Making Image Mosaics Infinitely Zoomableto Explore Large Image Collections

Romain Vuillemot Beatrice RumplerUniversite de Lyon

LIRISF-69621, France

romain.vuillemot@insa-lyon.fr

ABSTRACTImage Mosaics are images formed from series of smallerimages. They are inspiring, intuitive and trigger curios-ity. But remain still images. In this paper we introducea pure content-based interaction model to make theminteractive and infinitely zoomable. Our goal is to pro-vide users a smooth and multi-scale information orga-nization to better visually explore large non-structuredimage collections. Experiments on a non-familiar datasetindicates it is a smooth an efficient narrative environ-ment. Guidelines and implementation notes are givento extend to any mosaic interface, such as non-squaredmosaics and collages.

ACM Classification KeywordsH5.2 Information interfaces and presentation: User In-terfaces. - Graphical user interfaces.

General TermsImage Mosaic, Vizod, ZUI

Author KeywordsImage Mosaic, Zoomable User Interface, Visualizationon-demand

INTRODUCTIONInformal observations of people walking down the streetshowed that when passing by an image mosaic, peopletend to stop and pay attention to it. Catching thisattention span is priceless in our information society,where relentless streams of information are unwillinglydumped into our brain, through our eyes. Image mo-saics are part pieces of art, part technique, and havetraveled through the ages, and regained of interest dur-ing last decade with the rise of the digital era. Evenif this spatial organization of images is sophisticated,everybody knows how to get information out of it, re-gardless age, culture or education. Source images canbe fetched in a 2D way as tiles, clustered by color orshape, according to the big or target image that is vis-ible by stepping backward.

Meanwhile, it has never been so easy to capture imagesand make them available to a global community of peo-ple. Images tend to gain better quality and higher size

since displays and captors follow a restless trend in be-ing better and cheaper. But studies [18, 20] show thatpeople don’t invest much time classifying their photos,relying on automatic metadata such as photo transfertime or cameras timestamps. Internet and social web-sites now offer open repositories of images, that area goldmine for casual image lovers, but a frustratingexperience for people in need of a particular informa-tion. Even with an efficient information retrieval sys-tem, browsing large amount of results is not there yet.For instance, seeking for a landscape picture, users willstart by a broad term and then rely on browsing to findthe right image. But a query using that keyword isresulting in Flickr by:

2,341,040 results matching landscape.

How is it possible for a human being to explore such alarge amount of data in a limited time span? Becausescreens size remain limited, a better use of human visualbandwidth has to be found. While users are eager ofimage tiles [18, 6], our conviction is one step beyond,that image overload is a key to better use human visualability, and can be harnessed by a strong control of theinterface.

Filling screen space with images in a meaningful andunderstandable way, is the underlying challenge of anyimage manipulation interface. Our focus is on the worstcase scenario where images are not familiar and haveabsolutely no information that may index them, butby content. Spatial arrangement has been experimen-tally proven efficient [17], so are humans spatial mem-ory abilities [5]. But when it comes to deal with photosbecoming too small to recognize, reductions by repre-sentative photo [15] or density plot [13], the system maymiss image features. Collage are nice image display [10]but cluttering appears and remains a major issue. Andcluttering is not only about showing images with nooverlapping, but also not to lead to a degradation ofperformance at some task [19].

In this paper our hypothesis is that image mosaics arean intuitive and efficient spatial organization of images,that can pack a vast amount of images with minimaldegradation. Images are indexed based on a match-ing between their own content (shape, feature, color, ..)

Figure 1. We introduce MosaiZ, infinitely zoomable image mosaics. Starting from (a), the user pans mosaics to lookfor a target image that is a half dark, half clear. But no satisfactory image is found, so the user decides to make avisual query by zooming on a part of the current image that his fulfilling his needs. Border of the bridge with the skyseems to be a good candidate query, so the user is zooming on it. The image is becoming a mosaic itself (b). Andzooming again (c) is showing more results, and result images resolution gain in better quality (d) (user is hovering it,mouse is not drawn in the screen capture).

and the target image content: they can easily be visu-ally acquired by looking at the target image. Here arecharacteristics of image mosaics:

• Space filling organization with an optimal use ofscreen space. Every pixel holds informations on bothsource and target image.

