Interactive Non-Photorealistic
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INTERACTIVE NON-PHOTOREALISTIC TECHNICAL ILLUSTRATION by Amy Gooch A thesis submitted to the facultyof The University of Utah in partial ful llment of the requirements for the degree of Master of Science Department of Computer Science The University of Utah Decemb er 1998 c Copyright Amy Gooch 1998 All Rights Reserved THE UNIVERSITY OF UTAH GRADUATE SCHOOL SUPERVISORY COMMITTEE APPROVAL of a thesis submitted by Amy Gooch This thesis has b een read by each memb er of the following sup ervisory committee and by ma jorityvote has b een found to b e satisfactory. Chair: Peter Shirley Elaine Cohen Rich Riesenfeld THE UNIVERSITY OF UTAH GRADUATE SCHOOL FINAL READING APPROVAL To the Graduate Council of the University of Utah: Ihave read the thesis of Amy Gooch in its nal form and have found that 1 its format, citations, and bibliographic style are consistent and acceptable; 2 its illustrative materials including gures, tables, and charts are in place; and 3 the nal manuscript is satisfactory to the Sup ervisory Committee and is ready for submission to The Graduate Scho ol. Date Peter Shirley Chair, Sup ervisory Committee Approved for the Ma jor Department Rob ert Kessler Chair/Dean Approved for the Graduate Council David S. Chapman Dean of The Graduate Scho ol ABSTRACT Current interactive mo deling systems allow users to view mo dels in wireframe or Phong-shaded images. However, the wireframe is based on the mo del's parameteriza- tion, and a mo del's features may get lost in a nest of lines. Alone, a fully rendered image may not provide enough useful information ab out the structure or mo del fea- tures. Human technical illustrators follow certain visual conventions that are unlike Phong-shaded or wireframe renderings, and the drawings they pro duce are sub jectively sup erior to conventional computer renderings. This thesis explores lighting, shading, and line illustration conventions used by technical illustrators. These conventions are implemented in a mo deling system to create a new metho d of displaying and viewing complex NURBS mo dels. In particular, silhouettes and edge lines are drawn in a manner similar to p en-and-ink drawings, and a shading algorithm is used that is similar to ink-wash or air-brush renderings for areas inside the silhouettes. This shading has a lowintensityvariation so that the black silhouettes remain visually distinct, and it has a co ol-to-warm hue transition to help accent surface orientation. Applying these illustration metho ds pro duces images that are closer to human-drawn illustrations than is provided by traditional computer graphics approaches. To Bruce, for all of the love and supp ort. CONTENTS ABSTRACT ::: :::::: ::::: :::::: :::::: ::::: :::::: :::::: :::::: ::::: iv LIST OF FIGURES :: ::::: :::::: :::::: ::::: :::::: :::::: :::::: ::::: viii LIST OF TABLES :::: ::::: :::::: :::::: ::::: :::::: :::::: :::::: ::::: xii ACKNOWLEDGEMENTS : :::::: :::::: ::::: :::::: :::::: :::::: ::::: xiii CHAPTERS 1. INTRODUCTION ::::: :::::: :::::: ::::: :::::: :::::: :::::: ::::: 1 2. BACKGROUND : ::::: :::::: :::::: ::::: :::::: :::::: :::::: ::::: 4 2.1 Paint Programs and One-Shot Images ..... ...... ...... ...... ..... 4 2.2 One-Shot Images Conveying Shap e . ..... ...... ...... ...... ..... 8 2.3 InteractiveTechniques . ...... ...... ..... ...... ...... ...... ..... 11 2.4 Perception Studies . ...... ...... ..... ...... ...... ...... ..... 12 2.5 Background Summary . ...... ...... ..... ...... ...... ...... ..... 13 3. ILLUSTRATION TECHNIQUES ::: ::::: :::::: :::::: :::::: ::::: 14 3.1 Lines in Technical Illustration . ...... ..... ...... ...... ...... ..... 16 3.1.1 Line Weight..... ...... ...... ..... ...... ...... ...... ..... 18 3.1.2 Line Color and Shading . ...... ..... ...... ...... ...... ..... 20 3.2 Shading in Illustrations ...... ...... ..... ...... ...... ...... ..... 20 3.3 Illustration Summary . ...... ...... ..... ...... ...... ...... ..... 22 4. ALGORITHMS FOR ILLUSTRATION ::: :::::: :::::: :::::: ::::: 23 4.1 Algorithms for Finding Edge Lines . ..... ...... ...... ...... ..... 23 4.1.1 Algorithms for Finding Boundaries and Discontinuities ..... ...... ..... ...... ...... ...... ..... 23 4.1.2 Algorithms for Finding Silhouettes for NURBS ...... ...... ..... 24 4.1.2.1 Some De nitions . ...... ..... ...... ...... ...... ..... 24 4.1.2.2 Mesh Metho d ..... ...... ..... ...... ...... ...... ..... 25 4.1.2.3 Tessellated-Mesh Metho d . ..... ...... ...... ...... ..... 27 4.1.2.4 Srf-No de Metho d .. ...... ..... ...... ...... ...... ..... 30 4.1.2.5 Silhouette Finding Summary .... ...... ...... ...... ..... 34 4.1.3 Other Metho ds for Calculating Edge Lines . ...... ...... ..... 34 4.2 Shading Algorithms .. ...... ...... ..... ...... ...... ...... ..... 