The biological sciences need a generic image format suitable for long-term storage and capable of handling very large images. Images convey profound ideas in biology, bridging across disciplines. Digital imagery began 50 years ago as an obscure technical phenomenon. Now it is an indispensable computational tool. It has produced a variety of incompatible image file formats, most of which are already obsolete.
Several factors are forcing the obsolescence: rapid increases in the number of pixels per image; acceleration in the rate at which images are produced; changes in image designs to cope with new scientific instrumentation and concepts; collaborative requirements for interoperability of images collected in different labs on different instruments; and research metadata dictionaries that must support frequent and rapid extensions. These problems are not unique to the biosciences. Lack of image standardization is a source of delay, confusion, and errors for many scientific disciplines.
There is a need to bridge biological and scientific disciplines with an image framework capable of high computational performance and interoperability. Suitable for archiving, such a framework must be able to maintain images far into the future. Some frameworks represent partial solutions: a few, such as XML, are primarily suited for interchanging metadata; others, such as CIF (Crystallographic Information Framework),2 are primarily suited for the database structures needed for crystallographic data mining; still others, such as DICOM (Digital Imaging and Communications in Medicine),3 are primarily suited for the domain of clinical medical imaging.
What is needed is a common image framework able to interoperate with all of these disciplines, while providing high computational performance. HDF (Hierarchical Data Format)6 is such a framework, presenting a historic opportunity to establish a coin of the realm by coordinating the imagery of many biological communities. Overcoming the digital confusion of incoherent bio-imaging formats will result in better science and wider accessibility to knowledge.
Digital imagery and computer technology serve a number of diverse biological communities with terminology differences that can result in very different perspectives. Consider the word format. To the data-storage community the hard-drive format will play a major role in the computer performance of a community's image format, and to some extent, they are inseparable. A format can describe a standard, a framework, or a software tool; and formats can exist within other formats.
Image is also a term with several uses. It may refer to transient electrical signals in a CCD (charge-coupled device), a passive dataset on a storage device, a location in RAM, or a data structure written in source code. Another example is framework. An image framework might implement an image standard, resulting in image files created by a software-imaging tool. The framework, the standard, the files, and the tool, as in the case of HDF,6 may be so interrelated that they represent different facets of the same specification. Because these terms are so ubiquitous and varied due to perspective, we shall use them interchangeably, with the emphasis on the storage and management of pixels throughout their lifetime, from acquisition through archiving.
HDF5 is a generic scientific data format with supporting software. Introduced in 1998, it is the successor to the 1988 version, HDF4. NCSA (National Center for Supercomputing Applications) developed both formats for high-performance management of large heterogeneous scientific data. Designed to move data efficiently between secondary storage and memory, HDF5 translates across a variety of computing architectures. Through support from NASA (National Aeronautics and Space Administration), NSF (National Science Foundation), DOE (Department of Energy), and others, HDF5 continues to support international research. The HDF Group, a nonprofit spin-off from the University of Illinois, manages HDF5, reinforcing the long-term business commitment to maintain the format for purposes of archiving and performance.
Because an HDF5 file can contain almost any collection of data entities in a single file, it has become the format of choice for organizing heterogeneous collections consisting of very large and complex datasets. HDF5 is used for some of the largest scientific data collections, such as the NASA Earth Observation System's petabyte repository of earth science data. In 2008, netCDF (network Common Data Form)10 began using HDF5, bringing in the atmospheric and climate communities. HDF5 also supports the neutron and X-ray communities for instrument data acquisition. Recently, MATLAB implemented HDF5 as its primary storage format. Soon HDF5 will formally be adopted by the International Organization for Standardization (ISO), as part of specification 10303 (STEP, Standard for the Exchange of Product model data). Also of note is the creation of BioHDF1 for organizing rapidly growing genomics data volumes.
