This library allows encoding and decoding custom chunks in PNG images.

These chunks can contain arbitrary binary data (of length at most 2^32-1), placed before or after image data.

If they are properly marked as ancillary (see PNG specification, section 11.3), good decoders will simply ignore them.

Currently, the encoder always places the custom chunks before the image data, which allows reading them without even decoding the image.

Usage

Rust

The [Encoder] struct implements the [image::ImageEncoder] trait, so that it can be used to encode [image::DynamicImage] images:

// Create custom chunk
// The name implies that this is a private non-critical chunk that is safe to copy
// only when non-critical chuks have not been modified.
let chunk_type = png_achunk::ChunkType::from_ascii(&"teST")?;
let chunk = png_achunk::Chunk::new(chunk_type, vec![4, 5, 6])?;
// Encode an `image::DynamicImage`
image.write_with_encoder(
    png_achunk::Encoder::new_to_file("test.png")?.with_custom_chunk(chunk.clone()),
)?;

For decoding:

// Decode image and ancillary chunks
let (image, chunks) = png_achunk::Decoder::from_file("test.png")?.decode_all()?;
// Decode ancillary chunks without reading the image data
let chunks = png_achunk::Decoder::from_file("test.png")?.decode_ancillary_chunks()?;

Python

Only a primitive interface is implemented at the moment.

First, copy target/release/libpng_achunk_py.so to png_achunk.so.

import png_achunk
chunk = png_achunk.read_chunk("test.png", "teST")

C

Only a primitive interface is implemented at the moment.

The example in c/test.c can be compiled with

$ gcc test.c -L. -l:libpng_achunk.a -lm -o test

Implementation

PNG encoding and decoding use the [png] crate. For decoding, we need to decode ancillary chunks manually as [png::Decoder] does not expose them.