Imposing Geometric Constraints on Virtual 3 - D O b j ects w ith in an Immersiv e M od eler

This paper describes an intuitive and efficient way to desig n no t o nl y 3 -D shapes but al so their g eo m etric co nstraints within an im m ersive m o del er. The fundam ental co ncept o f the pro po sed m etho d is based o n o ur im m ersive m o del er V L E G O I I , which al l o ws m ul tipl e users to create 3 -D virtual o bj ects o nl y by assem bl ing sim pl e 3 -D g eo m etric shapes ( shape primitives) l ik e with real to y bl o ck s. I n o ur system , g eo m etric co nstraints are presented as prim itives ( c o n strain in g primitives) l ik e no rm al shape prim itives, and the g eo m etric co nstraints are im po sed o n virtual o bj ects sim pl y by assem bl ing shape and co nstraining prim itives. The system al so pro vides fl ex ibl e two -handed interactio n to suppo rt intuitive and efficient m anipul atio ns o f o bj ects. K e y w o r d s : virtual real ity, im m ersive m o del er, g eo m etric co nstraints, two -handed interactio n, m anipul atio n aid. 1 . I N TRO D U CTI O N N o wadays, the term 3-D c o mpu ter g raphic s d esig n im pl ies no t o nl y g eo m etric m o del ing , but al so creating g eo m etric co nstraints, k inetic co nstraints, anim atio n, behavio r and physical pro perties. O wing to such features, co m puter g raphics beco m e al ive and reactive: D ino saurs rag e in the screen, fig hters scuffl e with m o nsters in video g am es, and al l o bj ects in a virtual wo rl d react, ideal l y, as ex act as they were in the real wo rl d. Therefo re, recent to o l s fo r co m puter g raphics desig n are ex pected to have abil ity fo r describing these pro perties. O n the o ther hand, recentl y, easy-to -use, fast-turnaro und system s fo r 3 -D co m puter g raphics desig n have attracted m uch attentio n in term s o f rapid pro to typing in bo th pro fessio nal and ho bby uses. H o wever, m o st ex isting to o l s are to o co m pl icated fo r transient and whim use. There ex ist several system s to suppo rt rapid pro to typing with hig h l evel fl ex ibil ity, which em pl o y special script l ang uag es in which a desig ner describes g eo m etric co nstraints [ 1, 2] . These system s enabl e a desig ner to specify variabl e 3 -D o bj ects incl uding g eo m etric co nstraints, interactio n and anim atio n. H o wever, a desig ner m ust spend m uch tim e to l earn special l ang uag es to describe a variety o f co nstraints o n al l 3 -D o bj ects. W hat is wo rse is that these system s are hard to pro vide an intuitive interface because users cannot design 3-D ob j ects interactiv el y ; i.e., code editing and resul t conf irm ation are sep arated in a design cy cl e. I n th is p ap er, describ ed is a nov el ap p roach f or easy and rap id designing of reactiv e 3-D com p uter grap h ics. T h e k ey concep t of our m eth od is to em p l oy a v irtual real ity tech niq ue and to p rov ide a num b er of p rop erties to b e im p osed on 3-D ob j ects as v isib l e ob j ects in th e v irtual w ork sp ace. Designing 3-D ob j ects w ith in a v irtual env ironm ent rath er th an w ith 2-D p roj ection h as a num b er of adv antages. R eal -tim e h ead-track ed stereoscop ic v iew im p rov es th e understanding of sh ap e and sp atial rel ationsh ip s. Direct m anip ul ation im p rov es th e accessib il ity to 3-D ob j ects in a q uick and intuitiv e w ay . F or th e l ast decade, a num b er of im m ersiv e m odel ers h av e b een dev el op ed [ 3, 4, 5, 6], th ough m ost of th em are onl y ab l e to construct th e sh ap e of v irtual ob j ects, not to describ e th eir b eh av ior or geom etric constraints. W e f ocus on geometric constraints am ong a set of p rop erties of reactiv e 3-D ob j ects as th e f irst step , since m ost real ob j ects h av e geom etric constraints on th eir m ov ab l e range in rel ation to oth er adj oining ob j ects. F or ex am p l e, a rev ol v ing door turns round its p il l ar, a draw er m ov es al ong its cab inet. G eom etric constraints are p resented as v isib l e constraining p rim itiv es. E ach constraining p rim itiv