Multiblending: displaying overlapping windows simultaneously without the drawbacks of alpha blending

Transcription

1 Multilending: displying overlpping windows simultneously without the drwcks of lph lending Ptrick Budisch Microsoft Reserch One Microsoft Wy, Redmond, WA 98052, USA ABSTRACT Alph lending llows the simultneous disply of overlpping windows such s plette windows in visul workspces. Although lph lending hs een used in some pplictions, such s gmes, it hs not een widely dopted. One reson for the limited cceptnce is tht in mny scenrios, lph lending compromises the redility of content. We introduce new lending mechnism clled multilending tht uses vector of lending weights, one for ech clss of fetures, rther thn single trnsprency vlue. Multilending cn in most cses e utomticlly optimized to preserve the most relevnt fetures of oth the plette nd the ckground window. We present the results of user study in which multilended plettes provided higher recognizility of oth the ckground nd the plette thn the est prticipting version of lph lending. Ctegories & Suject Descriptors: H5.2 [Informtion interfces nd presenttion]: User Interfces. - Grphicl user interfces. Generl Terms: Humn Fctors, Design. Keywords: Alph lending, semitrnsprency, windows.lutwurst INTRODUCTION Overlpping windows nd 2½-D interfces were developed to let pplictions use limited screen spce multiple times. However, since overlpping windows occlude ech other, users hve to switch ck nd forth etween windows in order to ccess the different tools nd informtion. This switching ecomes especilly cumersome when the overlpping windows elong to the sme ppliction. Mny visul pplictions mke tools nd controls ville in permnently visile interfce components, such s tool plettes. Plettes cn e positioned on top of the workspce to llow tools to e closer to the re where the work is eing done. However, this cuses the plettes to occlude n re of the underlying dt in the workspce (e.g. Figure 1). Plettes re trdeoff of 2½D interfces, etween the vilility of ojects in the foreground nd ckground lyers. The more ville (i.e. lrger or closer) the tools in plette, the more difficult it is to see nd use the visul dt in the workspce elow. In the imge processing ppliction of Figure 1, tsks such s selecting lrger o- Permission to mke digitl or hrd copies of ll or prt of this work for personl or clssroom use is grnted without fee provided tht copies re not mde or distriuted for profit or commercil dvntge nd tht copies er this notice nd the full cittion on the first pge. To copy otherwise, or repulish, to post on servers or to redistriute to lists, requires prior specific permission nd/or fee. CHI 2004, April 24 29, 2004, Vienn, Austri. Copyright 2004 ACM /04/ $5.00. Crl Gutwin Computer Science, University of Ssktchewn 57 Cmpus Drive, Ssktoon, SK S7N 5A9, Cnd crl.gutwin@ussk.c Figure 1: () An imge processing ppliction with selection of frequently used tool plettes. () Alph lended plettes give etter ide of the periphery, ut color, rightness, nd contrst of the semioccluded res re still ffected. ject, checking whether retouched elements still mtch the overll lighting sitution, or getting n overview of the picture will require the user to hide plettes efore proceeding. In order to del with the mny plette windows tht come with mny professionl pplictions, such s CAD or softwre development environments, users often use dditionl screen spce such s second monitor [7]. The drwck of this solution is tht plettes re further wy, nd cquiring them thus tkes more time. Also, dding screen spce is not n option for users on moile computing devices. Semitrnsprent plettes hve een proposed nd implemented s solution to the prolem of occlusion (e.g. [9]). Semitrnsprency, using technique clled lph lending [15], llows two windows to e displyed on the sme piece of screen spce. Semitrnsprent plettes show the contents of oth windows, reducing the need for switching etween overlpping windows. However, semitrnsprency still seems fr from reching its potentil. While it hs een dopted into some gming pplictions such s EverQuest [8], nd is ville on window level in some operting systems (e.g., Linux nd McOS X), it hs not een implemented s prt of ny multi-window/multi-plette ppliction even though it seems to e n ovious nswer to n ongoing prolem. One reson for the limited cceptnce seems to e tht for mny scenrios, lph lending ffects the redility of window contents too much (Figure 1). In this pper, we rgue tht we cn mke window lending pplicle to wider rnge of pplictions y extending lph lending to selectively preserve imge fetures. The new technique, clled multilending, lends the individul color nd texture fetures of plette nd window seprtely, using rnge of imge processing techniques. This llows

