A look at the on-goingproblems file formatincompatibilitiescause for pros in allforms of imaging, inevery stage of theprocess-and alook at what's beingdone about it

It'sinteresting to note that when lifewas described as "a remorselessstruggle for existence", no one hadyet invented an image file format=or

tried to work with one. There are over 100broad formats competing for dominancenow. How does an imaging practitionermake sense of all this? This article willexamine some of the key issues involved,present the results of recent research ontills issue-and invite you to participate inwhat could be the biggest survey on imagefile format usage ever conducted.

Roots of the problemHistorically, the image community was

composed largely of highly trained engi-neers and scientists quick to create theirown software and image formats to solvetheir problems. Given the diversity of enduse applications (e.g., astronomy, medi-cine, publishing, etc.) and a tendency forresearchers to use their own specialistconferences and journals for sharing newson developments, it is perhaps under-standable that these pioneers of the imagefield created such a profusion of formats.

And file formats are of central impor-tance to all electronic imaging applica-tions, as they govern how easy it is toprocess, move or exchange imagesbetween different systems. Images areobtained, of course, from a variety ofsources, ranging from scanners to digitalcameras. Even at the point of capture, the

Victor Callaghan is with the University ofEssex. Dept. of Computer Science, Colch-ester UK. He can be reached at: UK# +44-206-872682.

44

question immediately arises as to whichformat the image should be stored in. Thechoice of format will be determined by fac-tors such as the intended use of theimage, how large it is (i.e., its spatial, con-trast and color resolution), whether it is tobe compressed (and how e.g., lossless)what auxiliary information is to be storedalong with the image (e.g., grabbing con-ditions, author, etc.).In scientific applications, the value of

the picture may lay in subtleimage data variations whichmay not be perceivable tothe eye without the aid ofsome sort of image process-ing. Seeing, accessing andcomparing text or drawingsmay be the key impulse indocument imaging. In imag-ing arts, the value of the pic-ture may be in the estheticcomposition of the imageitself (like any art work). Itis clear that imaging applica-tions can have vastly differ-ent requirements, in termsof the likes of tolerance todata loss and required auxil-iary information.

BY VICTOR CALLAGHAN

comfortable living by seiling file formattranslation products!

Image file format leadersTable 1 summarizes the results of an

informal "format popularity" poll which Iconducted on Usenet in early 1993. It'sprobably not surprising that TIFF(Tagged lrnage File Format) came out topof this list. It was developed in 1985 to ser-vice the needs of scanning and desktop

FORMAT

The Image eoMess: What~sYour TIFF?

Table I: CAP Survey

USAGETIFFGIFVIFFPBMfamilyHomebrewTargaSunRasterSGIEPSFITSPCX

Unnatural selectionTools used for the manip-

ulation of images at the vari-ous stages of the image lifecycle are frequently madeby differing companies. Forexample, a complete systemused by an imaging practitioner may com-prise an image acquisition system, com-puter, image processing package, publish-ing package and printer-all made by dif-ferent vendors.

In addition, there may often be therequirement to exchange imagery withother systems. For each of these prod-ucts, the manufacturer concerned has hadto decide which subset of all the formatsavailable will be supported. Given theinevitable differences that will arise fromsuch free choice, it is hardly surprising tofind that the unfortunate implementer ofsuch a system is left to juggle with thecombinations of supported formats, in thehope of finding one in common to all thecomponents that will provide all the func-tionality required. A few moments consid-eration of this problem will lead you tounderstand bow some companies make a

[Percentage of respondents who reported use of agiven fonnat; many chose multiples.I

48%33%30%27%24%21%18%15%9%6%6%

publishing companies. Its aim was to helpprevent the introduction of competing pro-prietary standards by offering a fairly uni-versal level of functionality which, in turn,has made it relatively complex. (It's alsoworth noting that at last count there are atleast 130 flavor variations on what mightbe called a "TIFF file", reason in itself forsome of today's confusion.)

