ArtDirectedWatercolorShaderforNon-Photorealistic

RenderingwithaFocusonReflections

YolandaChengJanuary10,2017

Abstract

In this research, I will develop a shader, a surfacematerial that can be assigned to 3D objectswithina scene to create computergeneratedwatercolor style renderings.Myshaderwillhave theapplicationtoreflections,specularity,shadow,diffuse,contoursandwithanemphasisonreflections. ThemethodologyIwilldevelopcanbesummarizedas:(1)ABarycentricshaderthatisbasedondegreezeroB-splinebasisfunctions;(2)Asetofhand-drawnwatercolorcontroltextureimagesthatnaturallyprovidethetargetingwatercolorlook.(3)Anartistichierarchyforwatercolorpaintings. Keywords:non-photorealisticrendering,watercolorrendering,watercolorreflections,reflections

A.IntroductionandMotivation

(a)ThewindmillsinwatercolorwiththreedifferenttypesofreflectionbyAndyWalker[7]

Nowadays,moreandmore traditional artworks are transforming todigital forms,which provide artistswith amore efficient, flexible platformwith easy accessing,changing and sharing features. For example, today’s Japanese anime use anincreasing amountof the3Delements for an easier control in their2Danimationproduction.Toachieveavarietyof2Dstylesinthe3Denvironmentisanimportanttopicinthefilmandanimationfields.

Watercolorislikenoothermedium.Itsvibrantcolorsandspontaneousshapesgiveit a distinctive charm. [8] Watercolor offers a very rich medium for graphicalexpression. As such, it is used in a variety of applications including illustration,image processing and animation. [5] Among all kinds of traditional media,watercolor has its own charm of the unpredictable freeform quality,whichmanyartists are fond of. Personally, I find it shares a similar nature with the Chinesetraditional ink painting/sumie painting. I have had many years of experience inChinese traditional ink painting and have always been fascinated by watercoloralthoughmystudyfocusisshadingandtexturinginCG.Thus,Ifinditinterestingtoworkonsomethingthatcombinesbothmyinterests:watercolorandCGtogether.Infact,watercolor look rendering in computer graphics is a verypopular topic overtheyears.However,there’snoresearchstudythereflectionsofwatercolor.

(b)PhotographyofthereflectionsonthelakesintheSouthIslandsofNewZealandbyYolandaCheng

The reflection in the natural world is a special form of beauty as shown in thefigures (b). Many artists tend to study and express this kind of beauty in theirartwork.Infigure(a)AndyWalkerdemonstratesthreedifferenttypesofreflectionsofthewindmillsinwatercolorpaintings.Theyalllookbelievablewhilerepresentingthe same object, thewindmills,with different looks of the reflections on the lakesurface in different weather conditions eg: sunny, windy, cloudy/gloomy. In myresearch,Iwillrecreatethewindmillssceneinthethree-dimensionalspaceandwillthenstudyandcreatethereflectionsofthewindmillsfordifferentreflectionlooksby using my watercolor shader. My goal is to achieve a believable watercolorpainterlylookwhenthesceneisrendered.

B.LiteratureReviewB.1WhyNPR? The advent of photography and computers has not replaced artists, illustrators, or draftsmen, despite rising salaries and the decreasing cost of photographic and computer rendering technology. [10] Although photorealistic rendering in CG is achieving amore and more advanced level of realism, which could fool human eyes, artistsneverstopexploringnon-photorealisticwork.ThegoodexamplecouldbeDisney’sshorts Paperman as in figure (c) and Feast in figure (d). Paperman (stylized aspaperman)isa2012black-and-white3Dromanticcomedyshortfilm.ProducedbyWalt Disney Animation Studios and directed by John Kahrs, the short blendstraditional animation and computer animation. [6] So does the Feast the 2014American3Dhand-drawn/computer-animatedromanticcomedyshortfilm.There’satrendforthesekindsofmorestylizedpainterlylookinganimationintheanimatedfilmfield,similarly,ReelFXstudiohasbeencreatingtheirnewfull lengthfeaturedfilmWishPolicewithaverystylizedpainterlylookrenderingasshowninfigure(e)therecentlyreleasedconceptartofthefilm.

(c)CGrenderedsceneinPaperman (d)CGrenderedsceneinFeast

(e)ConceptartofthecomingfeaturedfilmWishPolicebyReelFXstudio

B.2CurrentStudiesandLimits

(f)ChineseinkscenebyLiu (g)CharcoalscenebyDu (h)WatercolorscenebyMontesdeocaExamples of NPR uses: (f) render of a scene in Chinese ink painting style withreflections on the river. [3] (g) render of a scene in charcoal drawing style withreflectionsonthevases.[2](h)renderofahedgehogcharacterinwatercolorstyle.[4] A new approach, called Barycentric Shaders, is developed to simplify shaderdevelopmentthroughamoreintuitiveandstreamlinedprocess[1]Thismethodhassuccessfully been used in some specific artistic styles and has achieved somepromisingresultsinChineseinkpaintinglook[3]andcharcoaldrawinglook[2]inthecomputerrendered3Dscenes.ArecentstudyconductedbyscholarsinNanyangTechnological University presents some advanced results in watercolor stylerenderingwithart-directedcontrol.[4]However,theresultisachievedthroughthereal-time Autodesk Maya viewport render system developed. In my research, I’llfocusonachieving therenderresultsofflineas in themainstreamfilmproductionpipelineandaddingtheapplicationofwatercoloreffectstothereflectionsofthe3Dobjectswiththeconsiderationsofglobalillumination.

