Add DWA Compression

[johannesvollmer]

Why

Supporting the last missing compression format will increase reliability of the library. Currently, you can never be sure if an arbitrary image can be opened.

Implementation Approach

All code will go into a new dwa module that has to be created inside the src/compression/ folder. Look at ./piz/mod.rs and ./b44/mod.rs as a starting point.

The functions in the new module will be called from src/compression/mod.rs. To be exact, it's called here for compression (probably two times, because DWA has two variants, DWAA and DWAB):

exrs/src/compression/mod.rs

Line 175 in 66a2477

B44A => b44::compress(&header.channels, &uncompressed_native_endian, pixel_section, true),

and also here for decompression, probably also two times:

exrs/src/compression/mod.rs

Line 216 in 66a2477

B44 | B44A => b44::decompress(&header.channels, compressed, pixel_section, expected_byte_size, pedantic),

Here's the reference implementation, in C++:
https://github.com/AcademySoftwareFoundation/openexr/blob/main/src/lib/OpenEXR/ImfDwaCompressor.cpp

Note that the original implementation uses a Compressor class with mutable state. In the current Rust compression implementations, we only have two simple stateless functions instead: pub fn decompress(bytes, ...) -> bytes and pub fn compress(bytes, ...) -> bytes.

What the code should look like

There's some functionality in the compression::optimize_bytes module that you might be able to re-use, especially convert_little_endian_to_current and friends. Note that the C++ implementation has a lot of code duplication, as each compression method implements this algorithm again, look out for that part.

Of course, don't use unsafe code. For byte manipulations, have a look at the exr::io::Data trait. Also be sure to check out the existing piz and b44 Rust implementations if you are stuck. You can add unit tests in your new module. But even without any, you can still check your code simply by running cargo test. Big-endian support is not of high priority, so having little-endian alone would be great.

[johannesvollmer]

some work started in a branch, feel free to join