GIF (Graphics Interchange Format) wasdeveloped by Compuserve, an on-line infor-mation service, to give its users a hardwareindependent way of exchanging color imagefiles. VlFF (Visualization/Image File For-mat) is the image file format for the Khorosimage processing package, popular in scien-tific imaging circles. In addition to support-ing Khoros, it was intended to aid theexchange of imagery between institutions.PBM (portable Bit-Map) is a family of for-mats which support the PBM plus file for-

ADVANCED IMAGING

mat interchange package. The format wasdeveloped originally to allow bitmaps to besent by mailers unable to handle pure bina-ry. Homebrew refers to formats specificallydeveloped for in-house applications. Targa(or TGA), SunRaster and SGI were, obvi-ous;y enough, introduced by TruevisionInc., Sun and Silicon Graphics, respectively,for use on their range of workstations andvideo graphics products. EPS (EncapsulatedPostScript) originates from Adobe and dif-fers radically from conventional formats inthat it is really a page description languagerather than a bit map or array. FITS (flexi-ble Image Transport System) image filestandard was developed to service the spe-cialist needs of the astronomical communi-ty. ZSoft Corporation's PCX (pC graphiX)and the variant PCC (pC Clip art) are promi-nent in ruM PC applications.

A universal formatWhile in theory it should be possible to

define a universal format which could beused by all imaging disciplines, in prac-

tice, providing the full gamut of featuresrequired by every application would leadto such a standard being highly complex.This in itself might be a deterrent to manydevelopers who would have to weigh sim-plicity and performance considerationscarefully against the benefits of easyimagery interchange.

Considering the need to create a com-mon standard, TIFF (Tagged Image FileFormat) has proved itself to be more uni-versal than most formats. It has massiveflexibility built into it in the form oflabelled fields (i.e., tagged fields) that canbe added as required to create a very flexi-ble format. To manage complexity, TIFFuses a notion of classes which split thefields into groups, forming minimum setsfor certain applications. This allows TIFFwriters to be fairly simple although read-ers remain relatively complex.

For some time, there have been calls todevelop a single, vendor independent, uni-versal standard. As this magazine hasreported over the many years of its develop-

ment, such an international standard is cur-rently being developed under the auspicesof the ISO and is known as the IPI (ImageProcessing and Interchange) standardOSO-12087). It started life in 1990, after anISO letter ballot, and is slightly unusual inthat it addresses both image transfer andprocessing. The standard is organized intothree parts: generic architecture, program-mers' imaging kernel system (PIKS) andthe image interchange format (IIF).

Like TIFF, it uses the concept of sub-classes (called Profiles) to limit the com-plexity for a given application. IPI is still atleast 2 years away from being validated asa full standard. Finally, for those interest-ed in general purpose solutions, theNational Center for Super-ComputingApplications (NCSA) at the University oflllinois at Urbana-Champaign has devel-oped a system HDF (Hierarchical DataFormat) which can be used for imagesand other data. If the number of compet-ing image file formats is to be reduced,universal formats will need to be capable

of widespread application while keepingthe associated programming complexitylow enough to ensure easy usability.

Life in the jungleIn the absence of a international stan-

dard, one is left having to select whichsub group of all the available formats agiven application should support. Thisrequires a comparative evaluation ofimage file formats, which is a dauntingtask. The main difficulties arise from thelarge diversity and quantity of image fileformats in use. This, coupled with lack ofany standard way of describing formats,leaves the unfortunate inquirer with anunenviable job.

The principal objective of a goodimage file format is to provide a structurewhich maximizes the utility of image dataacross a set of applications. Given thediverse nature of applications it is hardlysurprising that there is such a profusionand variety of image formats in theworld. Since a digital image is essentially

sense for a truly innovative, cutting edgeapplication developer or hardware manu-facturer to devote their resources toimage format support.