2 us to ssign higher visiility to the fetures most relevnt to the user s tsk t hnd. Using multilending, we cn crete lended plettes tht etter preserve the visiility of oth the ckground nd foreground windows (Figure 2). Figure 2: This multilending style ( glss plettes ) llows higher fidelity in the ckground imge, nd still shows relevnt fetures of the plettes. (More close-ups: Figure 3, Figure 7, nd Figure 8). In this pper, we introduce multilending nd report on user studies showing tht multilending improves redility compred to lph lending. First, we review relted work on the prolem of window spce in 2½D interfces. RELATED WORK: OVERLAP IN 2½D INTERFACES In pplictions tht use plettes nd other floting windows, severl techniques hve een proposed to help mnge the trdeoff etween the visiility of plette nd ckground. Although some reserch hs een done in techniques for plcing plettes into res tht result in the lest mount of conflict [4], the mjority of work looks t vrious methods for diminishing plettes in situ. These techniques reduce either the plette s size or opcity. The techniques tht reduce plette size either shrink the entire plette (e.g., expnding trgets [11]), eliminte everything ut the plette s title r (e.g., window shdes [13], prticulr instnce of semntic zooming [14]), or hide the plette ltogether (e.g., Adoe Photoshop [2]). The techniques tht reduce opcity either remove some pixels entirely or ll pixels y certin mount. Overlys mke prt of the plette s surfce trnsprent. They hve een used for dding titles to video (i.e. chrom-keying), in hed-up displys to show flight dt (e.g. [16]), in gmes to show mps nd messges (e.g. Dilo II), for workspce tools such s mgic lenses [5], nd in workspces to show glol overviews [6]. Alph lending [15] lso known s semitrnsprency) lends plette windows with the ckground y computing weighted sum of pixel colors. This cn e done on per-pixel sis, s defined y the soclled lph mp. This llows content from oth plette nd ckground window to remin visile to some degree. Alph lending hs een used in numer of scenrios, including popup menus [9], tool glsses nd mgic lenses [5], nd multiple representtions of the sme imge [10]. Severl operting systems including Linux/KDE nd Mc OS 10 lso llow for semitrnsprent windows, nd dd-ons for window mngers re ville tht mnge the trnsprency of ppliction windows (e.g., [1]). Since ll diminishing techniques ffect the redility of the plette, they typiclly provide restored representtion in ddition to the diminished version tht is esier to red nd mnipulte. The individul techniques switch etween representtions either mnully or utomticlly. Mnul techniques include doule-clicking the diminished plette [13], selecting restore opertion from menu, or hitting keyord shortcut [2]. The utomtic mechnisms proposed so fr re sed on the proximity of the mouse cursor [8]. Moving the mouse cursor towrds the plette restores the plette either grdully or ruptly, s threshold distnce is pssed. All these techniques fully restore plettes when the mouse reches the plette; the oundries of the plette in motor spce therey remin unchnged, iding cquisition. Of ll the diminishing techniques, lph lending hs een pid the most ttention, nd severl results hve ppered tht consider the usility of semi-trnsprent tool windows. Hrrison nd collegues [9] found tht 50% trnsprent plettes cn gretly improve workspce visiility without degrding icon selection performnce. Other studies hve shown tht users cn perform trgeting tsks well with 25% or even 10% opcity, depending on the complexity of the workspce dt [8]. Unlike techniques tht reduce the size of the plette, lph lending hs the enefit tht ll plette content remins on the screen lthough in diminished form. Alph-lending thus potentilly llows users to monitor chnging plette content, such s disply of the cursor coordintes or wrning signs. The drwck of lph lending is tht it cuses the contents of plette nd ckground to interfere with ech other. This interference will often mke oth plette nd ckground hrd to red nd mkes it hrd to sy whether two fetures elong to the sme window. We will discuss these issues in more detil s we introduce our proposed lending technique in the following section. MULTIBLENDING To ddress the drwcks of lph lending, we hve developed new technique clled multilending tht llows the use of different lending functions for different visul fetures. Multilending is sed on the oservtion tht for prticulr tsk, users typiclly need only suset of the visul fetures in the involved windows or plettes. Multilending modifies the lending process to enle it to preserve more of these relevnt fetures if necessry t the expense of reducing other, less relevnt fetures. To explin this, we first descrie wht we men y fetures.

