The drk side of gloss Juno Kim, Phillip J Mrlow & Brton L Anderson Our visul system relies on the imge struture generted y the intertion of light with ojets to infer their mteril properties. One widely studied surfe property is gloss, whih n provide informtion tht n ojet is smooth, shiny or wet. Studies hve historilly foused on the role of speulr highlights in modulting pereived gloss. Here we show in humn oservers tht glossy surfes n generte oth right speulr highlights nd drk speulr lowlights, nd tht the presene of either is suffiient to generte ompelling perepts of gloss. We show tht pereived gloss delines when the imge struture generted y speulr lowlights is lurred or misligned with surrounding surfe shding nd tht pereived gloss n rise from the presene of lowlights in surfe regions isolted from highlights. These results suggest tht the imge struture generted y speulr highlights nd lowlights is used to onstrut our experiene of surfe gloss. Retinl imge struture ontins n entngled mixture of informtion out three-dimensionl (3D) shpe, surfe refletne nd the illumintion field. Although the veridil seprtion of these sene ttriutes from imges is formlly intrtle, we nonetheless experiene the world s n rrngement of 3D ojets with distint surfe nd mteril properties. One of the fundmentl gols of mid-level vision is to understnd how our experiene of surfe ttriutes is derived from retinl imges. Here we fous on understnding the informtion the visul system uses to generte our experiene of gloss, surfe ttriute tht most nturl surfes hve to vrying degrees. Extensive studies hve demonstrted tht the ddition of few ppropritely positioned nd oriented highlights n trnsform surfe s pperne from mtte to glossy 1 4. In n initil study 1 reserhers hd oserved orreltion etween pereived gloss nd the intensity, shpe nd orienttion of speulr highlights. They found tht pereived gloss depends on the positions nd orienttions of highlights reltive to the surrounding surfe shding, onlusion tht hs een supported y onsiderle ody of susequent work 4 7. More reent work hs shown tht the pereption of gloss is gretly enhned when glossy surfes re emedded in illumintion fields tht ontin seondry illumintion from light refleted y other surfes in the environment 8 12. These studies hd foused on the pereption of gloss in smooth surfes tht reflet distorted imges of the environment in whih they re pled. The lrity or shrpness of the refleted imge hs een shown to e powerful ue to surfe gloss 8,9,13,14. Indeed, one group of reserhers 9 hd demonstrted tht gloss n still e pereived when ler imge of the surrounding sene is visile in the ody of glossy surfe, even when the speulr highlights generted y the primry light soures hd een eliminted. Although the presene of shrp imge in the ody of refleting surfe n e powerful soure of informtion out surfe gloss, mny nturl surfes hve omplex urvtures tht strongly distort the struture of speulr refletions, suh s the umpy ojet depited in Supplementry Figure 1. The refletions of the environment in tht imge re not ler imges ut insted re shrp, nested ontours 13,14 tht tend to e elongted long diretions of miniml surfe urvture 6,7,1,15. The struture of these speulr refletions rises from the luminne vritions in the surrounding environment nd depends on the mirostruture of the refleting surfe. Mtte nd low-sheen surfes hve mirostruture tht genertes rod speulr loe tht stters light from the surrounding illumintion field, whih redues the ontrst nd lrity of speulr refletions in the ody of the refleting surfe. In ontrst, highly glossy surfes hve smooth surfes tht generte nrrow speulr loes, whih preserve the struture of the illumintion field. The nrrower speulr loe lso inreses the ontrst nd lrity of speulr ontours, inluding lolly drk speulr refletions. In Figure 1 we show two identilly shped umpy surfes tht were oth pinted with the sme low-gloss pint. The ojet on the right ws susequently oted with ler glossy polyurethne vrnish, nd oth ojets were photogrphed in the sme position under identil illumintion. The glossy oting smoothes the surfe s mirostruture, using its speulr loe to nrrow 16, whih inreses the ontrst of speulr refletions. The nrrowing of the speulr loe genertes not only more intense highlights ut lso speulr lowlights, whih re lolly drker luminne extrem (Fig. 1,). Although the role of speulr highlights in modulting pereived gloss hs een extensively studied, the ontriution of speulr lowlights to the pereption of gloss hs not een studied previously. We designed experiments to determine whether the pereption of gloss n e modulted y the imge struture generted y oth lowlights nd highlights. We seprted speulr refletions into their drker nd righter omponents y omputing the differene etween mtte nd glossy imges of the sme surfe (Online Methods). Brighter refletions inlude speulr refletions of the primry light soure ( highlights ) nd right seondry refletions ( midlights ). Drker refletions ( lowlights ) re generted y Shool of Psyhology, University of Sydney, Sydney, Austrli. Correspondene should e ddressed to J.K. (juno@psyh.usyd.edu.u) or B.L.A. (rt@psyh.usyd.edu.u). Reeived 6 Jnury; epted 17 August; pulished online 23 Septemer 212; doi:1.138/nn.3221 159 VOLUME 15 NUMBER 11 NOVEMBER 212 nture NEUROSCIENCE