Successful format conversion alsotakes experience. 11lC' technical difficultyinvolved in reading and writing formatsoccurs because there is no structural rea-son to include any information in theimage header beyond what the originalprogrammcr(s) fell was necessary for thatsystem. (Formats arc generally created aspart of an system rather than as a stand-alone entity.) Often critical informationsuch as whether the image is read top-down or bottom-up is left undocumented.

Conversion utilitiesConverting your image from its current

format directly to the format you need isthe fastest and safest way to convert animage. Sometimes vendors collaborate tomake sure that image conversion to theirspecific formats is successful. When theydon't, your other option is to purchase animage conversion utility.

A conversion utility should support acomprehensive list of formats from asmany different plat forms and operatingsystems as you are likely to encounter.(For example, a customer of ours atHandmade Software receives images gen-erated un Macintoshcs, PCs and UNIXworkstations originating from many dif-ferent applications-induding the ancientHarvard Graphics version 1.0. He thenprepares slideshows by converting thesefiles to Scodl for Agfa's Slide Recorder.)

Even conversion utilities arc not neces-sarily committed to thoroughly support-ing each variation of difficult formats likeTIFF. Be sure to compare apples toapples: some conversion utilities imply

MARCH 1994

Table 2: Examl.le of some CAP MetI·ies

FORMAT Popularity Complexity TaxonomyIS012087 5 HEHDF 5 HETIFF 1 5 HEPhoto CD 4 HETarga 6 _4 -~HIGIE 2 3 =-..::: • .: FI

r '

PBMfamily 4 2 JISunkaster 7 3 '""FI

just a two dimensional array, such anabundance of file types indicates thereare many differing needs or opinions asto the best way of structuring image data.Thus, the selection of an appropriate

comprehensive support of format "varia-tions" by simply listing a number of massmarket applications which they support;of course each application doesn't neces-sarily have a different variation. Also. ifyou use public domain software, theseconversion routines are often written tosolve a specific conversion proble-m andprobably will not offer complete formatsupport. They may be worth a try if youhave the lillie: the price is right. For retailproducts. check for guarantees. and lookfor technical support reputations.A conversion utility should allow the

user control over colors pace (e.g., I~C;P,.

CMYK), and a variety of dithering choir-es optimized for specific situations. Ithe-lps if the manual teaches you what thepar arnctcr s mean. Controlling theseoptions may including the ability to gen-erate a palette, or to optimize a palettefor a group of images when creating acatalog or manual which uses imagesfrom various sources. (This kind of con-trol is critical for our high-end servicebureau customers, but it's also necessaryfor our engineering customers who plotcolor images on the Encad Novajct orthe HP 650c, shown ill PIIO/o.)

Final considerationsThe last technical pier« of the puzzle is

to make sure the the scaling algorithmsare first-rate when you resize an image.Enlarging an image requires literallyspreading the image apart, and reducingit means omitting information. and thesoftware has to deduce what happens tothe image. This procedure is mathemati-cally complicated. and poor scaling leadsto pixelation (aliasing). The simplest typeof scaling is Nearest Neighbor scaling,but most people will find the results unac-

image file format can be a most difficult task.

Which image format for you?So, the question of how to select a for-

mat for a particular application arises. In

ceptable and need at least linear interpola-tion scaling algorithms. You may need todemo the software to COIl vince yourself ofthe quality of the scaling abilities. espe-cially if your reputation rests on the quali-ty of your images.

There are a few other potentiallyimportant features: the ability to com-press images (and again to control till'compression If it is lossy, iike jPEG) , theability to view the images in the conver-sion utility, and the ability to do balchprocessing. These features may be criti-cal 10 your operations, they may becomeimportant as your needs change, or theymay be irrelevant. Except for compres-sion, they do not impact the technicalaspect of converting the images, but theymayor may not be included in an imageconversion utility.

