ba­sics: ex­trude

In the first of our new se­ries of CG ba­sics we look at the ex­trude func­tion

3D World - - CONTENTS - 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.cre­ative­

This month we take a close look at the ex­trude tool

I f you’re new to CGI there are far too many tools to choose from in a dizzy­ing ar­ray of soft­ware. This se­ries aim is 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. The jour­ney be­gins with the ex­trude func­tion. the root of 3D cre­ation Ex­tru­sion can change a sin­gle poly­gon into a box, a disc into a sim­ple cup, or if re­versed, a hole in a wall. Achiev­ingt a suc­cess­ful ex­trude, which can then be built on to form more com­plex mod­el­ling func­tions, de­pends on how many edges or loops are added af­ter the ex­tru­sion, as well as the source poly­gon it­self.

Some soft­ware can per­form mul­ti­ple ex­trudes at once on a re­cur­ring com­mand, which can cre­ate curved and twist­ing shapes with one click.

There are some things to be aware of when ex­trud­ing, as with ev­ery­thing in 3D, best prac­tice pays div­i­dends. Make sure the ex­truded poly­gon is ideally a quad (four-sided poly­gon). While most CG ap­pli­ca­tions do sup­port ex­tru­sions on n-gons, poly­gons with more than four sides can cre­ate is­sues. This in­cludes the dreaded over­lap if the n-gon is be­ing re­duced in scale at the same time as it is be­ing ex­truded.

It is not just poly­gons that can be ex­truded, edges can be and so can points/ver­tices. The mileage can vary from ap­pli­ca­tion to ap­pli­ca­tion, but edge ex­tru­sion is great for build­ing out poly­gons when re­topol­o­gis­ing ex­ist­ing meshes from a 3D scan, for ex­am­ple, into a more or­dered form for an­i­ma­tion or ren­der­ing without arte­facts.

Ex­tru­sions can be ma­nip­u­lated when they are be­ing per­formed, ei­ther in ro­ta­tion, trans­la­tion or scale, this is how most ba­sic ‘box’ mod­el­ling is done. Box mod­el­ling is a tech­nique of start­ing any model, whether it is a dragon or a Fer­rari from a six-sided cube, us­ing the ex­trude tool to cre­ate ba­sic topol­ogy (an ar­range­ment of poly­gons in a mesh). Mas­ter­ing the tech­niques of ex­tru­sion is the foun­da­tion for any good model.

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