e h as its sh ap e, w h ich v isual l y rep resents its attrib ute, l ik e oth er v irtual ob j ects. H ence, th e attrib utes of geom etric constraints are com p reh ensib l e. M oreov er, it is easy and ef f icient to im p ose geom etric constraints on v irtual ob j ects, b ecause a user h as onl y to com b ine constraining p rim itiv es w ith norm al v irtual ob j ects using sim p l e tw o-h anded sp atial m anip ul ations. O ur sy stem h as th e f ol l ow ing tw o ch aracteristics: 1) T h e sy stem h as th e ab il ity of designing b oth 3-D ob j ects and its geom etric constraints, 2) T h e sy stem p rov ides f l ex ib l e tw o-h anded m anip ul ation. O w ing to th ese ch aracteristics, it b ecom es p ossib l e to design b oth 3-D ob j ects and th eir geom etric constraints in an intuitiv e and ef f icient m anner. T h is p ap er is organiz ed as f ol l ow s. F irst, p rev ious rel ated w ork s f or 3-D grap h ics w ith constraints are b rief l y rev iew ed. N ex t, th e adv antages of im m ersiv e m odel ing are describ ed, and an im m ersiv e m odel er, V L E G O I I , is al so sum m ariz ed. T h en, our ap p roach f or im p osing geom etric constraints on v irtual ob j ects w ith in an im m ersiv e env ironm ent is describ ed. F inal l y , th e im p l em entation of our dev el op ing sy stem is ex p l ained. 2. R E L A T E D W O R K S A num b er of sy stem s sup p ort designing b oth 3-D sh ap es and th eir b eh av iors. F or ex am p l e, T B A G [ 1] is a p aradigm and tool k it f or rap id p rototy p ing of interactiv e, anim ated 3D grap h ics. T B A G is b ased on tw o b roadl y ap p l ied design p rincip l es: grap h ical ab stract data ty p es and ex p l icit f unctions of tim e. S ince T B A G is p rov ided as C + + l ib raries, a user is ab l e to describ e v ariab l e 3-D ob j ects incl uding interaction, anim ation and constraints in C + + . O b l iq -3D [ 2] is a h igh -l ev el , f ast-turnaround sy stem f or b uil ding 3D anim ations. O b l iq -3D consists of an interp reted l anguage th at is em b edded into a 3D-anim ation l ib rary . T h ese sy stem s p rov ide a l arge num b er of f unctions and p rocedures in sp ecial l anguages f or descrip tion of a variety of properties of 3-D ob j ec ts. H en c e, a sk il l ed prog ram m er is ab l e to d esc rib e 3-D ob j ec ts w ith c on sid erab l y c om pl ex g eom etric c on strain ts an d an im ation . H ow ever, u sers m u st d esc rib e al l of properties an d sh apes of 3-D ob j ec ts in a spec ial l an g u ag e, an d th e d esc ription c an n ot b e m od ified in terac tivel y. T h erefore, th ese system s d on ' t provid e in tu itive in terfac es an d req u ire train in g tim e to u se. A l arg e n u m b er of c on ven tion al c om pu ter aid ed d esig n ( C A D) tool s w h ic h u se 2-D in pu t an d ou tpu t d evic es su pport b oth 3-D sh apes an d c on strain ts. H ow ever, 2-D in pu t/ ou tpu t d evic es d on ' t provid e in tu itive perc eption an d operation s of 3-D ob j ec ts. O n th e oth er h an d , as an in tu itive an d effic ien t w ay for d esig n in g 3-D ob j ec ts, im m ersive m od el in g h as attrac ted m u c h atten tion b ec au se of its ad van tag es ex pl ain ed in th e n ex t sec tion . F or th e l ast d ec ad e, a n u m b er of im m ersive m od el in g system s h ave b een d evel oped ; for ex am pl e, V L E G O [4] , th e C on c eptu al Desig n S pac e ( C DS ) [5] , an d th e C h apel H il l I m m ersive M od el in g P rog ram ( C H I M P ) [6] . H ow ever, m ost of th ese system s su pport on l y c on stru c tin g 3-D sh apes, n ot im posin g g eom etric c on strain ts on th e 3-D sh apes. 1. I m m e r s i v e M o d e l i n g I m m ersive m od el in g is a tec h n iq u e to d esig n 3-D ob j ec ts in an im m ersive virtu al en viron m en t, w h il e trad ition al C A D tool s u se 2-D in pu t an d ou tpu t d evic es. I m m ersive m od el in g h as tw o k ey ad van tag es over trad ition al C A D tool s: th e stereosc opic view of a virtu al w orl d w ith a h ead -trac k in g fac il ity an d th e spatial d irec t m an ipu l ation s of 3-D virtu al ob j ec ts. B oth of th