Figure 1 Glossy surfes generte speulr highlights nd lowlights. () Imges of two surfes with identil relief tht were initilly pinted with the sme low-gloss pint nd with glossy oting dded to the surfe on the right. () Luminne differene etween the entrl regions of the high-gloss (left) nd low-gloss (middle) surfe results in differene imge (right), in whih red indites regions where luminne is greter in the high-gloss imge thn in the low-gloss imge (speulr highlights), nd green indites regions where the luminne is lower in the high-gloss imge. () Mgnifition of the indited region, showing the shding profile for the sme olique surfe lotions on the low-gloss nd high-gloss surfes. Compred with the low-gloss surfe, the highgloss surfe exhiits steeper shding grdients, whih form shrper edge ptterns nd lolly drker luminne extrem (speulr lowlights ). = drk seondry light soures nd nrrowing of the speulr loe on smooth surfes (s in Fig. 1). Although this differene-imge tehnique defines speulr lowlights with respet to speifi genertive model, our gol ws to use this model to study how mtte nd glossy surfes n e distinguished on the sis of the imge struture they generte. We found tht dding the drker omponent of speulr refletions to mtte surfe generted ompelling pperne of surfe gloss eyond tht ptured y speulr highlights lone. To determine whether pereived gloss depends on the luminne struture of these speulr lowlights, we lurred nd rotted the drker speulr refletions reltive to the surfe s diffuse shding. We found tht lurring or rotting lowlights rpidly olished the pereption of gloss. We lso found tht the perept of gloss n e generted y the ddition of isolted speulr lowlights (in n nlogous fshion to the speulr highlight mnipultion pioneered y erly reserhers 1 ). Our results suggest tht the pereption of gloss, ross rod rnge of surfe geometries nd illumintion onditions, Proility glossier.4 = Rw Mthed Inflted.4.5.1.4 Simulted lowlight level Low gloss High gloss depends on the presene nd identifition of pproprite imge struture formed y speulr highlights nd lowlights. RESULTS Speulr lowlights ontriute to the pperne of gloss In Figure 2 we show glossy nd mtte surfe, with identil shpe nd underlying ody olor, rendered in the sme nturl illumintion field. The olored differene imge depits regions tht re righter nd drker for the glossy surfe thn the mtte surfe. We tested whether the drker speulr refletions (lowlights) n modulte the pereption of gloss y dding the negtive vlues of the differene imge k to the originl mtte surfe (Fig. 2). This mnipultion eliminted ll of the right speulr highlights nd midlights from the glossy imge, retining only the speulr struture generted y the drker lowlights. The psyhophysil results (Fig. 2) reveled tht inresing the simulted gloss level of these drker lowlight refletions monotonilly inresed pereived gloss. The effet of the lowlights on pereived gloss nnot e explined y differenes in men luminne, imge ontrst or the histogrm skew of the different gloss levels euse we otined the sme pttern of dt when we equted these vriles or even inflted them to reverse their order (Fig. 2). We ompred the reltive ontriutions of drker nd righter speulr refletions to pereived gloss y dding either the drker (negtive) speulr refletions only ( drk ), righter (positive) speulr refletions only ( right ) or oth right nd drk speulr Figure 2 Speulr lowlights generte the perept of gloss. () The differene etween glossy (left) nd mtte (middle) versions of the sme surfe geometry is olored differene imge (right). Red indites positive vlues, regions ontining the rightest speulr refletions; green indites negtive vlues, regions of drker speulr lowlights. () A mgnifition of the imge on the right in, whih shows how the luminne vrition of drker refletions produes strutured ontours long the ody of the surfe. () Mens nd 95% onfidene intervls of pereived gloss estimtes for rw surfe imges (some imges re shown) s funtion of inresing speulr lowlight level ( Rw ). Mthed dt were otined with imges tht were mthed in imge sttistis with ommon men luminne of, imge ontrst of.18 nd histogrm skewness of.1. Inflted dt were otined from onditions with inflted sttistil prmeters, where imges hd the sme men luminne ut with the imge of the mtte surfe djusted to hve 2% greter skew nd ontrst thn the imge with the gretest speulr lowlight level. nture NEUROSCIENCE VOLUME 15 NUMBER 11 NOVEMBER 212 1591