3 Feture selection is sed on humn vision Our current pproch to multilending is sed on four clsses of fetures: three color clsses nd one clss of texturl fetures. We focus on pprent fetures tht is, those tht re prticulrly esily perceived y humns. The three clsses of color feture used y multilending re the ones encoded y the CIE L color model [12]. CIE L is perception-oriented color model tht represents color s luminnce, red-green nd lue-yellow difference. We picked this model ecuse it lrgely mtches the color model the humn visul pprtus uses when sending color informtion to the rin. Multilending uses only single texturl feture clss the presence of high-frequency dt, such s edges or other res of high contrst in the imge. The humn visul pprtus hs specilized cells tht perceive contrst (e.g. receptive fields [16]). When nlyzing Figure 1 with respect to colors nd high frequencies, we see tht the screen is not s crowded s it seems. Most prts of the plettes re gryscle, i.e., contin no red/green nd lue/yellow informtion; the photogrph, on the other hnd, contins lrge res tht contin no visile edges. It is these unpopulted res tht we exploit in our pproch. The limits of lph lending Hving defined this set of fetures, we cn now etter express the limittions of lph lending. In terms of color, lph lending computes the output imge s weighted sum of the two source windows. This mkes the red, green, nd lue chnnel, s well s the chnnels of the CIE L representtion, into weighted sums ech weighted y the sme rtio (the lending prmeter lph of tht pixel). In other words, ll color chnnels get diluted y the respective contriution of the other window. In terms of texture, lph lending is suject to interference effects, since imge frequencies in the two imges will reinforce ech other in some cses, while they will cncel out in others, depending on how the two imges re ligned. If feture is sensitive to tht dilution or interference, it disppers. If tht feture would hve een relevnt to the user s tsk, lph lending ecomes unsuitle. A further drwck tht ffects ll fetures is tht lph lending introduces visul miguity. Looking t lended imge, it is hrd to sy which lyer specific feture elongs to (see, for exmple, Figure 6). Also, the reltionships etween fetures ecome oscured: it cn e difficult to determine whether two oserved fetures elong to the sme window nd thus re semnticlly relted. Overcoming those limits with multilending In order to prevent relevnt fetures from disppering, multilending ssigns ech clss of fetures n individul weight, insted of using the single glol weight used y lph lending. Even though theoreticlly, multilending llows weights to rnge from zero to one, it will often e eneficil to limit the technique to weights tht re either zero or one. In other words, one of the two windows will give up feture clss entirely in order to llow the respective feture in the other window to stnd out. A plette window my, for exmple, e desturted in order to not ffect color in the ckground imge. Limiting multilending to Boolen weights elimintes visul miguity; now ech type of feture cn stem from only one of the two windows, nd ll fetures of the sme type elong to the sme window, which clrifies their grouping. Interference tht is cused y the sme fetures stemming from different windows is eliminted. Converting regulr opque plette into multilending plette thus minly requires deciding for ech of the four feture clsses which windows to tke these fetures from. The ctul computtion is then strightforwrd. Given tht color is represented using the CIE L model, computing output color only requires picking ech chnnel from either plette or window nd ressemling them. The removl of texture informtion is ccomplished with filters tht lur or shrpen imges. These filters generlly do not interct with the overll color of window; whenever mking one pixel drker, these filters mke some nery pixel lighter. MAKING MULTIBLENDED PALETTES In this section, we detil the steps used to crete type of multilended plette introduced erlier s the glss plette, using Boolen weights for ech feture clss. These Boolen vlues re provided y the designer, n effort roughly equivlent to the