The HDF Group's digital preservation efforts make HDF5 well suited for archival tasks. Specifically their involvement with NARA (National Archives and Records Administration), their familiarity with the ISO standard Reference Model for an Open Archival Information System (OAIS),13 and the HDF5 implementation of the Metadata Encoding and Transmission Standard (METS)8 developed by the Digital Library Federation and maintained by the Library of Congress.
An HDF5 file is a data container, similar to a file system. Within it, user communities or software applications define their organization of data objects. The basic HDF5 data model is simple, yet extremely versatile in terms of the scope of data that it can store. It contains two primary objects: groups, which provide the organizing structures, and datasets, which are the basic storage structures. HDF5 groups and datasets may also have attributes attached, a third type of data object consisting of small textual or numeric metadata defined by user applications.
An HDF5 dataset is a uniform multidimensional array of elements. The elements might be common data types (for example, integers, floating-point numbers, text strings), n-dimensional memory chunks, or user-defined compound data structures consisting of floating-point vectors or an arbitrary bit-length encoding (for example, 97-bit floating-point number). An HDF5 group is similar to a directory, or folder, in a computer file system. An HDF5 group contains links to groups or datasets, together with supporting metadata. The organization of an HDF5 file is a directed graph structure in which groups and datasets are nodes, and links are edges. Although the term HDF implies a hierarchical structuring, its topology allows for other arrangements such as meshes or rings.
HDF5 is a completely portable file format with no limit on the number or size of data objects in the collection. During I/O operations, HDF5 automatically takes care of data-type differences, such as byte ordering and data-type size. Its software library runs on Linux, Windows, Mac, and most other operating systems and architectures, from laptops to massively parallel systems. HDF5 implements a high-level API with C, C++, Fortran 90, Python, and Java interfaces. It includes many tools for manipulating and viewing HDF5 data, and a wide variety of third-party applications and tools are available.
The design of the HDF5 software provides a rich set of integrated performance features that allow for access-time and storage-space optimizations. For example, it supports efficient extraction of subsets of data, multiscale representation of images, generic dimensionality of datasets, parallel I/O, tiling (2D), bricking (3D), chunking (nD), regional compression, and the flexible management of user metadata that is interoperable with XML. HDF5 transparently manages byte ordering in its detection of hardware. Its software extensibility allows users to insert custom software "filters" between secondary storage and memory; such filters allow for encryption, compression, or image processing. The HDF5 data model, file format, API, library, and tools are open source and distributed without charge.
X-ray crystallographers formed MEDSBIO (Consortium for Management of Experimental Data in Structural Biology)7 in 2005 to coordinate various research interests. Later the electron4 and optical14 microscopy communities began attending. During the past 10 years, each community considered HDF5 as a framework to create their independent next-generation image file formats. In the case of the NeXus,11 the format developed by the neutron and synchrotron facilities, HDF5 has been the operational infrastructure in its design since 1998.
Ongoing discussions by MEDSBIO have led to the realization that common computational storage algorithms and formats for managing images would tremendously benefit the X-ray, neutron, electron, and optical acquisition communities. Significantly, the entire biological community would benefit from coherent imagery and better-integrated data models. With four bio-imaging communities concluding that HDF5 is essential to their future image strategy, this is a rare opportunity to establish comprehensive agreements on a common scientific image standard across biological disciplines.
The following deficiencies impede the immediate and long-term usefulness of digital images:
It would be desirable to adopt an existing scientific, medical, or computer image format, and simply benefit from the consequences. All image formats have their strengths and weaknesses. They tend to fall into two categories: generic and specialized formats. Generic image formats usually have fixed dimensionality or pixel design. For example, MPEG29 is suitable for many applications as long as it is 2D spatial plus 1D temporal using red-green-blue modality that is lossy compressed for the physiological response of the eye. Alternatively, the specialized image formats suffer the difficulties of the image formats we are already using. For example, DICOM3 (medical imaging standard) and FITS5 (astronomical imaging standard,) store their pixels as 2D slices, although DICOM does incorporate MPEG2 for video-based imagery.