• No clutter or image overlapping. Even with manyimages, they remain strictly adjacent to each other.

• Both overview and details at a glance. Image mo-saic can be considered as a non-photo realistic render-ing process of the target image by the source images.

Reificating real world metaphor is a common and ad-vised practice in UI design [4]. While Informative Art[16] is using artistic visualization to dynamically dis-play updated information, image mosaics can be anadvanced content browsing system. Even if throughthe years mosaics have been studied and improved us-ing various angles (shape optimization, algorithm opti-mization, ..), as far as we know they have never beenstudied as a browsing metaphor. And since the digitalworld is not limited by space and distance, it is pos-sible to go one step beyond reality and make mosaicsinfinitely zoomable. Using a Zoomable User Interface(ZUI) is straightforward and has already been success-fully used with image management [2]. Zooming intomosaics is based on the property that images are madeof images themselves, becoming an infinite two dimen-sion plane. Semantic zoom [14] is automatic : whenthe source image miniature becomes too small, the tar-get image appears. Here are how we are tackling majordesign challenges:

• Consistent and predictable layout due the nest-edness property of the zoomable mosaics. The tran-sitional step (zoom), from source to target (or theopposite) is a global linear geometric scaling -and nohyperbolic deformation such as Fisheye. Magnifica-

tion is the same for every images. Humans know wellto deal with and anticipate such deformation.

• Scalability. A target image is summarizing the sourcedataset. In the scenario where source images are onepixel by one pixel, nearly 2 million images can bepacked in a 1600 × 1200 pixel image. Also mosaicscan be easily exploded into manageable chunks of im-ages, allowing progressive streaming over a network.

• Users know how the system works. Because im-age mosaics are based on visual perceptions proper-ties, there are no cultural or context based knowledge(such as classification or clustering).

MOSAIZOur contribution is called MosaiZ, a pure content-basedinteraction model to make current image mosaics inter-active and infinitely zoomable (See Figure 1). A MosaiZis a non-linear interaction space, where users can selecta source image that will become the target image, andso on. Maximizing information display is the priority,to display as much information as possible. The designhas to be pure content-based with no menu or buttons.Interactions aim at helping users to visually formulatehis query (image he is looking for) using target imagefeatures. This way users express their needs in an un-derstandable way for the machine, with no sign or code.Smooth transitions (continuous zooming) helps keepingcontext. We describe guidelines to create a MosaiZ andthen the interaction design.

Creating a MosaiZThe backbone of a MosaiZ is a scene graph (similar asa space scale diagram) connecting image mosaics alto-gether. Every nodes of the graph is a mosaic, and willbe connected to all the sources that are composing thatmosaic. Here are the steps to follow:

1. Select a dataset and a first image as scene graph root

2. Select one grid type, and one or many sizes

3. For each grid size:

(a) Generate mosaic

(b) Assign original image to source images

4. Check connectivity of the MosaiZ graph

Interaction DesignWe combined interaction techniques in a meaningfuland consistent way to help users to make the best de-cision at every level in the graph. Animation helps tobetter understand relationships by maintaining contextawareness in a single view using short term memory:there are no attention split or gap between views. Theuser interaction workflow is described on Figure 1. Hereare some specific design details regarding mouse andkeyboard:

MouseWe implemented a classic panning (translation) andzooming (uniform scale changes) navigation. Scroll upis to zoom in a location, and scroll down to go back-ward in the multi-scale space. When the mouse exitsthe application window, original image is displayed toease the feature seeking process (Figure 1 (a)). Whenthe mouse enters, the mosaic is on but with a 50% alphalevel couple with the original image (this is a commontrick used in image mosaics) that helps to better per-form visual query (Figure 1 (b)). This cheat feature(so called in the metapixel library we will be using inthe implementation) may be disturbing and biais userdecision. So when mouse is hovering the target image,the original source image is displayed (Figure 1 (d) butwithout vicinity) and its vicinity as well in a transitionalway (Figure 2).