Drk Bright Proility glossier.4 Both Bright Drk Mtte Figure 3 Drk speulr refletions generte glol pperne of surfe gloss. () Imges showing surfes with relief ontining the drker (negtive) speulr lowlight omponent ( drk ), righter (positive) speulr refletne omponent ( right ) or oth drk nd right speulr omponents ( oth ). () Mens nd 95% onfidene nds of proility estimtes of pereived gloss for surfes tht were purely mtte (mtte) or with the dded speulr omponents of the drk, right or oth onditions. () Mens nd 95% onfidene intervls for the proility tht surfes ppered glolly glossier in the mtte, drk, right nd oth imges. Both Proility glolly glossier.4 omponents to the mtte surfe ( oth ) (Fig. 3). To estimte the strength of pereived gloss for these surfes, we omputed the proility tht oservers seleted surfe depited in given imge s glossier ross ll trils where tht imge ws presented in pired omprisons tsk. We plotted the proility tht oservers seleted given surfe s glossier s funtion of simulted gloss level (Fig. 3). Oservers judgments of gloss monotonilly inresed s funtion of gloss level for ll speulr refletne onditions. Although surfes ontining drker refletions were rted less glossy overll thn surfes ontining highlights, mny oservers reported tht the surfes with lowlights ppered glossier over the entire surfe thn the surfe with only speulr highlights, whih oservers desried s ppering glossy only in the surfe regions in the viinity of the highlights. We therefore performed n dditionl experiment in whih we instruted the oservers to selet the surfe tht ppered more glolly glossy, tht is, uniformly overed in gloss over the entire surfe of the ojet. The dt otined with the new instrutions (Fig. 3) showed tht the surfe with drker refletions ppered more glolly glossy thn oth the mtte ontrol surfe nd the surfe with righter refletions. Surfes with the drker refletion only ppered s glolly glossy s did surfes with oth righter nd drker refletions. This suggests tht the struture of drker refletions n generte the perept of gloss not only in regions where they re dded ut lso in djent surfe regions. The differene in the glol pperne of gloss in the surfes with right refletions only (right) nd drk refletions only (drk) is evident in Figure 3. The righter region in the drk (lowlights only) imge ppers s rod speulr refletion, wheres the surfe regions tht lk speulr highlights in the right imge pper mtte. The pereived gloss tht is generted y surfes with lowlights does not pper to depend on ny inrese in pereived ontrst of the righter surfe regions tht re mispereived s highlights, s the pereption of gloss ws experiened glolly over the entire surfe, inluding in surfe regions tht ppered drker. In follow-up experiment we ssessed whether oservers seleted the surfe tht hd the smller ontrst differene etween the upper nd lower regions s the surfe tht ws glolly glossy. The gol of.5.1.15 Simulted gloss level Mtte Drk Bright Both this experiment ws to ensure tht oservers were not merely judging surfes tht were more uniform in their pereived gloss s more glolly glossy ut rther seleted the surfe tht ppered glossy over the entire surfe. To test this hypothesis, we redued the ontrst vrition of surfes with right refletions in the right ondition y multiplitively reduing their luminne nd presented oservers with these imges in rndom rrngement ompnied y the imge of surfe with drk speulr refletions (Fig. 4). We instruted oservers to pik the surfe tht ppered most glolly glossy in the set. Eighty-three perent of oservers hose the imge with drk speulr refletions over ll of the imges depiting surfes with right speulr refletions, wheres the remining 17% seleted the imge with unttenuted right refletions (tht is, no oservers seleted ny surfe with ttenuted right refletions). Tken together, these dt suggest tht drker speulr refletions ontriute independent, nonredundnt informtion for the pereption of gloss eyond tht ontriuted y speulr highlights. Pereived gloss depends on speulr lur nd orienttion One of the most slient differenes etween the mtte surfe nd the surfe with drk speulr refletions is the formtion of shrp speulr refletions in the glossy surfe round regions of high urvture nd regions tht reeive only seondry illumintion. In Figure 5, we show the effets of lurring nd rotting speulr lowlights reltive to diffuse shding on the pereption of gloss. Previous studies with speulr highlights hve shown tht pereived gloss is influened y the shrpness of speulr highlights 8,9,13,17..125 5.5 % % %.75 Drk % 17% 83% Figure 4 The glol pperne of gloss is not explined y uniformity in imge ontrst. Surfes generted y ttenuting the intensity of speulr highlights in the right imge ondition. Indited re the sling ftor used to ttenute right speulr refletions (top left of eh imge) nd perentge of oservers tht seleted given imge s ppering more uniformly overed in gloss in the stimulus set (ottom right of eh imge). The surfe with lowlights only ( drk ) is shown t the sme sle on the lower right. 1592 VOLUME 15 NUMBER 11 NOVEMBER 212 nture neuroscience