The ability to tile (2D), brick (3D), or chunk (nD) is required to access very large images. Although this is conceptually simple, the software is not, and must be tested carefully or risk that subsequent datasets be corrupted. That risk would be unacceptable for operational software used in data repositories and research. This function and its certification testing are critical features of HDF software that are not readily available in any other format.
The objectives of these acquisition communities are identical, requiring performance, interoperability, and archiving. There is a real need for the different bio-imaging communities to coordinate within the same HDF5 data file by using identical high-performance methods to manage pixels; avoiding namespace collisions between the biological communities; and adopting the same archival best practices. All of these would benefit downstream communities such as visualization developers and global repositories.
Performance. The design of an image file format and the subsequent organization of stored pixels determine the performance of computation because of various hardware and software data-path bottlenecks. For example, many specialized biological image formats use simple 2D pixel organizations, frequently without the benefit of compression. These 2D pixel organizations are ill suited for very large 3D images such as electron tomograms or 5D optical images. Those bio-imaging files have sizes that are orders of magnitude larger than the RAM of computers. Worse, widening gaps have formed between CPU/memory speeds, persistent storage speeds, and network speeds. These gaps lead to significant delays in processing massive data sets. Any file format for massive data has to account for the complex behavior of software layers, all the way from the application, through middleware, down to operating-systems device drivers. A generic n-dimensional multimodal image format will require new instantiation and infrastructure to implement new types of data buffers and caches to scale large datasets into much smaller RAM; much of this has been resolved within HDF5.
Interoperability. Historically the acquisition communities have defined custom image formats. Downstream communities, such as visualization and modeling, attempt to implement these formats, forcing the communities to confront design deficiencies. Basic image metadata definitions such as rank, dimension, and modality must be explicitly defined so the downstream communities can easily participate. Different research communities must be able to append new types of metadata to the image, enhancing the imagery as it progresses through the pipeline. Ongoing advances in the acquisition communities will continue to produce new and significant image modalities that feed this image pipeline. Enabling downstream users easily to access pixels and append their community metadata supports interoperability, ultimately leading to fundamental breakthroughs in biology. This is not to suggest that different communities' metadata can be or should be uniformly defined as a single biological metadata schema and ontology in order to achieve an effective image format.
Archiving. Scientific images have a general lack of archival design features. As the sophistication of bio-imagery improves, the demand for the placement of this imagery into long-term global repositories will be greater. This is being done by the Electron Microscopy Databank4 in joint development by the National Center for Macromolecular Imaging, the RCSB (Research Collaboratory for Structural Bioinformatics) at Rutgers University, and the European Bioinformatics Institute. Efforts such as the Open Microscopy Environment14 are also developing bio-image informatics tools for lab-based data sharing and data mining of biological images that also are requiring practical image formats for long-term storage and retrieval. Because of the evolving complexity of bio-imagery and the need to subscribe to archival best practices, an archive-ready image format must be self-describing. That is, there must be sufficient infrastructure within the image file design to properly document its content, context, and structure of the pixels and related community metadata, thereby minimizing the reliance on external documentation for interpretation.
Implementing a new unified image format supporting legacy software across the biological disciplines is a Gordian knot. Convincing software developers to make this a high priority is a difficult proposition. Implementation occuring across hundreds of legacy packages and flawlessly fielded in thousands of laboratories is not a trivial task. Ideally, presenting images simultaeously in their legacy formats and in a new advanced format would mitigate the technical, social, and logistical obstacles. However, this must be accomplished without duplicating the pixels in secondary storage.
One proposal is to mount an HDF5 file as a VFS (virtual file system) so that HDF5 groups become directories and HDF5 datasets become regular files. Such a VFS using FUSE (Filesystem-in-User-Space) would execute simultaneously across the user-process space and the operating system space. This hyperspace would manage all HDF-VFS file activity by interpreting, intercepting, and dynamically rearranging legacy image files. A single virtual file presented by the VFS could be composed of several concatenated HDF5 datasets, such as a metadata header dataset and a pixel dataset. Such a VFS file could have multiple simultaneous filenames and legacy formats depending on the virtual folder name that contains it, or the software application attempting to open it.