Figure 2. Mouse pointer is an extra layer that helps usersto get better source images resolution. Multiple shapesare available with variations in size and transparencylevel (the darker the square is, the better the qualityof image is).

KeyboardArrow keys are to pan around, but zooming is difficultbecause target acquisition is not easy since the systemhas no information about users visual focus. Our so-lution is by using the numeric pad, users progressivelyselect areas of the screen (Figure 3). We introduce inthe next section automatic touring that is an automaticstraight zooming, and that can change direction accord-ing to the key pressed, such as a to pilot a plane: itreduces users interaction, but requires users attention.

Figure 3. Image target acquisition through the keyboardis performed by pressing numeric pad keys (7, 9, 1 and3). Keys are tied to one quarter of the screen. Everytime a key is pressed, the region available is a quarter ofthe previous one.

FeaturesWe now describe four major features to complete ba-sic interactions we described above. Other features aregiven in the users experimentation section.

HistoryAny system must give users the opportunity to makemistakes and go backward. Users can right click to goto the previous state (previous scene graph node). Butalso visual clues such as red squared images or makingtarget image reddish shows already visited ones. His-tory or statistics can also be exported such as list orgraph to be browsed in another interactive environment.

Mosaic GridThe grid is the mosaic pattern making the basic sup-port to construct mosaics. Dynamically changing themosaic grid size is a powerful feature since it will al-low to change the query bounds. Grid resizing is doneby using mouse scroll and pressing control key at thesame time. Grid sizes have to be set during the imagemosaics generation step.

Rendez-Vous PointEvery exploratory tasks involve non predictable, con-fuse and irrational behaviors. Sometimes users justwant to find by serendipity or to have fun with thetool, rather than following beaten tracks. For that pur-pose, we allow users to set a Rendez-Vous Point : userspress a key at some point, explore, and by releasingthe key will go back to the Rendez-Vous. No history isrecorded. The way back to the Rendez-Vous is a fast au-tomatic animation to quickly remember what has beendone during that recreation time. Letting users press-ing the key is important to remind them that this isjust a short term image exploration process.

I am Feeling LuckySystems -and especially new ones- should not let usersstuck in a situation where they can’t do anything ordon’t know what the next step is. In a MosaiZ, spatialfreedom and zoomable process require lots of user visu-alization and concentration. Situations such as desertfog [9] -being lost in the multi-scale space with notenough information to take a decision- have to be tack-led. We introduced a system-based strategy to pre-vent it. By double clicking (instead of single click),

the system will look for the next image, regarding auser-selected heuristic that may be browsing new im-ages only, performing the same query as the previousbut on a new image, browsing by chronological order,image size, social popularity.. This automatic touringfeature will free users from interactions. It can also beseen as a shortcut to repetitive actions users perform.

We also introduce a semi-automatic approach, auto-matic straightforward zooming, users pressing arrowkeys or moving mouse to set the direction. The re-sult is a time dependent interface such as Dasher [23]which is using letters flow to compose phrases. In thatcase user’s attention is required and is parametrable:ttouring = s × (t − tstart) with time-lapse visualization(s < 1), real-time visualization (s = 1) or slow motionvisualization (s > 1).

