ba­sics: sub­di­vi­sion sur­faces

In our con­tin­u­ing se­ries of CGI ba­sics, we look at us­ing sub­di­vi­sion sur­faces

3D World - - CONTENTS -

Ex­plore the ben­e­fits of util­is­ing sub­di­vi­sion sur­faces

If you’re new to CGI, you may feel that there are far too many tools to choose from in a dizzy­ing ar­ray of soft­ware. This se­ries aims to break ev­ery­thing in CGI down to the very ba­sics, so that ev­ery artist can be armed with the knowl­edge of which tool is best. This time we take a look at sub­di­vi­sion sur­faces.

Cre­at­ing a re­al­is­tic CGI model is de­pen­dant on a large num­ber of fac­tors, and one of the main com­po­nents is nat­u­rally the poly­gons them­selves and how they are di­vided and flow to rep­re­sent the base mesh.

Smooth curves and rounded edges can be dif­fi­cult to rep­re­sent us­ing stan­dard mod­el­ling tech­niques, as tri­an­gles or quad poly­gons, by their in­her­ent na­ture, are not curved.

Sub­di­vi­sion sur­faces do the heavy lift­ing when it comes to rep­re­sent­ing these re­fined edges, as they take the ver­tex in­for­ma­tion and re-in­ter­po­late it via a va­ri­ety of math­e­mat­i­cal tech­niques to show a smoothed shape.

Us­ing sub­di­vi­sion sur­faces makes mod­el­ling or­ganic shapes, es­pe­cially for an­i­ma­tion, much sim­pler than us­ing a purely polyg­o­nal model, as less ver­tices are re­quired. This means sim­pler con­trol points and there­fore a sim­pler sys­tem to set up.

The key to us­ing sub­di­vi­sion sur­faces prop­erly is in un­der­stand­ing how a spe­cific 3D ap­pli­ca­tion im­ple­ments its smooth­ing par­a­digm. While there are a num­ber of stan­dards for sub­di­vi­sion sur­faces, not all ap­pli­ca­tions im­ple­ment them, which can make switch­ing meshes with sub­div sur­faces be­tween ap­pli­ca­tions un­pre­dictable. Edge weight­ing, where an edge loop can tighten a curve with­out the ad­di­tion of any more ver­tices, is a clas­sic ex­am­ple of a sub­div mod­el­ling tech­nique that is im­ple­mented dif­fer­ently across ap­pli­ca­tions, if at all.

Un­der­stand­ing how edge loops work and in­flu­ence topol­ogy en­sures that the base mesh from which the sub­di­vi­sion sur­faces are de­rived uses as many best-prac­tice polyg­o­nal-build­ing tech­niques as pos­si­ble. Min­imis­ing ngons with flow­ing edge loops can make us­ing sub­di­vi­sion sur­faces a straightfo­rward ex­pe­ri­ence, which ben­e­fits both the artist and the fin­ished scene.

Au­thor Mike Griggs Mike Griggs is a 3D and visual ef­fects artist with vast ex­pe­ri­ence across the in­dus­try, as both a cre­ator and a tech­ni­cal writer. www.cre­ative­bloke.com

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