JPEG2000 and JPIP
JPEG2000 is a wavelet-based image compression standard, developed by the Joint Photographic Experts Group committee with the intention to improve the existing JPEG standard. JPEG2000 provides many features that support scalable and interactive access to large-sized images such as virtual slides. These include an efficient and unified compression architecture, especially at low bit-rates, resolution scalability (Figure 1) and quality scalability (Figure 2), region of interest coding, spatial random access, and effective error resiliency.
JPEG2000 has been published as International Standard ISO/IEC 15444-1 | ITU-T Rec. T.800 (Part 1). The standard consists of several parts. Part 1 contains the core coding system, which is royalty and license-fee free. See the JPEG committee's introductory page for more details. Part 1 also specifies the JP2 file format ("jp2" as the common file extension), which offers, for example, the possibility of adding textual metadata into the image header. Part 2 specifies the JPX file format ("jpx" and "jpf" as the common file extensions), which contains more advanced features, such as building stacks of images (e.g., for specimens scanned at multiple focal depths) (Figure 3), and associating textual and graphical metadata with spatial regions.
Figure 3. The JPX file format allows multi-layered
image structure (example shows a focus stack).
With regards to virtual microscopy, the most important improvement over JPEG is the possibility to retrieve any part of the image with random spatial accessing. Moreover, due to the nature of the JPEG2000 data structure, different image resolutions (magnification levels in virtual slides) can be retrieved without compromising viewing speed. A single JPEG2000 file can thus contain the entire virtual slide data, and even data from multiple focus layers (by using the JPX format).
Part 9 of the JPEG2000 standard family defines the JPEG2000 Interactive Protocol (JPIP), which provides an efficient client-server architecture for network-based remote viewing of JPEG2000 family images. JPIP is transport-neutral and designed primarily on top of the HTTP/1.1 protocol for compatibility with existing WWW infrastructure, but it can run directly on top of lower-level transport protocols (such as TCP). In response to JPIP client image area requests, the JPIP server delivers a JPEG2000 image in streams of arbitrarily arranged data-bins, which comprise fractions of the compressed bit-stream. The data-bins are rearranged on the client side and decompressed for rendering. As the data-bins are self-contained, they engender fault-tolerant transmission: some data-bins can be absent without affecting the entire representation of the image. For detailed information on JPIP, please refer to paper published by Taubman and Prandolini.