NPR Non-photorealistic rendering Umesh .A Bhat {umesh@ut.ee} The - PowerPoint PPT Presentation

OCT 26, 2015 NPR Non-photorealistic rendering Umesh .A Bhat {umesh@ut.ee}

The “ Opposite ” of making things look real...

Watercolors SRC: Lunar Studio

Exploded view SRC: Wikipedia

Exploded view (complex) SRC: General Electric

Ink wash (Sumi-e) SRC: GamesRadar Sumiesociety

NPR is also known as... • Expressive graphics • Artistic rendering • Non-realistic graphics • Art-based rendering • Psychographics SRC: Yao et. al 2011

Why ?

Why ? SRC: Marc Benjamin Zinck Carnegie Mellon University

“A means of creating imagery that does not aspire to realism” - Stuart Green

Hue Perception of “distinct” colors by humans Tones Perception of brightness of a color by humans SRC: Wikipedia

Two fundamental cues Silhouette – the visible edges of a surface Hatching – the use of texture to indicate the local orientation (shading) of a surface SRC: Salisbury et al. 1997

NPR can also be classified as • Model based NPR • Image based NPR SRC: Doug DeCarlo and Anthony Santella 2002, Winkenbach and Salesin 1996

Model based NPR Visible polygons 3D Model Light Illumination Camera Textures

Image based NPR Pen-and-ink Water-color Cartoon style drawings paintings

NPR – rendering polygonal surfaces 3D Model Lighting Camera How much 3D Visible We will collect a reference image information do we polygons here to indicate desired tone preserve ? (lightness or darkness) at each pixel Procedural stroke texture Stroke clipping Outline drawing

NPR Categories • Pen-and-ink illustration • Painterly rendering • Cartoons • Technical illustrations • Scientific visualization

Pen-and-ink illustrations user input rendered output reference image SRC: Winkenbach and Salesin 1994

Pen-and-ink illustrations Strokes Curved lines of varying thickness and density Texture Conveyed by collection of strokes Tone Perceived gray level across image or segment Outline Boundary lines that disambiguate structure SRC: Winkenbach and Salesin 1994

Strokes and Stroke Textures Strokes are generated by moving along straight path Strokes are perturbed by • Waviness function (straightness) • Pressure function (thickness) Collected in stroke textures • Tone dependent • Resolution dependent • Orientation dependent SRC: Winkenbach and Salesin 1994

NPR – rendering parametric surfaces • Uses parametric directions as stroke directions • Maintain constant tone • Varying tone based on lighting • Can be combined with texture-mapping SRC: Winkenbach and Salesin 1996

NPR – rendering general surfaces • Placing strokes along a direction field • The field is either computed automatically or designed by user SRC: Hertzmann and Zorin 2000

Automatic painting Adding detail with smaller strokes Start from color image: no 3D information Blurring Paint in resolution-based layers • Blur to current resolution • Select brush based on current resolution • Find area of largest error compared to real image • Place stroke • Increase resolution and repeat Layers are painted coarse-to-fine Styles controlled by parameters SRC: Hertzmann

Painting styles Style determined by parameters • Approximation thresholds • Brush sizes • Curvature filter • Blur factor • Minimum and maximum stroke lengths • Opacity • Grid size • Color jitter Encapsulate parameter settings as style

Cartoon shading • Silhouette edge detection • Cartoon shading 2 texels The illumination is computed at discrete levels (often 2) between the fully lit illumination and shadow (ambient) color, determined by n.L SRC: [1]

Cartoon shading SRC: Lake et. al, NPAR 2000

Technical illustrations Level of abstraction • Accentuate important 3D properties • Diminish or eliminate extraneous details Does not represent reality Black edge lines Cool to warm shading colors Single light source Shadows rarely used SRC: SAM Aircraft

Technical illustrations Phong shading Metal shading Edge lines Gooch shading SRC: Gooch et. al 96

Technical illustrations Phong shading Metal shading Edge lines Gooch shading SRC: Gooch et. al 96

Cel shading • The back faces are drawn with thick lines • The object is drawn with a basic texture • Shading SRC: Wikipedia

Cel shading screen-sized color texture depth and world-space surface normal Sobel filter or similar edge-detection filter SRC: Wikipedia

Cel shading Result SRC: Wikipedia

Select references Book Presentations [1] http://classes.cec.wustl.edu/~cse452/lectures/lect14_NPR.pdf [2] http://graphics.cs.cmu.edu/nsp/course/15-462/Spring04/slides/21-npr.pdf [3] http://run.usc.edu/cs420-s15/lec24-npr/24-npr.pdf [4] http://www.cs.ucr.edu/~vbz/cs130f14-14.pdf [5] http://gfx.cs.princeton.edu/proj/sg05lines/course7-4-npr.pdf Papers [6] https://cseweb.ucsd.edu/~ravir/npr_materials.pdf Non-Photorealistic Rendering [7] https://www.academia.edu/2620342/State_of_the_art_non-photorealistic_rendering_NPR_techniques Bruce Gooch and Amy Gooch. A.K. Peters Ltd. Publishers ISBN: 1-56881-133-0 [8] http://www.cs.northwestern.edu/~bgooch/PDFs/gooch98.pdf

SRC: Reddit

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