1COS426Non-photorealisticRenderingLee MarkosianQ: What is computer graphics about?2A: Providing tools to help humandesigners produce images thatcommunicate visual information forsome purpose.Whether to use photorealismdepends on the purpose of theimage:• Documentation• Illustration• Story-telling• Expression3Qualities of hand-drawn images• Many details left out• Some details emphasized• Stylization / abstraction– used to evoke complex things• Recognizable individual styleDr. Seuss4Monet5Uri ShulevitzCharles Burns6Louise GordonVesalius7David Suter8Outline• Technical illustration• Pen & Ink• Painterly rendering for animation• Cartoonish renderingTechnical illustration• Saito and Takahashi, Siggraph 90• Purpose: render 3D models in styles that aremore “comprehensible”• Method:– Render various intermediate images– Do image-processing operations on them– Combine the results91011Problem• Parameters need careful tuning for goodresultsPen and Ink• Winkenbach and Salesin, Siggraph 94• Purpose: render 3D models as pen & inkdrawings• Method:– annotate model with procedural “textures”– Render tonal “reference image”– Use it to guide pen and ink textures12131415Pen and Ink• Salisbury, Anderson, Lischinski andSalesin, Siggraph 96• Purpose: define a scale-independentrepresentation for pen & ink images16Salisbury et al., cont’d• Method:– Store lo-res greyscale image annotated withdiscontinuities– filter greyscale image to desired size, run strokegeneration algorithm on it1718Problems• Only produces still images– Would not provide temporal coherence• What’s the application?19Painterly rendering• Meier, Siggraph 96• Problem: produce animations in a“painterly” style with temporal coherence ofstrokes• Method:– Populate surfaces with stroke “particles”– Render with the help of reference images2021videoProblem• Particles have fixed distribution– Need prescribed camera path22Cartoonish rendering• Kowalski, Markosian, Northrup, Holden,Bourdev, Hughes, Siggraph 99• Problem: render scenes like those truffulatrees I showed you earlier• Method:– Populate scene with detail elements “graftals”– Render with the help of reference imagesDr. Seuss23Detail elements (graftals)generated as neededGraftal texturesGraftals• Oriented in local frame• Can choose level of detail24Needed for placement of graftals:• Primarily at silhouettes (e.g.)• Controlled screen-space density• Placement on surfaces• Persistence of graftals25video26Problems• Temporal incoherence• Hard to author new scenes– You have to write C codeStatic graftals• Markosian, Kowalski, Meier, Northrup,Holden, Hughes, NPAR 00• Problem: improve temporal coherence• Method:– Graftals are fixed on surfaces– Draw with view-dependent LOD27Demo: night scene28Problems• Still hard to author scenes– You have to edit text files• LOD handling too restrictivePen & Ink: trees• Deussen and Strothotte, Siggraph 00• Problem: temporally coherent pen and inkrendering of trees• Method:– Draw leaf entities w/ controlled size/abstraction– Do image processing on depth buffer2930video31The future• More rendering algorithms• Better tools (UI)– NPR images need this especially• Prediction: big advances in our ability tomodel stylized characters (people) arecoming• Reason: the content creators will demand
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