saving TIFF tags in header?

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The TIFF format is very powerful and versatile. In addition to storing image metadata, it allows storing multiple channels, different data formats, etc.

Opencv's imwrite is a handy little function to save an image, but it won't allow you to do complex operations like metadata handling and exotic formats (like multispectral data).

Fortunately there is the libTIFF library, which is quite easy to use and it can handle any kind of TIFF data.

You can find several tutorials for using this library (e.g. here)