Figure 5 The pperne of surfe gloss depends on the lrity nd orienttion of speulr refletions reltive to diffuse shding. () Imges of surfes with inresing lur (s perentge of imge width) pplied to speulr lowlights efore they were dded to the surfe s diffuse shding. () Imges of surfes with inresing ngulr offset (in degrees) etween speulr lowlights nd the surfe s diffuse shding. An nnulus ws used to olude regions where ounding ontours of diffuse shding nd the rotted speulr lowlights eme misligned. Imges on the fr right ( nd ) show the extreme ses in eh ondition modified to hve 3% higher ontrst thn the untrnsformed imges on the left. () Mens nd 95% onfidene nds for pereived gloss estimted from psyhophysil tsks versus mgnitude of speulr lowlight lur (left) nd offset in ngulr orienttion of speulr lowlights reltive to diffuse shding (right). Speulr lowlight lur (% imge width) 6 12 12 Diffuse shding offset (degrees) 9 18 18 To ssess whether the shrpness of drker environmentl refletions ontriutes to the pereption of gloss, we onstruted imges in whih we progressively lurred ll the drker negtive refletions efore dding them to the diffuse shding profile. Reduing the shrpness of the speulr omponent deresed pereived gloss (Fig. 5); the drker refletions ppered less like speulr refletions nd more like diffuse shding. The effet of lur nnot e explined y redution in ontrst euse we found the sme deline in pereived gloss for lurry refletions with inflted ontrst (3% higher thn unlurred refletions fr right). We evluted these phenomenologil oservtions in pired omprison experiment in whih oservers seleted the glossier surfe from pirs of six different imges ontining different mounts of lowlight lur. The results of this experiment (Fig. 5) showed tht pereived gloss delined monotonilly with inresing lur of drker refletions, suggesting tht the shrpness of speulr refletions modulted the pereption of gloss in these imges. Other previous evidene suggests tht the pereption of gloss lso depends on physil orretness in the position nd orienttion of speulr highlights. Speulr highlights streth long lines of miniml surfe urvture nd ling to regions of high urvture 1,15. These geometri onstrints use speulr highlights to our pproximtely perpendiulr to the steepest diretion of the diffuse shding grdient, whih provides informtion the visul system uses to distinguish speulr highlights from vritions in surfe pigmenttion 1,4 7. To determine whether similr onstrints underlie the pereption of gloss in our lowlights-only imges, we progressively rotted the lowlights reltive to the surrounding surfe shding (Fig. 5). We onduted nother pired omprison experiment in whih we instruted the oservers to selet the surfe tht ppered glossier, for eh possile piring of five ngles of highlight rottion. Pereived gloss delined monotonilly s the ngle of offset etween drk environmentl refletions nd the surrounding surfe shding inresed (Fig. 5). We lso oserved deline in pereived gloss when imges with the rotted lowlights hd 3% greter ontrst thn tht of the imge with nonrotted lowlights (Fig. 5). These results suggest tht the pereived gloss rising from our lowlights-only imges depends on the pproprite lignment of speulr struture with the surrounding diffuse surfe shding. Isolted lowlights ontriute to the pereption of gloss The pttern of luminne vritions generted y the speulr refletion of seondry light soures formed imge regions tht were oth righter thn their equivlent mtte surfe ( midlights ) s well s regions tht were drker thn their mtte equivlent ( lowlights ). Although previous work hd reveled tht sense of gloss ould e experiened in smooth surfes tht do not ontin refletions of the primry lights soures 9, the imges in these studies still Proility glossier.4 3 6 9 12 15 15 45 9 18 Lowlight lur (%) Diffuse offset (degrees) ontined midlights. It is therefore unler whether isolted lowlights n indue the pereption of gloss. To ddress this question, we rendered new set of surfes in n rtifiil illumintion environment ontining drk ojets sprsely distriuted ginst homogeneous light kground (Fig. 6). We rendered ompletely mtte surfe, glossy surfe with speulr lowlights, glossy surfe with speulr highlights nd glossy surfe with oth speulr lowlights nd highlights. We onduted n experiment in whih we instruted oservers to selet the glossier of two surfes rndomly pired from different onditions. The dt reveled onsistent enhnement in the pereived gloss of surfes ontining isolted speulr lowlights ompred with the mtte ontrol surfe (Fig. 6). Surfes with speulr lowlights generted n pperne of gloss tht ws omprle to tht of surfes with speulr highlights, ut weker thn tht of surfes with oth speulr lowlights nd highlights. To test whether oservers were experiening the speulr lowlights s speulr refletions, we