IMPLEMENTATION NOTESWe implemented MosaiZ based on interaction designand features we described in a twofold approach (Fig-ure 4):

1. A local Java application using Picollo2D (opensource project based on [3]) which is a direct-manipulationgraphics library that supports constructing ZUI. Theapplication is a basic frame containing a layer onwhich three child nodes (mosaics) are tied (previous,current and next mosaic). Pointer layer is added andtied to mouse cursor, to display child images accord-ing to the pointer location, size and shape. A set ofevent listeners trigger node rotation when users getclose enough to an image (scale > 1) or click on it. Weused mipmaps (pre-calculated, optimized collectionsof images) as a cache to quickly provide images intothe application, and to prevent aliasing effects. Mo-saiZ implementation code contains a dozen of classesand a total of 1200 lines of code and 2 shell scripts. Amodular implementation makes it easy to add touringnew heuristics or other mosaic grids.

2. A remote server is generating the scene graph, basedon image descriptors uploaded the by client applica-tion. Server is keeping track of users activity, basedon [22] and is reached as a mosaic generation webservice. Since image mosaic generation is not fullypredictable, we wanted to avoid bottlenecks in userexperience, and generated the graph scene once forall in a offline mode. We used Metapixel to generatethe mosaics with different grid sizes. Activity logs(indicators such as path in the MosaiZ, frame rate,session times, features usage ..) are uploaded on theserver for analysis after every sessions, in order notto bias the performances.

EXPERIMENTATIONThe research question we want to answer is how MosaiZare useful in the context of image collection browsing?We performed a qualitative user study with both a dig-ital artist and knowledgeable users.

Figure 5. Control panel allows users to customize inter-actions with the MosaiZ.

Regarding the technical challenge that implementingMosaiZ is, we started using a relatively small datasetto circumnavigate technical limits (memory and mo-saics generation). We are using as dataset a selectionof 250 photos from New York City with heterogeneoussubjects (buildings, animal, people, textures, indoor,outdoor, flash, no flash, landscape, ..). Pictures are ho-mogeneous in resolution and compression, taken by thevery same person (which is none of the testers) with aSLR camera. Pictures were rotated if needed, resizedand cropped to 400 pixel by 400 pixels.

EquipmentA commonly available computer system was used, 3GhzIntel PCs with 2Go RAM and a 21” monitor, runningWindows XP. Screen resolution was of 1024 × 768. In-teractions were performed using a 3-button mouse withscroll wheel, and a basic keyboard. All users and resultsmentioned below are based on this environment.

Experiment #1 with a Digital ArtistA professional male digital artist, with a 3-year expe-rience background in design of websites and brochures,has been contacted and volunteered to give informalfeedbacks using the prototype. A time expensive andboring part of his daily routing is finding adequate pic-tures from professional image repositories. Image arehigh quality and copyright issues are a major concern.Seeking focus is not on a global scene, rather on partsof the image, or texture that will be used in a montageor as a backgrounds. We briefly introduced him theinterface and he instinctively started using it, for a 10minutes session. Having a teaching and collaborativework experience it was easy for him to tell his thoughtsat the same time while being focus on the application.

ResultsHis major satisfaction was not starting from scratch andhaving a basic ground of exploration, by starting with

Figure 4. Twofold MosaiZ implementation architecture. Local interactive client remains on users side (left). Aconnexion to a remote server (right) allows (strates from top to bottom): session opening, features upload, scenegraph download and finally users interactions. Logs are uploaded when session ended, and users access to theirbrowsing history using an ordinary web browser.

one image. Not having that blank sheet fear, was a cre-ative first step to him. He suggested taking himself apicture or drawing as a good start. As a game lover,he enjoyed the gameplay and extensively used scrollingand panning. Features such as history and feeling luckyhaven’t been used at all, because he was mostly relyingon his good visual and spatial memory he developedover his experience. He liked the rendez-vous, which isa common feature in Photoshop where drawing layerscan be removed or aggregated, allowing history visual-ization and transformation. The only customization hemade was replacing clicking from target acquisition, toprogressive zooming, which gave him more control overthe interface. He thought it was cool but needed betterquality, larger and more familiar dataset to really com-pare to his needs. But he would use it as a presentationmedium because of its innovative design and continuousprocess.

