PIPE­LINE TECH­NIQUES: rig faces in Maya with de­tached blend shapes

Tech­niques for smarter face rigs

3D Artist - - CONTENTS -

There are many tech­niques for rig­ging a char­ac­ter’s face in Maya. Whether your fa­cial rig con­tains a ba­sic sys­tem of joints or many com­plex nodes, chances are you’ll want to use a blend shape de­former.

Fa­cial rigs for high-end cg of­ten re­quire a se­ries of sculpted polyg­o­nal meshes, which can range from a few blend shapes to a few hun­dred. if your char­ac­ter has 60,000 faces (rel­a­tively low for pro­duc­tion stan­dards), ev­ery time you du­pli­cate the geom­e­try, your file size gets heav­ier and your scene per­for­mance de­creases. But if we’re only sculpt­ing fa­cial shapes for fa­cial ex­pres­sions, keep­ing the whole body ge­ome­tries and fill­ing up our hard drives can be too much.

Let’s say your char­ac­ter has a small head and the en­tire geom­e­try res­o­lu­tion is dense. if this char­ac­ter needs more than 70 blend shapes for the fa­cial rig, it needs a copy of 70 but the body won’t be changed a bit. how­ever, what if you could de­tach its head, sculpt all the blend shapes you want and then put it back on its body with­out adding any other Maya de­form­ers? in this tu­to­rial, we will demon­strate the proper work­flow of us­ing a ‘cut-out’ piece of geom­e­try to drive a con­tigu­ous char­ac­ter mesh. spe­cial thanks to Tighe Rzankowski for his as­sis­tance with VEX script­ing. 01

Man­age com­po­nent IDS When at­tempt­ing to sep­a­rate the head geom­e­try from the body, you must not change any of the orig­i­nal com­po­nent ids. in Maya, each com­po­nent, such as a ver­tex, edge or face, is as­so­ci­ated with a unique nu­meric value start­ing at zero and end­ing with last num­ber of the to­tal range of the com­po­nents – this is of­ten re­ferred to as point or­der. Your char­ac­ter is es­sen­tially a list of points in space and we need to make sure each one ends up in the right place. Briefly, Maya re­mem­bers where the com­po­nent was in lo­cal space of the geom­e­try trans­form and moves it to an as­signed po­si­tion that’s driven by de­for­ma­tion nodes. it works like each of the marching band play­ers, who are mov­ing to cer­tain po­si­tions on the foot­ball field they are per­form­ing on. in the im­age, cylin­der (A) and (B) have iden­ti­cal com­po­nent ids. how­ever, if you look at the cylin­der (c), you’ll no­tice the point or­der has changed. if you were to ap­ply the two cylin­ders (B) and (c) as tar­gets of the cylin­der (A), the out­put geom­e­try would look dif­fer­ent for each shape. 02

Ex­am­ples of blend shapes in the im­age, we are suc­cess­ful be­cause the two ge­ome­tries share the same com­po­nent ids. each ver­tex of the base cylin­der (A) has moved to the cor­rect po­si­tion rel­a­tive to the orig­i­nal shape, cylin­der (B). This is an­other case of a blend shape. un­like the first ex­am­ple, it doesn’t quite look like what i wanted. ex­actly the same thing hap­pened to this cylin­der but the only dif­fer­ence is it had a dif­fer­ent com­po­nent id

as­signed on each ver­tex. it looks a lit­tle bro­ken but tech­ni­cally there is noth­ing wrong with this out­put – this can eas­ily hap­pen if you do not de­tach the head geom­e­try prop­erly. As i have men­tioned, com­po­nent ids of a geom­e­try start at zero and end at the last num­ber of the range. Any ac­tion that adds or re­moves com­po­nents like ex­trud­ing, merg­ing, sep­a­rat­ing, cut­ting or in­sert­ing loops will change the length of the com­po­nent range, thus af­fect­ing the point count. These types of model ed­its will of­ten cause un­pre­dictable changes to the point or­der. 03

De­tach head from the body There is no way to avoid a se­ri­ous change in point or­der – how­ever, we can ma­nip­u­late the point or­der in a way that lets us work with the body and head sep­a­rately. se­lect the faces of the char­ac­ter’s head that you wish to de­tach and use Maya’s sep­a­rate com­mand. We now have two ge­ome­tries with com­po­nent ids start­ing at zero. This would be a great time to du­pli­cate the de­tached head geo. We can clone this geom­e­try for ev­ery fa­cial tar­get shape we wish to create. 04

Com­bine the head into body it is im­per­a­tive that we se­lect the head first as this func­tion re­builds the com­po­nent id. The first se­lected ob­ject will re­tain its orig­i­nal point or­der; the points, in re­la­tion to the sec­ond ob­ject, will be renum­bered start­ing from one above the last id of the first ob­ject. Af­ter merg­ing two ge­ome­tries, the to­tal num­ber of the com­po­nents is more than the orig­i­nal geom­e­try’s com­po­nent num­ber be­cause there are un­closed com­po­nents in be­tween the com­bined ge­ome­tries of where the sec­tion was. if the num­ber ver­tex of the orig­i­nal geom­e­try was ‘n’, the to­tal num­ber of the ver­tex be­comes ‘n’ plus the num­ber of ver­tices from the seam. it is hard to tell just by look­ing at it but these ver­tices need to be merged as well. Keep­ing the com­po­nent id af­ter merg­ing ver­tices is the same as merg­ing ge­ome­tries. se­lect the com­po­nents from the head first, then the com­po­nents of the body parts and merge! The out­put geo has an iden­ti­cal shape and com­po­nent num­ber but has a dif­fer­ent com­po­nent or­der. how­ever, the out­put geom­e­try shares the same num­ber and or­der of com­po­nents of the du­pli­cated head geo. The head geom­e­try can de­form the body with­out ex­plod­ing. 05

Fi­nalise the geom­e­try Af­ter com­bin­ing head and body, you may have no­ticed a lit­tle mark where the sec­tion was com­pletely sealed. You can sim­ply re­move these hard edges by smooth­ing the nor­mal of the mesh. This ac­tion won’t make any change to the com­po­nent ids so it is safe to do af­ter the whole process. how­ever, it still leaves con­struc­tion his­tory un­less you turn it off when you are run­ning this com­mand. Re­mov­ing nodes that aren’t needed for your rig per­for­mance helps op­ti­mise your scene. now open your out­liner and node edi­tor to find and clean all the ex­tras. Af­ter adding this fi­nal touch, this spe­cial body geom­e­try is ready to be driven by its de­tached head. 06

Ap­ply blend shape on mesh There’s still one more small ex­tra step be­fore ap­ply­ing the blend shape on the body. if you sim­ply create blend shapes with de­fault op­tions, you might re­ceive an er­ror read­ing, ‘no de­formable ob­ject se­lected’. You can avoid this by sim­ply uncheck­ing ‘check topol­ogy’ in the tools op­tions or turn­ing it off in your com­mand line. Don’t for­get about this when you’re adding more blend shapes, too.

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