Convert reference to binary data (header_value)

[kipcole9]

Thanks for building this library, amazing work! I'm not an experienced libvips user so apologies if this is a very naive question. I'm aiming to decode the "exif-data" chunk of an image.

iex> Image.header_value img, "exif-data"
{:ok, #Reference<0.2004606776.2701787139.175100>}

If there a way to convert the reference to the underlying binary data? I know I can:

iex> Image.header_value_as_string img, "exif-data"
{:ok,
 "RXhpZgAASUkqAAgAAAAF.....
}

I assume I can then decode_base64 back to the binary blob but that seems more conversion work than necessary.

Is there any way to return the underlying binary blob for a header field?

[akash-akya]

Hi @kipcole9, thanks for the complements :)

Agree with you. I think just returning the binary blob for Image.header_value makes more sense. Currently, I don't see the need to return ref. I will make changes.

Just curious, you want to just fetch the exif-data blob and parse it outside? I'm asking because you can get parsed exif data fields as strings directly (it uses libexif).

{:ok, field_names} = Vix.Vips.Image.header_field_names(img)

field_names
|> Enum.filter(&String.starts_with?(&1, "exif-if"))
|> Enum.map(fn field_name -> 
  {:ok, value} = Vix.Vips.Image.header_value(img, field_name)
  IO.puts(field_name <> " = " <> value)
end)

I'm not an experienced libvips user so apologies if this is a very naive question

No need, I'm beginner to libvips too :)

[kipcole9]

Oh, now I feel a little silly - thanks for the pointer. Yes, I have been doing my own exif decoding (leaning on prior work).

Your example code is very helpful however the return from Vix.Vips.Image.header_value/2 is quite noisy:

iex> {:ok, value} = Vix.Vips.Image.header_value(img, "exif-ifd2-ShutterSpeedValue")
{:ok, "exif-ifd2-ShutterSpeedValue = 8321928/1000000 (8.32 EV (1/320 sec.), SRational, 1 components, 8 bytes)"}

Which seems to be the inherent noisiness of libexif?

Anyway, thanks for the engagement and feel free to close the issue! (look forward to a raw binary return from get_header_value/2 when you get around to it).

[akash-akya]

I see. Agree that current output is noisy. Probably this is relevant: kleisauke/net-vips#74 (comment)

If possible, can you try https://github.com/akash-akya/vix/tree/vips-blob branch for the getting the blob as binary?

Tried this

Mix.install([
  {:vix, git: "git@github.com:akash-akya/vix.git", branch: "vips-blob" }
])

{:ok, img} = Vix.Vips.Image.new_from_file("~/Downloads/Canon_DIGITAL_IXUS_400.jpg")
{:ok, exif_data} = Vix.Vips.Image.header_value(img, "exif-data")
File.write!("exif.dat", exif_data)

[kipcole9]

Thats great news, thank you. Will try that out in a few hours. I'll stick with doing my own Exif decoding for now since I also need to decode XMP data (and maybe later IPTC data). And it may actually be more efficient since its only one NIF call.

Are you open to PRs to add some additional functions like vips_image_remove() and vips_image_set()?

[kipcole9]

Have implemented the vips-blob version. Its perfect for my use case and very fast. Much appreciated.

[akash-akya]

Nice! I'll close the issue once the change is merged.

Are you open to PRs to add some additional functions like vips_image_remove() and vips_image_set()?

That's great. Yes, all kinds of contributions are welcome.

Thank you for taking time to raise the issue and testing it.