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

Oct 20, 2022 •245 likes •597 views

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

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

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

  3. Watercolors SRC: Lunar Studio

  4. Exploded view SRC: Wikipedia

  5. Exploded view (complex) SRC: General Electric

  6. Ink wash (Sumi-e) SRC: GamesRadar Sumiesociety

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

  8. Why ?

  9. Why ? SRC: Marc Benjamin Zinck Carnegie Mellon University

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

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

  12. 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

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

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

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

  16. 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

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

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

  19. 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

  20. 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

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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]

  26. Cartoon shading SRC: Lake et. al, NPAR 2000

  27. 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

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

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

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

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

  32. Cel shading Result SRC: Wikipedia

  33. 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

  34. SRC: Reddit

  35. Typos ? Corrections ? Left something out ? Feel free to ping me at {umesh@ut.ee}

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