sked eight oservers to selet regions on the surfe tht ppered to ontin refletions of ojets in the surrounding world nd regions tht ppered to e inditive of the diffuse ody olor of the ojet. We overlid oservers responses on the imges in Figure 6. The men luminne of points tht oservers reported s the pereived ody olor (58.7 d m 2, s.d. = 13.8) ws higher thn the luminne of pereived ojet refletions (25.6 d m 2, s.d. = 8.7). We plotted surfe normls for points where oservers pereived ojet refletions reltive to the norml of the surfe region with the rightest diffuse shding pointing t the primry light soure (zero degrees; Fig. 6d). The verge norml of pereived environmentl refletions ws 9, sustntilly greter thn the verge norml of regions pereived s losest to the true ody olor (59.3 ) nd the verge norml of regions tht generted speulr highlights (45. ). These dt show tht oservers pereived drker surfe regions with speulr lowlights s glossy environmentl refletions tht were displed from righter regions where speulr highlights were generted. The ove results suggest tht the pereption of gloss does not depend ritilly on the rightness of speulr refletions ut rther the struture of speulr refletions tht re omptile with the diffuse shding profile. If this nlysis is orret, then it should e Proility glossier.4 nture NEUROSCIENCE VOLUME 15 NUMBER 11 NOVEMBER 212 1593
Figure 6 Isolted speulr lowlights provide informtion eyond speulr highlights. () Exmples of surfes used in the experiment with different omintions of speulr lowlights nd highlights. () Mens nd 95% onfidene intervls for the pereived gloss estimtes of surfes tht were ompletely mtte, with dded speulr lowlights (low), with dded speulr highlights (high) nd with oth omponents (oth). () Oservers identifition of surfe regions tht ppered s the true ody olor of the surfe (lue dots) or regions tht ppered to ontin glossy ojet refletion (green dots). (d) Orienttion of surfe normls for regions pereived s the surfe s ody olor (ody) or independent ojet refletions (lowlight) nd regions tht generted speulr highlights (highlight). Angles in degrees of surfe normls were omputed reltive to the normls of surfe regions with the rightest diffuse shding (zero degrees, rrow pointing upwrd towrd the primry lighting diretion). The vertil dshed line shows the men surfe norml of the seleted regions with ojet refletions. possile to generte lowlight displys nlogous to those in pioneering highlight demonstrtions 1. We photogrphed the sme glossy red up with nd without speulr lowlight refleted in its front fe (Supplementry Fig. 2). Oservers onsistently reported tht the up in the imge with the speulr lowlight ppered glossier thn the up in the imge without the lowlight. This result demonstrtes tht speulr lowlights, like speulr highlights, n generte ompelling pereption of gloss. DISCUSSION The omputtion of surfe gloss is prtiulrly diffiult euse the struture of speulr refletions depends on the illumintion field, the 3D geometry of the refleting surfe nd n oserver s viewing position. The physil interply etween these sene ttriutes omplites the tsk of identifying ues to gloss tht re stle ross hnges in illumintion, surfe shpe nd viewing position. For smooth, onvex surfes with smll vritions in surfe urvture (suh s spheres), the pereption of gloss n e onstrued s form of trnspreny pereption suh tht the imge of the refleted world is seen s seprte lyer situted ehind the ody of the refleting surfe. The lrity nd ontrst of imges refleted y these surfes provide strong ues to glossiness. However, suh ues nnot redily generlize to surfe geometries tht do not form ler imges of the surrounding environment. The omplex vritions in the struture of speulr refletions ross hnges in illumintion nd surfe geometry hve led some to suggest tht the visul system omputes gloss using imge sttistis 9,18, ut susequent work hs not provided support for this view 5,12,19,2. The gol of the work reported here ws to identify imge ues tht provide informtion out gloss tht re roust to hnges in illumintion, surfe shpe nd viewing position. We used differene-imge tehnique to identify potentil soures of imge struture tht distinguish mtte nd glossy surfes, whih reveled tht glossy surfes produe imge regions tht re sustntilly righter (highlights nd midlights) thn their orresponding mtte surfes s well s regions tht re sustntilly drker (lowlights). Our dt show tht pereived gloss n e enhned not only y inresing surfe luminne in pproprite wys s previously thought 1 ut lso y deresing surfe luminne ppropritely (y dding negtive pttern of surfe luminne to mtte surfe). The ppropriteness of this rightening (or drkening) depends on its physil orretness with respet to other informtion in the imge. We found tht lurring or rotting speulr lowlights reltive to the surfe s shpe nd shding profile redued pereived gloss. These results suggest tht the