Experiment #2 with Knowledgeable UsersFour Computer Science PhD candidate volunteer tookpart in a second experiments. They have respondedto requirements of using image manipulation softwareat least once a week and with large image collections.Participant were asked if they were familiar with thedataset, one claimed he knew the city. Some statisticshave also been recorded and are discussed in the discus-sion section. An interface demonstration has been given

to the participants, they were observed during the exe-cution, and asked to think loudly during a 10 minutessession.

ResultsThe first impression of all participants was that it wascool and always wanted to do it.

The participant who claimed to be familiar with NYCwas the one who developed the most observation, tellinghimself stories based on memories. His exploration pro-cess was based on the semantic of the image. Whileother participants rushed to try the usability of the in-terface, and focused on low level features (shapes, color,..). One participant particularly interested in skyscrap-ers found the interface very useful: by clicking on hori-zons or building borders with the sky, another buildingpictures appeared every time.

Another candidate used the Rendez-Vous point as aquick magnification tool by being quickly activating anddeactivating it. This was also used as a zooming out,which is similar to the Expose feature in MacOS X.

Participants found the export feature useful because ithelped them to remember and get details on the imagethey were looking for. They wanted to annotate andrecord Rendez-Vous Point, and a replay feature that

would display text or voice annotation based on theirMosaiZ path.

But even with history visualization, participants com-plained that it was hard to be sure to complete theirown personal challenge of complete browsing. All par-ticipants extensively used the feeling lucky button be-cause they did not want to make decision and also be-cause it was hard to visually explore and make up theirmind. Participants comments were mainly about thelimit of query by existing content. But still were sat-isfied to have a continuous flow of information and notwaiting, always having the feeling to be in control. Aparticipant compared the interface as the iPod wherethe wheel makes the navigation enjoyable.

Participants used the control panel (Figure 5) to cus-tomize the interface. Displaying grid was one of thefavorite feature, that is a one-pixel black border aroundimages for delineation. Indeed images mapping blue skyare not easy to separate. Changing grid was a deceptionsince bigger sizes did not make recognizable mosaics.Still with the 10 ×10 pixel grid, participants liked hav-ing similar images in the same vicinity.

Some interesting behavior appeared, such a participantwanting to close and restart a session with no reason.Even if the space is infinite, some closure process orfresh started were needed.

DISCUSSIONSIn our two experiments, all five participants definitelyenjoyed the experience and the learning time was veryshort. Participants reaction was also based on theirown sensibility. Apart the digital artist, participantsfelt comfortable by reducing there degrees of freedom,being sure they were not missing something. Partici-pants seems to be better at examining lists rather than2D displays, but MosaiZ offer contextual awareness thatprovides more information overload.

Figure 6. Peripheral target visualized in a 10 minutessession. 2-peripheral image means we consider images 2grid units away from focus. In that case, since contentis of 250 images, 220 images (88%) have been browsed.

On Figure 6 we display the image browsed during one

participant session. Not all the images seem to havebeen browsed. One major reason is the distribution ofthe images in the mosaics: some images are good candi-dates and appear multiple times in mosaics. We checkedthe connectivity of the scene graph and some graphanalysis may also explain participants behavior. A so-lution would be to constraint the mosaic library, butthis is a trade-offs between time, aestheticity and use-fulness. Connectivity is also better difficult with smallergrids, but it already hold most of the pre-computationresources (Figure 7).

Figure 7. 250 images mosaics are generated according to6 different grid sizes. The smallest one, the 10-pixel sizegrid mosaics are the one requiring most the the compu-tation time.

