_path)
{
  try {
    // Open and read metadata from the original image
    auto src_img = Exiv2::ImageFactory::open(src_path);
    src_img->readMetadata();

    // Open the newly created watermarked image
    auto dst_img = Exiv2::ImageFactory::open(dst_path);
    dst_img->readMetadata();

    // Copy EXIF, IPTC, and XMP data
    dst_img->setExifData(src_img->exifData());
    dst_img->setIptcData(src_img->iptcData());
    dst_img->setXmpData(src_img->xmpData());

    // Copy ICC Color Profile (Crucial for correct colors in non-sRGB images)
    if (src_img->iccProfileDefined()) {
        dst_img->setIccProfile(*src_img->iccProfile());
    }

    // Write the metadata back to the destination file
    dst_img->writeMetadata();
  } catch (const Exiv2::Error& e) {
    std::cerr << "Warning: Failed to copy metadata: " << e.what() << std::endl;
  }
}

3. Call it after cv::imwrite:

  // Write the result
  if (!cv::imwrite (opt.output_img, watermarked))
    die (1, "Failed to write output image: %s", opt.output_img.c_str());

  // Inject metadata back into the saved file
  copy_metadata(opt.input_img, opt.output_img);

Option 2: Use ExifTool (The easy/CLI way)

If you don't want to add another C++ dependency to your project and you know exiftool is installed on the system, you can simply invoke it via a system call after saving the image. exiftool is incredibly thorough and will copy absolutely everything, including ICC profiles and maker notes.

#include <cstdlib>

static void
copy_metadata_cli (const std::string &src_path, const std::string &dst_path)
{
  // -TagsFromFile copies all metadata
  // -icc_profile copies the color profile
  // -overwrite_original prevents exiftool from creating a backup file (dst_path_original)
  std::string cmd = "exiftool -TagsFromFile \"" + src_path + "\" -all:all -icc_profile \"" + dst_path + "\" -overwrite_original -q";
  
  int ret = std::system(cmd.c_str());
  if (ret != 0) {
    std::cerr << "Warning: exiftool failed to copy metadata." << std::endl;
  }
}

A Warning About OpenCV and ICC Profiles (Color Spaces)

Because OpenCV strips the ICC Profile on load, it assumes every image is in standard sRGB.

If a user inputs an image in AdobeRGB or Display P3, OpenCV will blindly perform its YIQ math on those raw values as if they were sRGB. When you copy the ICC profile back into the output file (using either Exiv2 or ExifTool), image viewers will correctly display the colors again.

For a subtle watermark, doing math in the "wrong" color space usually doesn't cause visible artifacts, but it is technically mathematically incorrect. This is one of the major reasons why a port to libvips is vastly superior—libvips is fully color-space aware and handles ICC profiles natively during its math operations.