input required for lph lending, where only single, ut therefore rel-vlued opcity vlue needs to e provided. We will show methods for mnul improvement in lter section. Note tht mny of the following exmples require seeing color version of this pper. The scenrio for our wlkthrough is n imge editing/retouching session in Adoe Photoshop [2]. The gol of plette cretion is to support this tsk y showing s much of the surfces nd color of the photogrph. The finl result of this wlkthrough is shown in Figure 2. A wlkthrough in five steps Figure 3 shows prt of the screen tht contins severl plettes floting on top of photogrph the user intends to retouch. By defult, the tool plette is opque nd occludes the photogrph (Figure 3). When the plette is lphlended with the ckground (Figure 3), the photogrph shows through, ut its contrst nd colors re still ffected. Step 1: Desturting: The Photoshop tool plette contins no useful color informtion, so we eliminte it y desturting (Figure 3c). To fully preserve the color of the photogrph (red/green nd lue/yellow, ut not luminnce), we use lending function tht comines the plette s luminnce with the red/green nd lue/yellow informtion from the underlying photogrph. Ech of the color chnnels is tken from only one of the two windows, which prevents them from getting diluted. Step 2: Mking surfces trnsprent: The tool plette minly consists of icons, nd it is the icon s contours tht re most importnt for recognition. The icon s surfces, on the other hnd, seem to ply lesser role. We therefore

4 mke the plettes surfces trnsprent s shown in Figure 3d. In detil, this is done s follows. First, we pply highpss filter (here n emoss filter) to ring out edges. This produces gryscle imge with light nd drk edges. The edges cn e interpreted s 3D effect; ut most importntly, the effect mkes the edges stnd out ginst light nd drk ckgrounds. We then remp tht gryscle imge to trnslucency y using n pproprite lending function ( so-clled liner light lending function, for detils on filters nd lending functions, see [2]). ing res y monitoring the plettes. See [3] for survey of relted techniques. Step 5: Rempping chnnels: In most cses, steps 1-4 will suffice to produce stisfctory imge; however, some situtions of high interference require n dditionl step where informtion from one source is rempped to n lternte chnnel. Figure 6 shows worst cse scenrio two pieces of text in itmp formt, oth using the sme font nd font size. Both windows need to preserve the sme fetures in order to e redle nd given tht text contins less redundncy thn photogrphs, oth windows re more sensitive to mutiltion thn the imge content we hve looked t so fr. When lph lended, oth text segments ecome unredle (Figure 6). c d Figure 3: A tool plette lended using () opcity, () lph lending, (c) luminosity, nd (d) emoss. Step 3: Blurring noisy ckgrounds: Noisy ckgrounds interfere with multilended plettes s much s they do with lph-lended plettes (Figure 4 nd ), mking oth plettes virtully dispper. Multilending therefore elimintes high frequencies from the ckground imge y pplying lur filter to the ckground ehind the plette (Figure 4c). Multilending uses smrt filter tht moves with the plette, nd tht lurs only those res tht exceeds certin contrst threshold, concept similr to the unshrp msk filter in [2]. The resulting plette is esy to red; ll high contrst content is clerly on the plette, while ll low contrst content is in the photogrph ehind it. The resulting plette seems to e mde of piece of frosted glss 1, plette style we will refer to s the glss plette. Step 4: Are-sed opcity sed on usge dt. In Figure 5 nd, significnt prt of the plette consists of window decortion, unused icons, or lels tht never chnge, such s R, G, nd B. Once users hve lerned such sttic plette elements, they offer little informtion to the user. While vrying opcity cross lph plettes leds to noisy ppernce, diminishing prts of glss plettes works well nd cn e used to mke dditionl ckground spce visile (Figure 5c nd d). Frequently used res re determined utomticlly sed on click dt, frequently chng- 1 The plette cn lso e thought of s relief plette with the photogrph pressed onto it. This