Question: Writing EXIF

[jeromenelson]

Hello there,

Hoping this is a quick one. How to insert IFD2/Lens tags into an image? I was able to set many other tags without any issues, anything related to Lens info (exif-ifd2-LensModel, exif-ifd2-LensSpecification, exif-ifd2-LensSerialNumber, exif-ifd2-LensMake) does not seem to work.

            using var image = NetVips.Image.NewFromFile(Path.Join(_path, "img_no_exif.jpg"));
            image.Mutate(m => m.Set(GValue.GStrType, "exif-ifd2-LensMake", "Canon"));
            image.WriteToFile(Path.Combine(_path, "img_with_lens_info.jpg"));

            using var exifed = NetVips.Image.NewFromFile(Path.Join(_path, "img_with_lens_info.jpg"));
            bool hasLensMake = exifed.Contains("exif-ifd2-LensMake");

Side note: I tried both GValue.GStrType and GValue.RefStrType with no luck. For other tags, I figured out how to successfully set certain values through trial and error - for example, while working with other numeric fields (say exif-ifd2-FNumber), instead of directly using GValue.GDoubleType I had to convert it to a rational string and then use the type GValue.RefStrType to successfully set. But this simple string field, I just can't get to the bottom of it.

Thank you in advance!

[kleisauke]

Hi @jeromenelson,

These ASCII tags were added in the EXIF 2.3 specification, which libvips is currently unaware of. I've just opened PR libvips/libvips#2795 that allows you to modify/create these tags in libvips. After that, this should work without any problems:

using var image = Image.NewFromFile("x.jpg");
using var mutated = image.Mutate(m => m.Set(GValue.GStrType, "exif-ifd2-LensMake", "Canon"));
mutated.WriteToFile("x2.jpg");

using var image2 = Image.NewFromFile("x2.jpg");
Debug.Assert(image2.Contains("exif-ifd2-LensMake"));

[jeromenelson]

Thank you for the quick response, any idea how long would it take for this PR to make it into the next release?

[kleisauke]

We aim for a release in late May, see: libvips/libvips#2790. Setting exif-ifd2-LensSpecification should be possible after PR libexif/libexif#85, e.g.:

using var image = Image.Black(100, 100);
using var mutated = image.Mutate(m => m.Set(GValue.GStrType, "exif-ifd2-LensSpecification", "5/1 10/1 100/1 200/1"));
mutated.WriteToFile("x2.jpg");

using var image2 = Image.NewFromFile("x2.jpg");
Debug.Assert(image2.Contains("exif-ifd2-LensSpecification"));

[kleisauke]

NetVips v2.2.0 and NetVips.Native v8.13.0 is now available.

Note that setting exif-ifd2-LensSpecification would require libexif to be built from the master branch, the pre-built binaries provided by NetVips are built against libexif v0.6.24.

[jeromenelson]

Thank you, super excited about the other new features as well!