pereption of gloss depends on the formtion of d Proility glossier.4 Mtte Low High Both Mtte Low High Both Highlight Body Lowlight 2 4 6 8 1 12 14 Surfe norml (degrees) Body olor Refletion physilly orret luminne inrements nd derements generted y the speulr refletne of glossy surfes. It is urrently unknown preisely wht imge ues re used y the visul system to distinguish speulr imge struture from other soures of imge vrition. Previous reserh, inluding our own, hs shown tht highlights only pper s speulr refletions when they re ppropritely positioned nd oriented reltive to the surfe s diffuse shding profile 1,5 7. We oserved similr results for our lowlight imges presented here, suggesting tht ommon proess my underlie our ility to distinguish speulr imge struture from other soures of vrition in luminne. One possile soure of informtion tht is ommon to lowlights nd highlights tht ould ount for pereived gloss reported here nd in erlier studies is the position nd orienttion of speulr edge struture. The inresed glol pperne of gloss in our lowlights-only imges (Fig. 3) my rise euse the lowlights generte edge ontours tht surround regions where some highlights would hve formed, whih provide soure of informtion out the lotions of speulr highlights. However, the edges do not hve to shre oundries with physilly orret highlights; the edge struture of speulr lowlights n lso enhne pereived gloss in regions tht only reeive seondry illumintion, devoid of speulr highlights. The extent to whih pereived gloss depends on the omputle ppropriteness of speulr edges ompred with other spets of speulr refletions remins topi for further reserh. Methods Methods nd ny ssoited referenes re ville in the online version of the pper. Note: Supplementry informtion is ville in the online version of the pper. Aknowledgments We thnk M. Niemel for 3D printing of physil surfe models, R. Fleming for dvie on rendering tehniques, K. Grdy for rtisti inspirtion, nd n nonymous reviewer for insight out the optis of pints. This projet ws funded y n Austrlin Reserh Counil Disovery Projet wrded to B.L.A., J.K. nd R. Fleming, nd n Austrlin Reserh Counil fellowship to B.L.A. 1594 VOLUME 15 NUMBER 11 NOVEMBER 212 nture neuroscience
AUTHOR CONTRIBUTIONS J.K. reted the initil demonstrtion of speulr lowlights. J.K., P.J.M. nd B.L.A. designed the experiments nd visul stimuli. J.K. rendered the imges, olleted the dt nd performed the sttistil nlyses. J.K., P.J.M. nd B.L.A. interpreted the results nd wrote the pper. COMPETING FINANCIAL INTERESTS The uthors delre no ompeting finnil interests. Pulished online t http://www.nture.om/doifinder/1.138/nn.3221. Reprints nd permissions informtion is ville online t http://www.nture.om/ reprints/index.html. 1. Bek, J. & Przdny, K. Highlights nd the pereption of glossiness. Perept. Psyhophys. 3, 47 41 (1981). 2. Blke, A. & Bülthoff, H. Does the rin know the physis of speulr refletion? Nture 343, 165 168 (199). 3. Wendt, G., Ful, F. & Musfeld, R. Highlight disprity ontriutes to the uthentiity nd strength of pereived glossiness. J. Vis. 8, 14 (28). 4. Todd, J.T., Normn, J.F. & Mingoll, E. Lightness onstny in the presene of speulr highlights. Psyhol. Si. 15, 33 39 (24). 5. Anderson, B.L. & Kim, J. Imge sttistis do not explin the pereption of gloss nd lightness. J. Vis. 9, 1 (29). 6. Kim, J., Mrlow, P. & Anderson, B. The pereption of gloss depends on highlight ongruene with surfe shding. J. Vis. 11, 4 (211). 7. Mrlow, P., Kim, J. & Anderson, B. The role of rightness nd orienttion ongruene in the pereption of surfe gloss. J. Vis. 11, 16 (211). 8. Pellini, F., Ferwerd, J.A. & Greenerg, D.P. Towrd psyhophysilly-sed light refletion model for imge synthesis. Pro. SIGGRAPH 55 64 (2). 9. Fleming, R.W., Dror, R.O. & Adelson, E.H. Rel-world illumintion nd the pereption of surfe refletne properties. J. Vis. 3, 347 368 (23). 1. Fleming, R.W., Torrl, A. & Adelson, E.H. Speulr refletions nd the pereption of shpe. J. Vis. 4, 798 82 (24). 11. Doershner, K., Mloney, L.T. & Boyi, H. Pereived glossiness in high dynmi rnge senes. J. Vis. 1, 11 (21). 12. Olkkonen, M. & Brinrd, D.H. Pereived glossiness nd lightness under rel-world illumintion. J. Vis. 1, 5 (21). 13. Hunter, R.S. & Hrold, R.W. The Mesurement of Apperne 2nd edn. (New York, Wiley, 1987). 14. Ferwerd, J.A., Pellini, F. & Greenerg, D.P. A psyhophysilly sed model of surfe gloss pereption. Pro. SPIE Humn Vis. Eletroni Imging 4299, 291 31 (21). 15. Koenderink, J.J. & vn Doorn, A.J. Photometri invrints relted to solid shpe. Opt. At 27, 981 996 (198). 16. Klinker, G.J., Shfer, S.A. & Knde, T. The mesurement of highlights in olor imges. Int. J. Comput. Vis. 2, 7 32 (1988). 17. Wendt, G., Ful, F., Ekroll, V. & Musfeld, R. Disprity, motion, nd olor informtion improve gloss onstny performne. J. Vis. 1, 7 (21). 18. Motoyoshi, I., Nishid, S., Shrn, L. & Adelson, E.H. Imge sttistis nd the pereption of surfe qulities. Nture 447, 26 29 (27). 19. Kim, J. & Anderson, B. Imge sttistis nd the pereption of surfe gloss nd lightness. J. Vis. 1, 3 (21). 2. Wijntjes, M.W.A. & Pont, S.C. Illusory gloss on Lmertin surfes. J. Vis. 1, 13 (21). nture NEUROSCIENCE VOLUME 15 NUMBER 11 NOVEMBER 212 1595