underlines tht when lending windows, the notion of Z-order s mens for defining n occlusion order goes wy. Z-order is only needed to decide which window receives mouse input, nd if only one window cn receive mouse input, Z-order ecomes unimportnt. c Figure 4: (, ) Over high-frequency ckground, lph-lended nd emossed plettes ecome unredle. (c) A ckground lur solves this issue. c Figure 5: ( nd ) Borders nd unused icons use up screen spce (c nd d): Frequently clicked icons nd dynmic numeric informtion re preserved, ut decortion nd constnt text lels fde. When pplying steps 1 4, only lurring ctully ffects the plette. The prolem is tht oth windows use the sme color chnnel (luminnce) to convey their informtion. We ddress this y rempping the luminnce chnnel of the foreground text to different color chnnel, here the redgreen difference (Figure 6). Then we use lending function tht ssemles the finl imge from the hue of the foreground nd luminnce nd sturtion from the ck- d

5 ground. The resulting imge llows the lurry text to e red sed on its luminnce (hold the figure n rm s length wy), while the crisp text cn e red sed on its color (hold the pper close). Note tht this is worst cse scenrio. We creted multilending with grphicl mteril in mind; even with the enhncements of multilending, lending text will generlly remin undesirle. Nonetheless, pre-filtering text segments cretes limited mount of redility where lph lending does not. Figure 6: () Alph-lending text on text. () Blurring one text nd encoding the other text in hue (hue encoding is invisile in /w hrdcopy. Plese see the ACM digitl lirry for color version of this figure). Summry of the wlkthrough In this wlkthrough, steps 1-3 removed fetures in order to preserve the respective clss of fetures in the other window from interference. We pplied lur filter to remove texturl fetures nd we used customized lending function to selectively process color chnnels. In step 4, we extended the pproch y llowing different lending prmeters for individul res. In step 5, finlly, we solved collisions in requirements y rempping chnnel. As result, ech feture clss is now used y either plette or ckground. This elimintes visul miguity, s ech feture is clerly ssocited with only one window. Figure 7 gives n ide of the pplicility of the glss plette. In this exmple, we merge two windows tht hve identicl fetures, s the shown overview plette shows the sme photogrph s the ckground. We decide tht the overview plette contins less tsk-relevnt informtion thn the ckground photo nd thus turn the overview into glss plette. While the outline informtion in the overview is still sufficient for showing which prt of the photogrph is currently visile, this plette voids the visul miguity tht the lph plette introduces. Figure 7: () Alph overview plette. () The glss overview interferes less with the ckground. Mnul plette optimiztion When converting opque plettes to multilended plettes, the individul weights re est chosen such tht the window with the more prevlent fetures of tht clss wins. These initiliztions cn generlly e done utomticlly; nd loding different picture or moving plettes to different ckground cn even e used to trigger chnge in the plette s representtion. These initiliztions my, however, need mnul correction. For exmple, removing red-eye effect requires preservtion of color, even if the rest of the picture hs little sturtion. Allowing users to mnully switch etween plette representtions t runtime llows otining the est results for the tsk t hnd. Also during plette cretion, the qulity of multilended plettes cn e improved y mnul input. Figure 8 shows n exmple of mnul ckground removl. Alph lending color swtches results in diluted, thus inccurte colors (Figure 8). The swtches thus need to e rendered s fully opque. Mnul cropping of swtches (Figure 8) llows preservtion of the colors with miniml occlusion, while the decortion of the plette uses the known glss effect. Figure 8: () Alph lending dilutes colors in color plette. () Glss plette with mnully cropped swtches (see ACM digitl lirry for color version). IMPLEMENTATION We implemented n initil Jv version of the glss plettes descried