At Handmade Software. Inc. (Los CatesCAl, we've created the software packagesImage Alchemy. Image Alchemy PS, andImage Alchemy Prcpress to run on PCand compatibles and UNfX workstationsand efficiently convert some G7 image for-mats. including TIFF, Targa, Sun Raster,Silicon Graphics. MaePICT. Windows13M!" PCX, XWD, IFF/ILBM, AdobeAcrobat's PDF. Photo CD, and the printerand plotter formats HP RTt. Novajct, andCalcomp. Image Alchemy I'S is a Post-Script interpreter, and Image Alchemy"repress additionally supports Scitcx andIris formats. All of our tools weredesigned to deliver solutions on the keypoints I've described. •

Jessica Margolin Bailey is the Director of Mar-keting at Handmade Software, lnc For more.ntormation on Handmade Software, call(800) 358-3588 or +1 (510) ?52·0101. or callfax-on-demand at + 1 (510) 252 0303

47

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some cases, the format choice is dictatedby the use of a third party imaging pack-age (e.g., Khoros users will adopt VIFF).However, those writiog their own imagingsoftware will have a less constrainedchoice. For such developers, if portabilityis a prime issue, then a major factor mightbe the relative popularity offormats.

For others, reducing programmingeffort or increasing software perfor-mance might outweigh portability con-siderations. Popularity is sometimesobtained by making the format generalpurpose which can ill turn make it morecomplex, thereby increasing program-ming overheads or reducing perfor-mance. In these circumstances, a devel-oper might prefer to use his own inter-nal format (homebrew), which is welltailored to the application. The highposition of homebrew formats in popu-larity ratings is clear evidence of this. Itis apparent that in making the decisionon which format to adopt, informationon the popularity, complexity and func-tionality of formats would be useful.Whilst, in theory, this information couldbe extracted from file format specifica-tions, in practice the diverse origin ofthese formats means that they are fre-quently difficult to understand andmake any comparative analysis atedious affair.

Metrics to the rescueIn everyday life,high level specifications

are often found to be helpful when narrow-ing down choices prior to more detailedtechnical investigations (e.g., when buyinga car or stereo). Such high-level specifica-tions can equallybe used as an aid to imagefile format selection. Recently,a set ofhighlevel specificationsreferred to as CAP met-rics has been proposed which address theproblem of comparing the virtues of imagefile formats. CAP metrics describe suchcharacteristics as the popularityof a format,its relativecomplexityand potential flexibili-ty. For example, the followingtable lists themost popular formats (popularity rating),together with two other CAP metrics, com-plexityratings (referred to more formally asa trans/ormation index) and HI-FI tax-onomies.The complexity rating is a measure

loosely related to the number of transfor-mations a software reading routine mighthave to perform on a given format beforeit can be displayed (e.g., decompression).The taxonomy is based on the premisethat internal field hierarchy and labellinghave a fundamental effect on the overalllogical structure of a format This taxono-my categorizes image file format struc-tures as being one of four main types: HE(Hierarchical Structures with ExplicitLabels), HI (Hierarchical Structures withImplicit Labels), FE (Flat Structures withExplicit Labels) and FI (Flat Structureswith Implicit Labels). The use of field hier-

(continues on page 85)49

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CALLAGHAN(continued from page 49)

archy and labels increases the flexibility ofa format but places an overhead on a for-mat reader which must be able to dealwith these structures. Thus, as Table 2also illustrates, such structures increasethe complexity rating.

Popularity gives evidence of the poten-tial user base, whilst the taxonomy mightbe used to assess the flexibility of the for-mat. Complexity provides an indication ofthe relative programming difficulty. Suchinformation can be difficult to distil fromthe varied and frequently complex formatspecifications. Another factor to consideris bow well the features a format offers arematched to your application. A usefulgraphical representation known as e func-tionality profile is also included in CAPmetrics, and gives a simple pictorialimpression of the quantity and distribu-tion of features within a format. Inessence, a functionality profile is a his-togram describing the distribution of fea-tures within a format.

