BA­SICS: TRANS­PAR­ENT MA­TE­RI­ALS

We con­tinue our se­ries

3D World - - CONTENTS -

I f 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. Let’s con­tinue ex­plor­ing ma­te­ri­als and shaders by look­ing at trans­par­ent sur­faces.

Cre­at­ing a trans­par­ent ma­te­rial such as glass seems easy – just in­crease the trans­parency slider to 100 and the job is done, so why doesn’t the glass look right?

Trans­par­ent ma­te­ri­als are ac­tu­ally one of the most com­plex items in a 3D scene to get cor­rect, as they have a wide va­ri­ety of fac­tors that im­pact on their look. From the thick­ness of the ob­ject, to the way the ma­te­rial bends light and even the chem­i­cal com­po­si­tion of the glass, di­a­mond, oil or wa­ter amongst many other trans­par­ent ma­te­ri­als, there is a lot to fig­ure out.

As with many other ma­te­rial sys­tems, legacy fixes, or cheats, which were de­ter­mined by a lack of com­put­ing power, are now no longer rel­e­vant for trans­par­ent ma­te­ri­als due to the speed of to­day’s ren­der en­gines. That be­ing said, trans­par­ent ma­te­ri­als can still be com­pu­ta­tion­ally in­ten­sive, so un­der­stand­ing the core com­po­nents of how trans­parency works in a ren­der en­gine is crit­i­cally im­por­tant.

Learn­ing how to model for trans­parency can be of ben­e­fit, for ex­am­ple a solid glass ball trans­mits and re­fracts (mov­ing light through an ob­ject) very dif­fer­ently from a light bulb. This is be­cause the light bulb is a thin-walled glass sphere, which means that the light does a lot less bend­ing, but at the same time there are two lay­ers of trans­parency for light to go through, rather than the one with the solid glass ball.

As with many tech­niques in 3D, you should aim to get as much real-world ref­er­ence as pos­si­ble when work­ing with trans­par­ent ma­te­ri­als – ide­ally in sim­i­lar light­ing con­di­tions that you need to cre­ate. This is par­tic­u­larly im­por­tant as trans­par­ent ma­te­ri­als be­hav­ing in­cor­rectly are one of the eas­i­est ‘tells’ in CGI. That be­ing said, get­ting a trans­par­ent ma­te­rial cor­rect is one of the most sat­is­fy­ing ac­com­plish­ments a CG artist can master.

01 in­ci­dence of re­frac­tion

Ev­ery trans­par­ent ma­te­rial has a dif­fer­ent In­ci­dence of Re­frac­tion. This is the cal­cu­la­tion of how light passes through a ma­te­rial and af­fects how ob­jects be­hind the trans­par­ent ob­ject are seen by the eye (or cam­era). The great thing about learn­ing IOR val­ues is that they are sci­en­tif­i­cally ac­cu­rate, so in­putting a value of around 1.5 will get a good ap­prox­i­ma­tion of glass in any cur­rent ren­der en­gine. With the lat­est gen­er­a­tion of PBR ma­te­ri­als, IOR and re­flec­tion prop­er­ties now tend to be linked.

02 thin vs solid Glass

A ma­jor giveaway for trans­par­ent ob­jects is their thick­ness, as the thicker a trans­par­ent sur­face the more it will re­fract and bend light. But not all trans­par­ent ob­jects are solid – bub­bles, tra­di­tional light bulbs and even cam­era lenses are all ex­am­ples where thin­ness mat­ters. These types of ob­jects can be cre­ated by mod­el­ling a thin wall into the ob­ject, to check to see whether the ren­der en­gine be­ing used has the abil­ity to com­pen­sate for thin-walled glass.

03 trans­parency scale

Trans­parency and es­pe­cially light-re­frac­tive qual­i­ties are crit­i­cally de­pen­dant on model scale (if the ren­der en­gine be­ing used is phys­i­cally ac­cu­rate). For ex­am­ple, a wine glass that is 2m tall will re­fract and trans­mit light very dif­fer­ently to one that is nor­mally sized. For this rea­son, it is very im­por­tant to en­sure that you model at the cor­rect scale. The good thing about this is that a sin­gle glass ma­te­rial can look dif­fer­ent on a wide va­ri­ety of ob­jects.

04 dis­per­sion

An­other method of giv­ing a trans­par­ent ob­ject life is to play with the dis­per­sion, if it is avail­able in the cho­sen ren­der soft­ware. Dis­per­sion, in essence, hints at the chem­i­cal makeup of the trans­par­ent ma­te­rial by split­ting out the light wave­lengths, and can be a great way of highlighting the dif­fer­ences be­tween a whiskey con­tainer made out of glass and one made out of crys­tal. When taken to an ex­treme it can also be a great way of cre­at­ing a soap bub­ble.

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