ONLINE METHODS Constrution of simulted surfes. The 3D model of the rok surfe shown in Figure 1 nd Supplementry Figure 1 nd used in the experiments ws reted y reursively sudividing geodesi sphere into 655,362 verties. Eh vertex ws then displed outwrd long its norml (tht is, outwrd from the sphere s origin) y weight defined y the lol luminne vlue of loud texture generted in Blender 3D Version 2.49 (sle = ). The resulting surfe formed glolly round ojet with vritions in its inherent meso-struture. The mushroom-shped surfes presented in Figure 6 were reted using the sme initil geodesi mesh ut with the verties displed using mrle texture generted y n oriented sinusoidl pttern dded to loud texture. Physil surfe models. To rete the demonstrtion in Figure 1, we printed 3D physil models of our rok surfe using prototyping printer (Zprinter 15, Z Corportion). The surfes were then pinted with three ots of low-sheen, mid-gry ryli pint. The pint olor ws determined y lending different mounts of Titnium White nd Cron Blk ryli pints (Mtisse) to form uniform mixture. After suffiient drying time, two ots of wter-sed ler vrnish (Cothne, Cot s) were pplied to one of the surfes with re tken to prevent the pperne of rush strokes. We pled the two surfes ginst uniform drk-gry kground. A Cnon 1D digitl SLR ws used to otin the photogrphs of eh surfe. The sme mer ws used to photogrph the red up shown in Supplementry Figure 2. The up ws pled on homogeneous white A4 sheets of pper, whih provided reltively uniform mient illumintion soure in ddition to the diret, overhed fluoresent lighting. A single lk ojet (n insulted eletril le) ws pled in front of the up so tht its refletion ws lerly visile in the surfe of the up (tht is, n isolted speulr lowlight). Stimulus genertion. The 3D rok surfes were rendered in Rdine 21 using imge-sed lighting with the Eulyptus Grove light proe 22. The simulted ledo of the surfe (tht is, its diffuse refletne) ws set to vlue of.4 in the rendering softwre. The speulr omponent ws defined ording to the Wrd model 21. Renderings were performed with the rok surfe viewed from n elevtion of 45. Imge-sed lighting ws onfigured to simulte mient refletions with no mient interpoltion for physilly sed rendering. Output imges in 8 8 pixel high dynmi rnge (HDR) formt were resmpled to 4 4 pixels using uilt-in nti-lising proedures of Rdine (pfilt ommnd). Gloss ws simulted using the plsti mteril model in Rdine, whih determined eh pixel s intensity ording to the weighted sum of diffuse nd speulr omponents, s in eqution (1): I( x, y) = A rs( x, y) + (1 A) rd( x, y) (1) where I is the luminne of given imge pixel (x,y), A is the simulted speulrity level, ρ s is the orresponding intensity of the speulr omponent, nd ρ d is the orresponding intensity of the diffuse omponent. The nti-lised imges were tone-mpped using the Wrd model to onvert the imges from HDR to low dynmi rnge (LDR) 8-it imges. Tone mpping prevents intensity sturtion nd preserves the photorelisti pperne of the simulted ojets. A umultive histogrm ws generted sed on series of rw glossy nd mtte versions of the surfe using the phisto sript in Rdine. The umultive histogrm ws then used to perform the tone mpping of individul imges using the pond ommnd. Severl tone-mpped imges were rendered: one mtte version of the surfe, omprle glossy version nd view of the kground with the surfe msked out in lk. No tone mpping ws performed on the imges used in Figure 2 s the rwimge dt of surfes with lowlights only did not sturte the dynmi rnge of luminne vlues. In ll ses, the sene informtion in the kground ws then removed from eh imge y sutrting the imge of the sene with the msked-out surfe. Speulr lowlights nd highlights were seprted y tking the luminne differene etween imges of tone-mpped glossy nd mtte surfes. Imges of surfes with drk refletions only (tht is, lowlights) were generted y dding only the negtive vlues of the differene imge to the mtte surfe. Imges of surfes with right refletions only were generted y dding only the positive vlues of the differene imge to the mtte surfe. Simulted lowlight level ws ltered y inresing the speulrity of the originl glossy surfe efore performing the differening opertion. The stimulus imges used in Figure 2 were mthed in men luminne, histogrm skew nd imge ontrst using sequene of imge-proessing tehniques. The men luminne of eh imge ws initilly set to.5 efore we pplied suitle luminne gmm trnsformtions (γ ) to mth imges in histogrm skewness (eqution (2)). Root-men-squred imge ontrst ws djusted to.15 y multiplying the resulting luminne vlues y