As with most decisions, the choice offormat will usually be a compromisebetween conflicting ideals. Thus, whenchoosing an image format, you have tobalance such aspects as programmingcomplexity, flexibility and popularity. TheCAP metrics represent a convenient

MARCH 1994

mechanism of summarizing some of themain factors involved and should serve asa good starting point to those searchingfor a suitable application format beforethey expend too much time wadingthrough rather lengthy and complexdescriptions of the standards themselves.

The futureWhether any single image file standard

can stem the profusion of formats andbring order to this turbulent area is highlydebatable. Clearly, devising a format tosatisfy all imaging requirements, whilekeeping its level of complexity lowenough to ensure widespread usability, isa most difficult task. Many argue that the"Achilles heel" of all universal formats iscomplexity which will result in therealways being considerable opportunitiesfor tailored formats.

There is a clear tendency in the newestformats to sacrifice simplicity forincreased flexibility. In many respects,this is inevitable; if a file format is toassume the mantle of a real standard, itneeds to be able to adapt to the unavoid-able technological advances that willoccur over its lifetime. Some would sug-gest there is a dilemma that only the mar-ket can solve; programmers want simplici-ty but successful standardization demandsflexibility and thus increased complexity.Which view will predominate? The answeris in your hands! •

SUM of the PARTS(continued from page 72)

Program" has proven itself very successfulin field trials. There is a significant amountof enthusiasm that it will prove successful ona much broader basis-and the recent expo-sure in Washington is expected to provide amost beneficial boost to the companies'commercialization plans. As this columnregularly reminds you, a well-designed, well-marketed cooperative effort can be moreefficient than the "sum of the parts."

The futureCAEI has been most successful in a short

period of time bringing together privateindustry (both manufacturers and users),technology providers, academic institutions,and governmental entities for the commoncause of promoting electronic imaging.

CAEI has also been successful in facili-tating and encouraging the cooperativeefforts between member companies suchas Advanced Graphics and Creative Edu-cation Institute to the mutual benefit ofboth entities.If enthusiasm by participating entities is

any measure of future success, the leadersof the Center for Advanced Electronic Imag-ing (CAEI) have developed a model for aninfrastructure to effectively bridge the gapbetween technology providers and the ulti-mate user to the benefit of all. •

85

ADVANCED IMAGINGImage File Format Survey, March 1994We want to know how image file format issues are impacting your work-

so we can report it to all of you in an upcoming issue. Make a copy of this form,completed, and fax it or mail it to the addresses below by May 2, 1994.

1) Which image formats do you use? Circle all letters that apply-AND CIRCLE NAME of your main format.

a.BMP g. GIF m.PBM s.TARGAb. ccnT h.HDF n. PCX/PCC t.TlFFc. EPS i. HIPS o. Photo CD u. UTAHd.FBM j. Homebrew (In-house) p. PICT v. VIFFe. FITS kIFF q. SGI w.XBMf. GEM I. IPI (ISO-12087) r. Sun Raster Note others:

2) Have you personally experienced problems caused by the large number of formats?

b- SOMETIMESa-OFTEN c-NO

3) Do you believe an international standard like IPI/ISO 12087 will be widely adopted by vendors and developers?

a- YES b- NO

4) Which image file features do you consider most important in deciding to support or use one?(Circle 1 to 3 choices)

d. wide commercial acceptancee. acceptance in your disciplinef. set world standard

g. well-documented specificationh. no charge for specificationi. improves processing efficiency

a. simplicityb. universal functionalityc. tailored for your discipline

5) Which forms of imaging does your work entail. (Circle all that apply):

a. Document imagingb. Electronic still imaging for arts/commercial

c.Electronic still imaging for scientific/industriald. Digital video imaging

OPTIONAL:

Name: ___

JobTitle: ~~ ~~ ~ _

Firm or Institution: _

FAX BACK TO: Advanced Imaging, at U.S.# (516) 845-2797.Or Mail to: Advanced Imaging File Format Survey, 445 Broad Hollow Rd., Melville NY 11747.

50 ADVANCED IMAGING