Performances were a major concern in our approach.While interacting with a visual display, the frame rateis a good indicator about the smoothness of an applica-tion. We noticed below 15 frames per second a move-ment doesn’t seem to be smooth anymore. Results (Fig-ure 8) show some drops in the rate that are tied to auser zooming action to a new mosaic. There is a designenhancement to make in the transitional state (See Fig-ure 1) from (d) to (a). In (d) there are only two imagesbeing rendered whereas in (a) many other (surroundingthe target) and in bigger size and quality. But partic-ipant never complained about it. Offline performancesconcern image mosaic generation: it took 2h32mins togenerate all the 6 × 250 mosaics.

Globally with know extra device we learned on users eyegaze activity and feedback. The task to perform waslimited, dataset was not significant but users appreci-ated the browser-like design of the MosaiZ interactionmodel.

FUTURE WORKOur next iteration is to make the program technicallyscalable up to million of images. The bottleneck willbe mosaic generation, but can be outsourced and com-puted in parallel on a grid or a cloud, because they areindependent processes. Next research tracks are mainlybridging links with other communities such as imageanalysis, computer graphics and social networks:

Figure 8. Frame rate during a 60 second session sample.Every drop to 11s, 12s, indicate zooming interaction. 100is the maximal frame rate and is reach for all the otheractivities.

ContextAccording to participants, MosaiZ would to be goodwhen dealing with outputs from a previous system, suchas the results of a Information Retrieval system. Themajor limit is the very long offline mode that is requiredand would not make MosaiZ a real time exploratorytool. It will be efficient with large results dataset butwould not work below a number of results. Indeed aminimal quantity and quality of images are required tomake good mosaics.

Collaboration and social experienceA MosaiZ instance (i.e. path in a MosaiZ) can be sharedexperience, and used as photo presentation. Throughhistory it will be possible to adjust path, by un checkingunwilled photos. Using other modalities is an interest-ing path of study, such as vocal annotation, music. Ex-periences gathered through massive MosaiZ usage mayreinforce the feeling lucky heuristic. This social feed-back may also be integrated into social image mosaics(e.g. a color code is showing the most frequently clickedtargets and result in heat map). Above storing andretrieving tasks, communication and presentations arealso important features when it comes to deal with im-age collections. Storytelling [1] is a feature expected byimage authors. As recommended by our digital artist,a presentation tool can be made out of it, with content-based transitions. Using a zooming approach has beenalready been successful with CounterPoint [8].

Other grids and pointing shapesThe grid changing feature is interesting, but so manymosaics drawings and image segmentation algorithmsare available and can help to do better than squares.Plenty of contribution are available, from SIGGRAPHpapers to student projects. Numerous mosaic typeshave already been object of study [7, 21] during lastdecade. Anti-Mosaics are classic mosaics but imagesare the parts of a single image. Recently introducedJigsaw Image Mosaic [11] are based on a dataset thathas shape similarities and a shape homogeneity. Some

software are available and are free such as KMosaic is-sued from [12] and MetaPixel are fully usable. Collageare mosaics with pieces having different shapes, thatcan be rotated, overlapping, such as we took a scis-sor and cut magazine to make a new pictures. FinallyQuadTree based on image segmentation are well knownand implemented technique that can be integrated as agrid. And pointer shapes can segment areas, instead ofremaining constant shape, that will assist users provid-ing automatic target similarities detection.

CONCLUSIONWe described MosaiZ, an innovative interface to exploreimage collection by making image mosaics interactiveand infinitely zoomable. Even if packing a lot of imagesinto a delimited area is a challenge, we provided a scal-able and smooth interface in which can be done visualqueries. A wide range of optimizations and trade off areavailable, due to the wide design space. Our implemen-tation and experiments were limited but comforted us inour initial hypothesis that image mosaics are a promis-ing use of human visual bandwidth. Further work hasto be done to make it fully usable and solving real tasks.Another exciting challenge is to federate communities towork together to extend the generic interaction modelto other mosaic creation and image segmentation tech-niques.

ACKNOWLEDGMENTSWe thank all the testers, and developers of the Piccoloand metapixel libraries.

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