Parallel PNG Proposal

This is a proof-of-concept implementation of a parallel-decodable PNG format, based on ideas from https://github.com/brion/mtpng

It is fully backwards-compatible with existing PNG decoders.

However, it adds a new ancilliarry chunk, called pLLD, which advises compatible encoders that they may decode the file in parallel pieces.

This specific implementation isn't particularly fast (It's written in Python), but it demonstrates how the data can be split into independently decodable pieces. Another limitation of this implementation is that it does not support filters.

See samples/parallel_out.png for an example PNG, containing a pLLD chunk, and 8 IDAT chunks, each containing their own piece of the image.

The pLLD Chunk

Short for "Parallel Decodable".

It must appear before the first IDAT chunk.

This chunk has only two fields:

Piece height	4 bytes
Flags	1 byte

Piece height gives the height of each piece, in pixels. In instances where the image height is not evenly divisible by the piece height, the last piece may be smaller (but not larger).

A decoder should be able to determine the number of such pieces by calculating ceiling(image_height / piece_height). Each piece is stored in its own IDAT chunk.

If bit 0 of Flags is set, it indicates that the image is parallel-defilterable. This means that the filter type for the first row of each piece must never refer to the row above.

Flag bits 1-7 are reserved.