Exif support

[seaeagle1]

Curious whether this is (or can be turned into) something usable for more than just me.

I've added a way to access embedded Exif tags in a sort-of-standardized way. The basic idea is that allows access when decoding images for other parsing libraries and for recoding images with the Exif tag intact. Since Exif is essentially TIFF-formatted, all the basic tools were more or less in place and I just had to tweak them a bit. The things that were really missing was support for IFD sub-directories and encoding values with a dynamically derived type.

Decoder -> has a method to gather the Exif information, store the entries and feed them into an encoder for a TIFF-file without pixel data.

Encoder -> has a similar method to decode TIFF-file with-or-without pixel data and feed the Exif entries into the encoder itself.

[fintelia]

Thanks for sharing this!

I've been wanting to add better EXIF handling to image-rs and being able to convert the EXIF data in a TIFF file into the same form as the EXIF chunks in PNG or JPEG would certainly help with that. It'll probably be a little while before I have a chance to work on implementing that, so I'll leave this open for now

[spoutn1k]

Where is this at ? I am interested in writing Exif as well and would love to help.

[spoutn1k]

IFD data structures are defined twice. In encoder:

image-tiff/src/encoder/mod.rs

Line 161 in 6dc7a26

ifd: BTreeMap<u16, DirectoryEntry<K::OffsetType>>,

image-tiff/src/encoder/mod.rs

Lines 565 to 569 in 6dc7a26

	struct DirectoryEntry<S> {
	data_type: u16,
	count: S,
	data: Vec<u8>,
	}

And decoder:

image-tiff/src/decoder/ifd.rs

Line 699 in 6dc7a26

pub type Directory = HashMap<Tag, Entry>;

image-tiff/src/decoder/ifd.rs

Lines 334 to 338 in 6dc7a26

	pub struct Entry {
	type_: Type,
	count: u64,
	offset: [u8; 8],
	}

u16 can be subsituted for Tag or Type in the above. Why not create a ImaegFileDirectory object that contains the data in a cow to keep the possibility of having references and implementing methods to manipulate the IFD on it ?

[spoutn1k]

Does this require an image-exif crate to be used in more than tiff ?