RFC 24: GDAL Progressive Data Support

Author: Norman Barker
Contact: nbarker@…
Status: Development

Summary

To provide an interface for data streaming support to GDAL. The RFC focuses on JPIP but should be generic to apply to other streaming / progressive formats.

Definitions

JPIP: JPEG 2000 Interactive Protocol

Objective

To provide an interface to a streaming data source in a convenient manner for RasterIO operations within GDAL and to expose this interface via swig.

Note this RFC originally explored using an Observer pattern as per implementations in Java ​imageio update but after detailed discussions with ​Tamas Szekeres, Even Rouault and Adam Nowacki a simpler interface is proposed to reduce the potential for problems by having the callback implemented by another thread.

Each swig wrapper language will have its own design pattern for event handling (the image updates) and the proposed interface will allow that pattern implementation to be provided, even if simulated.

Implementation

The implementation is a definition of an interface for streaming support data support within GDAL. Concrete implementations of this interface will follow. Currently the most convenient JPIP streaming developer library is ​Kakadu however since GDAL is also a developer library, only stubs can be distributed to conform to Kakadu licensing (JP2KAK). Vendors are also interested in using GDAL for streaming support and a standard interface will allow these plugins to be incorporated. e.g. ​ECW, ​MrSID.

To support reading streaming data from a progressive server it will be necessary to communicate between different threads. In general it's safer not to do this by shared memory, but by explicit message passing using asynchronous queues. e.g. ​Glib Asynchronous Queues, ​Wikipedia Message Queues

This interface results in a subclass of GDALDataset, and currently supports dataset level access only, not raster band access. Access to the stream per band is not precluded by this design and can be added as a later RFC.

Class Diagram

Sequence of Events