ove. The progrm works y rendering n opque version of ech plette into n off-screen uffer, pplying ll required filters to tht off-screen imge, merging it with copy of the respective frgment of the screen uffers, nd then copying the resulting itmp ck onto the screen. Since our prototype does not yet use grphicl ccelertion its rendering performnce is firly limited; rendering is therefore not done while plettes re moving, nd there is noticele puse fter moving plette efore the multilending effect ppers. However, for sttionry plettes, our experience with this prototype suggests tht the technique is vile from n implementtion stndpoint; future versions in ntive code will esily overcome the current performnce limittions. USER STUDIES In order to vlidte the multilending pproch, we conducted two user experiments compring the glss plette to lph-lended plettes t different levels of opcity. Our min hypothesis ws tht the glss plette would simultneously deliver etter recognizility of foreground nd ckground thn ny lph plette. Ech of the two studies mesured one of these spects using distinct tsk. The

6 plette recognizility tsk required prticipnts to find nd click on series of plette icons. The ckground recognizility tsk required prticipnts to mtch the ckground picture to one of severl cndidtes. Bckground recognizility study The first study considered the recognizility of ckgrounds tht were covered with plettes. Methods Twenty-four prticipnts were recruited from locl university. All hd norml or corrected-to-norml visul cuity nd norml color vision. All hd extensive experience (more thn 10 hours/week) with pplictions tht used plettes nd visul workspces; 8 prticipnts hd experience (> 1 hour/month) with n imge-processing ppliction. The study ws conducted on Pentium4 Windows PC running custom-uilt Jv ppliction. The study system ws displyed on 21-inch monitor t 1280x1024 resolution. The study compred three lph plette types nd glss plette type (types shown in Figure 14). Plette visuls were tken from Adoe Photoshop [2] nd converted utomticlly. Alph-lended plettes computed pixel colors s weighted sum of plette nd ckground using opcities 10%, 25%, nd 50%, s suggested y [8]. Glss plettes computed pixel colors using the following four steps: emoss (2-pixel height) nd desturte to the plette, Gussin lur (1-pixel rdius) to the ckground underneth the plette, nd lending (using liner light ). The tsk sked prticipnts to look t source imge tht ws covered y plettes (Figure 9, top left qudrnt), nd click on the exct mtch of tht imge from mong set of three cndidte imges (other three qudrnts in Figure 9). This simulted the rel-world tsk of imge retouching, where the user must ssess the correctness of the overll imge fter every stroke. In two of the three cndidtes, one imge feture (the flowers in the top left of the picture) hd een ltered y chnging either its rightness or its contrst y 1, 2, or 3 steps in either direction. The modified feture ws either light (Figure 10) or drk (Figure 11). Prticipnts were given four prctice trils with ech of the plettes, nd then completed 10 test trils in ech condition. Most of the plette surfce (the ckground) ws light. Alph Plettes thus formed stronger contrst with the drk ckground fetures thn with the light ones. Since lph lending reduces contrst, we hypothesized tht it would ffect the recognizility of the drk fetures more. Mesures in this tsk included completion time nd error mgnitude tht is, the numer of steps difference etween the two imges when n error ws mde. The study used 4x2 mixed fctoril design. The fctors were plette type (glss plette, lph-50, lph-25, lph- 10) nd feture type (light or drk). Plette type ws within-prticipnts fctor, nd feture type ws etweenprticipnts fctor. Order nd sptil position in the qudrnts were counter-lnced so tht ech condition ws seen eqully in ech qudrnt. The study system collected completion time nd error dt. Figure 9: Bckground recognizility tsk (lph-50 condition). A source imge with overlid plettes is shown t the top left, nd three cndidte imges ner copies of the source imge re shown in the other three qudrnts. c Figure 10: The 4 conditions in the ckground recognizility tsk: (), lph-lended t 10% opcity, () lph 25%, (c) lph 50%, nd (d) glss plettes. c Figure 11: sme plettes over drk imge fetures d d