em h el p to avoid am b ig u ou s perc eption an d c om pl ic ated 2-D operation s. T h e stereosc opic view of a virtu al w orl d is g en eral l y provid ed b y a H M D ( H ead M ou n ted Displ ay) or a pair of l iq u id c rystal sh u ttered g l asses. T h e stereosc opic view h el ps u sers to u n d erstan d sh apes of 3-D ob j ec ts an d spatial rel ation sh ips am on g th e ob j ec ts in a virtu al w orl d . M oreover, real -tim e h ead -trac k in g fac il ity w ith a 3-D trac k er provid es m otion paral l ax , w h ic h m ak es it easier to perc eive 3-D ob j ec ts. S patial d irec t m an ipu l ation is u su al l y im pl em en ted b y 6DO F ( 3-D position an d orien tation ) in pu t d evic es ( 3-D m ic e, g l oves w ith a 3-D trac k er, etc . ) h el d in h an d s. S in c e a u ser is ab l e to in terac t d irec tl y w ith 3-D ob j ec ts in a virtu al w orl d b ased on proprioc eption , spatial d irec t m an ipu l ation provid es th e u ser w ith in tu itive an d q u ic k m ean s to ac c ess th e 3-D ob j ec ts. I n ad d ition to th e ad van tag es ab ove, a u ser c an easil y c on firm th e resu l ts of m od ific ation of th e 3-D ob j ec ts, b ec au se im m ersive m od el in g en ab l es th e u ser to d esig n 3-D ob j ec ts en tirel y w ith in a virtu al w orl d . H ow ever, m an ipu l ation in an im m ersive en viron m en t ten d s to b e aw k w ard an d fatig u in g ow in g to th e l ac k of forc e-feed b ac k , c om pu tation al d el ay an d so on . T h is d isad van tag e sh ou l d b e el im in ated an d c ertain softw are b ased m an ipu l ation aid is servic eab l e. 2. V L E G O I I We have been d evel o p i n g an i m m er s i ve m o d el er V L E G O I I [ 4], a s har ed vi r t u al en vi r o n m en t f o r c o l l abo r at i ve t w o -han d ed 3 -D m o d el i n g ( F i g u r e 1). V L E G O I I i s i m p l em en t ed o n o n e o r t w o S G I g r ap hi c s w o r k s t at i o n ( s ). T he vi r t u al w o r k s p ac e i s d i s p l ay ed s t er eo s c o p i c al l y t o a u s er t hr o u g h a H M D w i t h a head -t r ac k i n g f ac i l i t y . A u s er ho l d s a p ai r o f 3 -D i n p u t d evi c es i n hi s o r her han d s . E ac h d evi c e has a 6 D O F m ag n et i c t r ac k er 3 S P A C E ( P o l hem u s ) an d f o u r f eat her t o u c h s w i t c hes o n i t . T hes e d evi c es ar e u s ed t o m an i p u l at e vi r t u al o bj ec t s vi a t w o 3 -D c u r s o r s . I n V L E G O I I , al l o p er at i o n s o n 3 -D o bj ec t s ar e p er f o r m ed by u s i n g t he 3 -D c u r s o r s . I n V L E G O I I , al l vi r t u al o bj ec t s c o n s i s t o f o n e o r m o r e s i m p l e 3 -D g eo m et r i c s hap es ( s p her es , bo x es , c o n es , c y l i n d er s , et c .) c al l ed shape primitives. P r i m i t i ves ar e s t o r ed i n a p r i m i t i ve bo x i n ad van c e an d t hey ar e g en er at ed u n l i m i t ed l y f r o m t he bo x by p i c k i n g t he p r i m i t i ves i n t he bo x . S i n c e a u s er c an c o n s t r u c t 3 -D o bj ec t s s i m p l y by as s em bl i n g t he p r i m i t i ves l i k e r eal t o y bl o c k s , he o r s he d o es n o t n eed t o l ear n c o m p l ex o p er at i o n s . V L E G O I I has t he f o l l o w i n g t hr ee c har ac t er i s t i c s : 1) A n u m ber o f f l ex i bl e t w o -han d ed m an i p u l at i o n s s u c h as as s em bl y , d ec o m p o s i t i o n , c o l o r i n g an d s c al i n g o bj ec t s . 2 ) D i s c r et e c o n s t r ai n t s , w hi c h r es t r i c t t he p o s i t i o n an d o r i en t at i o n o f 3 -D o bj ec t s d i s c r et el y t o m ak e i t eas y t o ar r an g e. 3 ) C o l l i s i o n avo i d an c e, w hi c h d et ec t s c o l l i s i o n s am o n g 3 -D o bj ec t s an d ad j u s t s t hei r l o c at i o n s . V L E G O I I o f f er s n at u r al an d q u i c k w ay s f o r m u l t i p l e u s er s t o c r eat e 3 -D vi r t u al o bj ec t s c o o p er at i vel y i n a s har ed vi r t u al w o r k s p ac e. O u r n ew s y s t em f u n d am en t al l y em p l o y s t he d es i g n i n g c o n c ep t o f V L E G O I I . F i g u r e 1: V L E G O I I w o r k s p ac e.