C.MethodologyThescenewillbeconstructedinAutodeskMayaandrenderedwithMentalRay.Thenode editor system in Maya could create customized shading networks whichprovidesthemaximumflexibilitiesandpossibilitiesfortestingandtailoringshaderswithoutcoderequired.Itisveryartists-friendlyandeasyaccessing.Whencreatingmyshader,IusedthecharcoalshaderthatDu[2]createdasfoundationalbaseandmodifiedsomecrucialkeyelementstofitthenormofwatercolor.1. BarycenticBasisFunctionsforWatercolorPainting

2. ControlImagesforWatercolorPaintings Hand drawn watercolor textures are created manually. When painting the

texturemaps, Ipayextraattentiontothebrushstrokes,wateramount inordertohaveaconsistentlookingwhenthemapsappliedtothe3Dobjects.IwillthenscanthemapsandtransferthemintodigitalfileswithanadditionaleditinginPhotoshoptoensurethemapisseamlessandtileable.Currently,fortheinitialtesting,therearethreemain controlmaps,which are highlights,medium tone and shadowswith asinglecolorofthewatercolorpaint.Theshiftsofthecolorsfortheshadercouldbeachievedbyaddedahueshiftnodeinthewatercolor-shadingnetwork.Sotheuserscanchooseanycolortheywantbyaneasycontrolforthehue.Morecontrolmapswould be created for a smoother transfer from light to dark when the shaderresponsestothelights. 3. WatercolorPaintingHierarchyTheshaderwillbecreatedconsideringthereallifeorderofthewatercolorpainterswhen they are creating their painting.One of the most common techniques used in watercolor is the application of thin washes of watered paint to regions of the paper. [9] InAndyWalker’spaintingas shown in figure (a), the reflectionscome last,wheremanyartistwilluseanextrawaterdropstoblurthereflections.Similarly,Iwillusethesamehierarchyprincipleformywatercolorshaderparameters.

D.ConclusionThe barycentric shader method provides intuitive art-directed control, and itsframework allows the shaders to be included in any rendering pipeline withoutmajor changes. [1] In my research, the barycentric shader would be the idealmethod to start from and with adding the customized elements to eventuallyachieve the stylized watercolor look in 3D. The methodology makes the shaderhighlycontrollableastheartisthasthefullaccesstothediffuse,shadow,reflection,specularandcontours.Thekeypartisthatallthevaluescouldbecontrolbytexturemapsthatartistcreatedwhichmaybetimeconsumingbutallowstheartistshavethefreedomandauthorityovertheoveralllookthattheywanttoachieve.

References[1]ErgunAkleman,SLiu,andDonaldHouse.Barycentricshaders:Artdirectedshadingusingcontrolimages.[2] Yuxiao Du and Ergun Akleman. Charcoal rendering and shading with reflections. In ACMSIGGRAPH2016Posters,page32.ACM,2016.[3]SiranLiuandErgunAkleman.Chineseinkandbrushpaintingwithreflections.InSIGGRAPH2015:Studio,page8.ACM,2015.[4] S.E. Montesdeoca, H.S. Seah, H-M. Rall. Art-directed Watercolor Rendered Animation.

Non-PhotorealisticAnimationandRendering,Expressive2016.[5] Bousseau, Adrien, et al. "Interactive watercolor rendering with temporal coherence andabstraction." Proceedings of the 4th international symposium on Non-photorealistic animation andrendering.ACM,2006.[6] PapermanWikipediahttps://en.wikipedia.org/wiki/Paperman[7] AndyWalker,WatercolorPainting http://painting.about.com/od/watercolourpainting/ss/AndyW_Reflect.htm#step1[8]Curtis,CassidyJ.,etal."Computer-generatedwatercolor."Proceedingsofthe24thannualconferenceonComputergraphicsandinteractivetechniques.ACMPress/Addison-WesleyPublishingCo.,1997.[9]Lum,EricB.,andKwan-LiuMa."Non-photorealisticrenderingusingwatercolorinspiredtexturesandillumination."ComputerGraphicsandApplications,2001.Proceedings.NinthPacificConferenceon.IEEE,2001.[10]Gooch,Amy,etal."Anon-photorealisticlightingmodelforautomatictechnicalillustration."Proceedingsofthe25thannualconferenceonComputergraphicsandinteractivetechniques.ACM,1998.