ADD Droop­i­ness to a sin­glevol­ume char­ac­ter In Ziva

3D World - - FEATURE -

While ZIVA VFX can be pri­mar­ily used to cre­ate pho­to­re­al­is­tic char­ac­ters driven by physics-em­bed­ded anatomy, it is also use­ful in pro­duc­ing dy­namic fea­ture an­i­ma­tion char­ac­ters that might not nec­es­sar­ily have com­plex anatomy. This ex­am­ple shows you how to in­crease the vis­i­ble ‘droop­i­ness’ and ‘weight’ of a spe­cific body re­gion on a sin­gle-vol­ume char­ac­ter by al­ter­ing its ma­te­rial pa­ram­e­ters.

how ob­jects IN ZIVA Work 01

Here, we have a car­toon ele­phant that con­sists of a sin­gle, uni­form vol­ume and a ba­sic biped rig. Un­like a more com­plex anatom­i­cal char­ac­ter which would be com­prised of nu­mer­ous mus­cles, bones and cloth lay­ers, this car­toon ele­phant is a sin­gle solid ob­ject.

All ob­jects in ZIVA VFX have ma­te­rial prop­er­ties that cor­re­spond to real-world phys­i­cal prop­er­ties such as den­sity and elas­tic­ity. Th­ese prop­er­ties can be set to a fixed value through­out the ob­ject, or can be made to vary across the ob­ject. For our ele­phant, this means that we have the flex­i­bil­ity to eas­ily make part of its body ap­pear heav­ier or jig­glier.

con­trol The Mesh 02

To achieve this, first se­lect the char­ac­ter mesh by click­ing on any point of the char­ac­ter. The tet mesh will ap­pear as thin grey tri­an­gles (tets). Th­ese con­trol the res­o­lu­tion at which the ma­te­rial can de­form. From here, go to the Ziva menu on the top menu bar in Maya and then se­lect the Ma­te­rial Layer op­tion. This will au­to­mat­i­cally add a ma­te­rial layer to the char­ac­ter.

choose MA­TE­RI­ALS 03

Next, right-click on any area of your char­ac­ter and the Ma­te­ri­als menu will ap­pear. Here, hover over Paint and se­lect zma­te­ri­als. In the zma­te­ri­als sub­menu, you can pick from the var­i­ous ma­te­rial lay­ers that you have cre­ated. For the pur­pose of this ex­am­ple, there is only one op­tion avail­able to us: ‘zma­te­ri­al1_weights’. How­ever, if you had pre­vi­ously cre­ated mul­ti­ple ma­te­rial types, then they would all be se­lectable here and would be num­bered ac­cord­ingly.

In ZIVA, a ma­te­rial refers to the phys­i­cal prop­er­ties of a tis­sue or cloth. This in­cludes sim­ple prop­er­ties like den­sity, which af­fects mass and in­er­tia. It also in­cludes some more com­pli­cated ma­te­rial prop­er­ties, such as the model of the re­la­tion­ship be­tween strain (es­sen­tially dis­place­ment) and stress (es­sen­tially elas­tic forces).

AP­PLY MA­TE­RI­ALS 04

Upon click­ing ‘zma­te­ri­al1_weights’, a paint­brush cur­sor and menu will ap­pear. This tool en­ables us to eas­ily de­fine the ar­eas of the char­ac­ter mesh where we wish to ap­ply the ma­te­rial pa­ram­e­ter changes (white paint) or, al­ter­na­tively, se­lect the ar­eas we wish to leave un­af­fected (black paint). The in­tu­itive paint­brush tool is used through­out for fast, ef­fec­tive sec­tion se­lect­ing.

In this ex­am­ple, only the bot­tom por­tion of the ele­phant has been painted, since we would like to in­crease the weight and jig­gli­ness of the belly re­gion only. We’ve also se­lected the Flood op­tion lo­cated on the paint­brush menu, on the left-hand side. This func­tion will au­to­mat­i­cally smooth out the oth­er­wise rough paint­ing we just com­pleted. This cre­ates a more or­ganic, nat­u­ral tran­si­tion be­tween the painted and not-painted ar­eas.

MA­TE­RIAL QUAL­I­TIES 05

Once you’ve fin­ished paint­ing, se­lect your new ma­te­rial in the chan­nel box. This will prompt a list of ma­te­rial pa­ram­e­ter in­put fields to ap­pear. To show a sig­nif­i­cant ma­te­rial change around the lower half of the ele­phant, we can re­duce the Young’s Mo­du­lus to 10^2 and lower the Vol­ume Con­ser­va­tion to 10^6.

The Young’s Mo­du­lus pa­ram­e­ter con­trols the ‘stiff­ness’ of the ma­te­rial. We sug­gest us­ing the stan­dard real-world stiff­ness val­ues for mus­cle, ten­don, fat, etc. pro­vided in the ZIVA VFX Doc­u­men­ta­tion. Vol­ume Con­ser­va­tion, on the other hand, con­trols the liq­uid­ity and com­pres­sion re­sis­tance of a ma­te­rial. Typ­i­cally, the nu­meric value for Vol­ume Con­ser­va­tion is set to be 10^2 – 10^7 greater than that of Young’s Mo­du­lus. With a suf­fi­cient level of vol­ume con­ser­va­tion or com­pres­sion re­sis­tance in ZIVA VFX, you can even achieve a soft, goop-like be­hav­iour. In this ex­am­ple, though, we will set the stiff­ness and vol­ume con­ser­va­tion val­ues to be sim­i­lar to gelatin.

See The RE­SULTS 06

Once an­i­mated, the re­sults of the new ma­te­rial set­tings are very dis­tin­guish­able. By paint­ing only the lower por­tion of the ele­phant and mod­i­fy­ing the ma­te­rial pa­ram­e­ters in the re­gion, we are able to vis­i­bly in­crease the droop­i­ness of the char­ac­ter’s lower stom­ach re­gion while still pre­serv­ing its vol­ume.

Ad­ding pro­found sec­ondary dy­nam­ics, even on sim­ple fea­ture an­i­ma­tions, is eas­ily achiev­able with ZIVA VFX. Not only can this func­tion be ap­plied to sin­gle-vol­ume char­ac­ters made with the Maya plugin, but char­ac­ters cre­ated out­side ZIVA can ben­e­fit from the ro­bust physics sim­u­la­tions too.

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