7 Results ckground recognizility Prticipnts ccurcy in mtching the source imge using different plette types is shown in Figure 12. Anlysis of vrince (4 (plette) x 2 (feture type) ANOVA) with feture type eing etween sujects fctor ws used to test the effects of the two fctors. For errors, there ws significnt min effect of plette type (F 3,66 =11.14, p<0.001) ut not of feture type (F 1,22 =0.59, p=0.45). Interction etween plette style nd feture ws orderline significnt (F 3,66 =2.56, p=0.06). We crried out follow-up nlyses to compre individul conditions. Errors were significntly lower for the glss plette thn ny lph plette; (glss plette vs. lph-10 plette, F 1,22 =5.26, p<0.05). The only other significnt difference ws etween lph-10 nd lph-50 (F 1,22 = 11.15, p<0.005). error mount (steps) light fetures drk fetures were either locted over light or drk ckground feture, for 50% of the trils ech. Prticipnts were given 12 prctice trils when strting different plette type. Since prticipnts thus hd good generl ide of where ech icon ws (s would e the cse for n experienced user of progrm like Photoshop), the tsk did not test visul serch over lrge re, ut rther ssessed loclized serch, icon recognizility, nd trget cquisition. Mesures for this tsk were tsk completion time nd numer of incorrect clicks. This study used 4x2 within-prticipnts fctoril design with the sme fctors (plette type nd ckground feture type) used previously; however, in this study oth fctors were within-suject fctors lph-10 lph-25 lph-50 glss Figure 12: Men error mounts for ckground recognizility tsk. Error rs show std error. Completion time dt ws lso nlyzed using ANOVA, ut no min effect (F 3,66 =1.73,p=0.17) or interctions were found. On verge, ech tril took etween 40 nd 70 seconds. Experience with imge-processing pplictions did not hve ny effect on performnce. We will now first present the second study, which will llow us to discuss the results of oth studies in conjunction. Plette recognizility study The second study investigted the recognizility of the imges on the plettes themselves. Methods Twelve prticipnts were recruited in the sme wy s for the first study. The study ws conducted on the sme pprtus, with similr Jv ppliction. The sme four plettes types were used. Prticipnts were presented the pprtus shown in Figure 13. In ech tril, n icon ws shown in the middle of the screen. The prticipnts tsk ws to click on the mtching icon locted in one of the four six-icon plettes on the screen s quickly s possile. The tsk consisted of 24 trils per plette type. Ech icon ws presented once per condition; the sme icons nd plettes were used in ll conditions. We used the sme ckground imge s in the previous study. As with the previous study, trget plettes icons Figure 13: Plette recognizility tsk (lph 50% condition). The next icon to e selected is shown in the centre circle. Figure 14: Alph plettes t () 10% () 25%, nd (c) 50%, nd (d) glss plette used in user study. Results plette recognizility Using 4x2 ANOVA, the min result ws the cler difference etween lph-10 nd the other three plette types. There were min effects of oth errors (F 3,33 =9.15, p<0.001) nd completion time (F 3,33 =7.56, p<0.005). Where the error rte with the other three types ws out one in 25 trils, the rte for lph-10 verged more thn one in three for light ckgrounds, nd more thn 1.5 per tril for drk (see Figure 15). Completion time rnged from more thn five seconds on verge for the lph-10 condition, to less thn two seconds for ll the other plette types. Post-hoc nlyses confirmed tht these differences were c d