onstnt (A), followed y n dditive shift in luminne (C) to re-equte men luminne to n pproprite vlue tht kept the histogrms within the upper dynmi rnge of pixel intensities. The kground regions of the sene were not trnsformed nd held t zero intensity ross these trnsformtions I = A I g + C (2) The mushroom-shped surfes shown in Figure 6 (ledo = ) were rendered using diffuse illumintion field with pek luminne long the zenith. Speulr highlights were generted using the Cook-Torrne speulr shder 23. Speulr lowlights were generted y pling drker ue ojets (ledo = ) in the surrounding environment ginst the lighter mient surround (ledo =.491). The edge lengths of ues were pproximtely equl to the dimeter of the refleting surfe situted t pproximtely the sme distne wy. Speulr lowlights were lurred in the imge in Figure 5 using Gussin kernel with sigm proportionl to the width of eh 4 4 pixel imge. The nnulus pled round the surfes in the lowlight rottion ondition retined viewing ngle of ~1.9 for eh imge presented to the oservers. Grysle renderings of surfes were presented to oservers on 2.1-inh Dell LCD Monitor with luminne Gmm of 2.2 (5 d/m 2 to 128.3 d/m 2 ). The monitor ws linerized for onditions where the sttistil properties of imges were mthed or reversed in order (with men luminne = 28. d/m 2 in ll onditions, exept when drker speulr refletions were lurred nd rotted, where men luminne = 44.2 d/m 2 ). The disply ws generted y n nvidi GeFore 88 GT Grphis dptor instlled on Mintosh G5 Computer running Mirosoft Windows XP. Psyhophysil tsks. For surfes ontining only speulr lowlights, we inresed the simulted gloss level of the lowlights generted y the rok surfes (Fig. 2), nd estimted pereived gloss using the pired omprisons method, two-lterntive fored hoie (2AFC) psyhophysil proedure. Imges of surfes (15. m dimeter) were presented side-y-side ginst lk kground (5 d/m 2 ) on the omputer monitor with their inner edges seprted y distne of 4. m. Eh imge in eh ondition ws pired with the imges from ll other onditions (tht is, imges from the sme onditions were never ompred with one nother). Informed written onsent ws otined from the oservers who were nive first-yer psyhology students seted diretly in front of the disply t distne of ~6 m. Oservers mnipulted the left nd right rrow keys on stndrd 11-key PC keyord to indite their preferenes for one surfe ppering glossier ompred to the other surfe presented on the opposite side of the disply. Oservers were lso instruted to selet the surfe tht ppered more glolly glossy (tht is, more uniformly overed in gloss) when presented with imges shown in Figure 3 nd imges with the ttenuted right refletions shown in Figure 4. Pereived gloss ws estimted s the proportion of times surfe ws seleted s glossier (or more glolly glossy) divided y the totl numer of times it ws presented. Four sujets prtiipted in the initil experiment where we inresed speulr lowlight level with rw imges nd imges tht were sttistilly mthed or ltered to reverse in order. Seven oservers prtiipted in the experiment where we estimted pereived gloss s funtion of inresing the gloss level of surfes with drk, right or oth drk nd right speulr refletions. Twelve oservers prtiipted in the experiment where we sked them to selet surfes tht ppered more uniformly overed in gloss (tht is, glolly glossy). In the ontrol ondition with ttenuted right refletions, eh oserver seleted single surfe from the imge set rrnged rndomly in 2 3 mtrix (12 oservers). Seven sujets prtiipted in the lowlight-lur experiment, nd eight sujets prtiipted in the lowlight-rottion experiment. Fifteen oservers prtiipted in the experiment to ompre isolted lowlights with isolted highlights, with nture NEUROSCIENCE doi:1.138/nn.3221
eight oservers instruted to use mouse to point to regions on the surfe tht ppered s independent environmentl refletions (or the surfe s true ody olor). Six oservers performed the tsk of seleting the imge of the red up (either with or without the lowlight) tht ppered glossiest. Sttistil nlysis. After onverting rw dt into proility estimtes for the pereived gloss of surfes in eh ondition, we performed sttistil nlyses using repeted-mesures nlyses of vrine (ANOVA). We used repetedmesures t-tests to ompre the men proility estimtes of pereived gloss etween surfes with lowlights nd surfes with other speulr omponents. Bonferroni orretion ws used to ontrol the type-i error rte for ll pir-wise omprisons (with α =.5). A Wiloxon signed-rnk test ws performed to ompre surfe norml dt for pereived ody refletions nd lowlights in the pointing tsk. 21. Wrd, G.J. The RADIANCE lighting simultion nd rendering system. Pro. SIGGRAPH 28, 459 472 (1994). 22. Deeve, P. Imge-sed lighting. IEEE Comput. Grph. Appl. 22, 26 34 (22). 23. Cook, R.L. & Torrne, K.E. A refletne model for omputer grphis. Pro. SIGGRAPH 15, 37 316 (1981). doi:1.138/nn.3221 nture NEUROSCIENCE