35 4.2.1 Traditional Shading of Matte Ob jects . ...... ...... ...... ..... 35 4.2.2 Tone-based Shading of Matte Ob jects . ...... ...... ...... ..... 36 4.2.3 Shading of Metal Ob jects ...... ..... ...... ...... ...... ..... 44 4.3 3D Illustration . ..... ...... ...... ..... ...... ...... ...... ..... 46 5. IMPLEMENTATION AND RESULTS : ::: :::::: :::::: :::::: ::::: 49 5.1 Edge Lines .... ..... ...... ...... ..... ...... ...... ...... ..... 49 5.2 Approximation to New Shading Mo del . .... ...... ...... ...... ..... 49 6. CONCLUSION : :: ::::: :::::: :::::: ::::: :::::: :::::: :::::: ::::: 53 6.1 Future Work ... ..... ...... ...... ..... ...... ...... ...... ..... 56 REFERENCES :::::: ::::: :::::: :::::: ::::: :::::: :::::: :::::: ::::: 57 vii LIST OF FIGURES 1.1 An few examples of a NURBS-based mo del displayed in wireframe. The large numb er of isolines makes distinguishing key features dicult. ..... 1 1.2 Hand-tuned Phong-shaded image versus technical illustration. ..... ..... 2 2.1 Non-photorealistic one-shot images with a high level of abstraction. ..... 5 2.2 Computer-generated p en-and-ink illustrations. ...... ...... ...... ..... 7 2.3 One-shot computer-generated p en-and-ink images conveying shap e by Winken- bach. Copyright 1996 Georges Winkenbach [39]. Used by p ermission. ..... 8 2.4 Another example of one-shot image conveying shap e. Saito enhances a shaded mo del by drawing discontinuity and contour lines. Copyright 1990 Saito [29]. ...... ..... ...... ...... ..... ...... ...... ...... ..... 9 2.5 One-shot images conveying shap e by Do oley and Elb er...... ...... ..... 10 2.6 Image from a frame of Markosian et al. [24] real-time 3D interactive system for illustrating non-self-intersecting p olygon mesh-based mo dels. Copyright 1997 Lee Markosian. Used by p ermission. ... ...... ...... ...... ..... 11 3.1 Technical illustration conventions. Copyright 1995 Volvo Car UK Lim- ited [28]. ...... ..... ...... ...... ..... ...... ...... ...... ..... 15 3.2 An example of the lines an illustrator would use to convey the shap e of this airplane fo ot p edal. Copyright 1989 Macdonald & Co. Publishers Ltd. [25]. 17 3.3 Changing which isolines are displayed can change the p erception of the surface. The image on the right lo oks as if it has a deep er pit b ecause the isolines go thru the maximum curvature p oint on the surface. Images courtesy of David Johnson. .... ...... ..... ...... ...... ...... ..... 18 3.4 Illustrators use lines to separate parts of ob jects and de ne imp ortant shap e characteristics. This set of lines can b e imitated for NURBS mo dels by drawing silhouettes, b oundaries, and discontinuities, shown ab ove drawn over the wireframe representation. ... ..... ...... ...... ...... ..... 18 3.5 De nition of a silhouette: At a p oint on a surface, u; v and given E u; v as the eyevector and nu; v as the surface normal, a silhouette p ointis de ned as the p oint on the surface where E u; v nu; v =0ortheangle between E u; v and nu; v is 90 degrees. ...... ...... ...... ..... 19 3.6 Three line conventions suggested by Judy Martin [25]. Left: single line weight used throughout the image. Middle: heavy line weight used for out edges and parts with op en space b etween them. Right: vary line weight to emphasize p ersp ective. Copyright 1989 Macdonald & Co. Publishers Ltd. [25]. ...... ..... ...... ...... ..... ...... ...... ...... ..... 19 3.7 This photograph of a metal ob ject shows the anisotropic re ections and the white edge highlights which illustrators sometimes depict. .. ...... ..... 20 3.8 Illustrators sometimes use the convention of white interior edge lines to pro duce a highlight [25]. ...... ...... ..... ...... ...... ...... ..... 21 3.9 Illustrators combine edge lines with a sp eci c typ e of shading. Shading in technical illustration brings out subtle shap e attributes and provides information ab out material prop erties. Left: Compare this shaded image of airplane p edal to the line drawing in Figure 3.2. Copyright 1989 Macdonald & Co. Publishers Ltd. [25]. Right: Engine. Courtesy Macmillan Reference USA, a Simon & Schuster Macmillan Company [28]. ...... ...... ..... 22 4.1 Interp olating silhouettes: After two neighb oring surface p oints with di erent 's are found, the p oint where E u; v nu; v = = 0 can b e found by linearly interp olating in u or v with the two angles as calculated in Equation 4.1. Note: = E n > 0 and = E n < 0. .. ...... ..... 25 1 1 1 2 2 2 4.2 Calculating the mesh normals: The four mesh normals which corresp ond to m are n , n , n , n , where for example n = vec vec , with i;j 1;3 1;2 4;3 4;2 1;3 1 2 vec1= m m and vec2= m m ............. ...... ..... 26 i;j i1;j i;j 1 i;j 4.3 Envision the mesh metho d as a table of signs, where can b e +, -, or 0. .. 26 4.4 These