8 significnt (p<0.05). The lrge difference in errors etween light nd drk ckgrounds for the lph-10 plette lso resulted in significnt min effects of ckground type on errors (F 1,11 =6.32, p<0.05), nd on completion time (F 1,11 =8.77, p<0.05). There were lso interctions etween ckground nd plette type (for errors, F 3,33 =5.25, p<0.01; for completion time, F 3,33 =4.57, p<0.05). errors per tril over light ckground over drk ckground lph-10 lph-25 lph-50 glss Figure 15: Men trgeting errors for plette recognizility tsk. Error rs show std error. Preferences Prticipnts in the first study were lso shown the trgeting tsk t the end of their session, so they could compre the conditions oth for ckground nd foreground visiility. We then sked them which technique they felt est supported oth tsks, considered together. Of the 24 prticipnts, 20 chose the glss plette nd 4 the lph-25 plette. DISCUSSION The trdeoff of lph lended plettes is tht incresing opcity to perform etter on foreground tsks necessrily implies worse performnce on ckground tsks. The studies showed tht multilended plette is le to offer etter trdeoff, nd perform well on oth tsks. Glss plettes were t lest s good s the est lph plettes for oth tsks, nd were lso significntly etter thn the est overll lph plette (25% opcity) for certin imge types. Moreover, the mjority of prticipnts preferred the glss plettes. Although our study tested only comprly smll smple of lph vlues, it seems unlikely tht different choice of lph vlues would hve led to different outcome of the experiment: opcities ove 50% should perform even worse in the ckground recognizility tsk tht lph-50; opcities elow 10% should e even less recognizle in the foreground tsk thn lph-10. The etter performnce of the glss plettes seemed to e cused y two min properties of this plette style. First, y mking most of the plette surfce completely trnsprent they provide n unltered view on lrger prts of the ckground. This llows users to see nd check importnt imge ckground fetures, such s color nd rightness. Second, the emoss effect pplied to plettes produces outlines with oth light nd drk components, mking edges stnd out on vriety of ckground color nd rightness. CONCLUSIONS By eliminting the drwcks of lph lending, such s visul miguity, loss of contrst, nd unfithful reproduction of colors, multilending helps optimize the redility of plettes nd ckground. For the tsks exmined in our user study, multilending mintined recognizility of plette nd ckground significntly etter thn ny of the tested lph-lended plettes. On the other hnd, multilending is computtionlly more expensive nd optimiztion of plettes requires certin understnding of the ppliction scenrio. For future work, we pln to test multilended plettes in vriety of pplictions scenrios, rnging from gmes, imge editors nd CAD systems to instnt messengers, udio plyers, nd tsk rs. Acknowledgements Thnks to Christopher Jmes Fedk for his work on the implementtion nd to Mry Czerwinski, Andy Wilson, Steven Drucker, nd Ed Cutrell for their comments. REFERENCES 1. ActulTools Corportion, Actul Trnsprent Windows Softwre, 2. Adoe Photoshop 3. Budisch, P., DeCrlo, D., Duchowski, A., nd Geisler, B. Focusing on the Essentil: Considering Attention in Disply Design. CACM 46(3), pp Bell, B. Feiner, S., Höllerer, T. View mngement for virtul nd ugmented relity. In Proc. UIST 01, pp Bier, E., Stone, M., Pier, K., Buxton, W., nd De Rose, T. Toolglsses nd Mgic Lenses: the See-Through Interfce. In Proc. SIGGRAPH 93, Cox, D., Chugh, J.S., Gutwin, C. nd Greenerg, S. The Usility of Trnsprent Overview Lyers. In CHI 98 Compnion, Grudin, J. Prtitioning digitl worlds: focl nd peripherl wreness in multiple monitor use. In Proc. CHI 01, pp Gutwin, C., Dyck, J., nd Fedk, C. The Effects of Dynmic Trnsprency on Trgeting Performnce, In Proc. Grphics Interfce 2003, Hrrison, B.L., Kurtench, G., nd Vicente, K.J. An Experimentl Evlution of Trnsprent User Interfce Tools nd Informtion Content. In Proc. UIST 95, Lieermn, H. Powers of ten thousnd: Nvigting in lrge informtion spces. In Proc. UIST'94, McGuffin, M., Blkrishnn, R. Acquisition of expnding trgets. In Proc. CHI 2002, McLren, K. The development of the CIE 1976 (L***) uniform colour-spce nd colour-difference formul, Journl of the Society of Dyers nd Colourists, 92, 1976, pp Plm, W. WinRoll, Perlin, K. nd Fox, D. Pd: An lterntive pproch to the computer interfce. Proc. SIGGRAPH 93, Porter, T. nd Duff, T. Compositing Digitl Imges, Computer Grphics 18, 3, July 1984, pp Wndell, B. Foundtions of Vision. Sinuer Assoc, Wickens, C. Engineering Psychology nd Humn Performnce, Hrper Collins, 1992.