The design and function of an HDF-VFS has several possibilities. First, non-HDF5 application software could interact transparently with HDF5 files. PDF files, spreadsheets, and MPEGs would be written and read as routine file-system byte streams. Second, this VFS, when combined with transparent on-the-fly compression, would act as an operationally usable compressed tarball. Third, design the VFS with unique features such as interpreting incoming files as image files. Community-based legacy image format filters would rearrange legacy image files. For example, the pixels would be stored as HDF5 datasets in the appropriate dimensionality and modality, and the related metadata would be stored as a separate HDF5 1D byte dataset. When legacy application software opens the legacy image file, the virtual file is dynamically recombined and presented by the VFS to the legacy software in the same byte order as defined by the legacy image format. The fourth possibility is to endow the VFS with archival and performance analysis tools that could transparently provide those services to legacy application software.
To achieve the goal of an exemplary image design having wide, long-term support, we offer the following recommendations to be considered through a formal standards process:
Out of necessity, bioscientists are independently assessing and implementing HDF5, but no overarching group is responsible for establishing a comprehensive bio-imaging format, and there are few best practices to rely on. Thus, there is a real possibility that biologists will continue with incompatible methods for solving similar problems, such as not having a common image model.
The failure to establish a scalable n-dimensional scientific image standard that is efficient, interoperable, and archival will result in a less-than-optimal research environment and a less-certain future capability for image repositories. The strategic danger of not having a comprehensive scientific image storage framework is the massive generation of unsustainable bio-images. Subsequently, the long-term risks and costs of comfortable inaction will likely be enormous and irreversible.
The challenge for the biosciences is to establish a world-class imaging specification that will endow these indispensable and nonreproducible observations with long-term maintenance and high-performance computational access. The issue is not whether the biosciences will adopt HDF5 as a useful imaging frameworkthat is already happeningbut whether it is time to gather the many separate pieces of the currently highly fragmented patchwork of biological image formats and place them under HDF5 as a common framework. This is the time to unify the imagery of biology, and we encourage readers to contact the authors with their views.
This work was funded by the National Center for Research Resources (P41-RR-02250), National Institute of General Medical Sciences (5R01GM079429, Department of Energy (ER64212-1027708-0011962), National Science Foundation (DBI-0610407, CCF-0621463), National Institutes of Health (1R13RR023192-01A1, R03EB008516), The HDF Group R&D Fund, Center for Computation and Technology at Louisiana State University, Louisiana Information Technology Initiative, and NSF/EPS-CoR (EPS-0701491, CyberTools).
Catching disk latency in the act
Better Scripts, Better Games
1. BioHDF; http://www.geospiza.com/research/biohdf/.
2. Crystallographic Information Framework. International Union of Crystallography; http://www.iucr.org/resources/cif/.
3. DICOM (Digital Imaging and Communications in Medicine); http://medical.nema.org.
4. EMDB (Electron Microscopy Data Bank); http://emdatabank.org/.
5. FITS (Flexible Image transport System); http://fits.gsfc.nasa.gov/.
6. HDF (Hierarchical Data Format); http://www.hdfgroup.org.
7. MEDSBIO (Consortium for Management of Experimental Data in Structural Biology); http://www.medsbio.org.
8. METS (Metadata Encoding and Transmission Standard); http://www.loc.gov/standards/mets/.
9. MPEG (Moving Picture Experts Group); http://www.chiariglione.org/mpeg/.
10. netCDF (network common Data form); http://www.unidata.ucar.edu/software/netcdf/.
11. NeXus (neutron, x-ray and muon science); http://www.nexusformat.org.
12. NFS (Network File System); http://www.ietf.org/rfc/rfc3530.txt.
13. OAIS (Open Archival Information System); http://nost.gsfc.nasa.gov/isoas/overview.html.
14. OME (Open Microscopy Environment); http://www.openmicroscopy.org/.
15. RDF (Resource Description Framework); http://www.w3.org/RDF/.
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