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Art-Based Rendering of Fur, Grass, and Trees Michael A. Kowalski, Lee Markosian, J.D. Northrup, Lubomir Bourdev, Ronen Barzel Overview Introduction Prior Work Method Results / Conclusion Introduction

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Art based rendering of fur grass and trees l.jpg

Art-Based Rendering of Fur, Grass, and Trees

Michael A. Kowalski, Lee Markosian, J.D. Northrup, Lubomir Bourdev, Ronen Barzel


Overview l.jpg
Overview

  • Introduction

  • Prior Work

  • Method

  • Results / Conclusion


Introduction l.jpg
Introduction

  • “Any art student can rapidly draw a teddy bear or a grassy field. But for computer graphics, fur and grass are complex and time consuming.”


Introduction4 l.jpg
Introduction

  • The Problem

    • How can we use current 3D graphics to create the effectiveness and persuasiveness or an artist’s few stroke drawings?


Introduction5 l.jpg
Introduction

  • Motivation

    • Expand 3D graphics by using techniques for depicting complexity from art and illustration.


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Introduction

  • Goals

    • Give designer of a scene control over the style of rendering.

    • Ease the burden of modeling complex scenes by treating the rendering strategy as an aspect of modeling.

    • Provide interframe coherence for the kinds of stylized renderings developed.


Introduction7 l.jpg
Introduction

  • Solution

    • To simulate making strokes on a 2D surface, they propose stroke-based textures.


Prior work l.jpg
Prior Work

  • Particle Systems

    • Reeves introduced particle systems that he used to create trees, fireworks, and other complex images.

    • Alvy Ray Smith used particle systems and L-systems to create graftals that he used to create more accurate biological structures


Prior work9 l.jpg
Prior Work

  • Particle System

    • Cartoon Tree by Badler and Glassner is the direct precursor that uses fractals and graftals to create surfaces through an implicit model that produce data.


Prior work10 l.jpg
Prior Work

  • Stroke Placement

    • Difference Image by Salisbury et al. had a stroke-placing algorithm that was modified to place procedural texture elements at specific areas.


Prior work11 l.jpg
Prior Work

  • Stroke Placement

    • Winkenbach and Salesin used “indication” for pen-and-ink rendering.

    • Strothotte et al. wrote about artistic styles that result in specific effects or perceptions.


Prior work12 l.jpg
Prior Work

  • Particle Based Strokes

    • Meier provided two insights in her particle-based brush stokes method.

      • Using particles to govern strokes in her Monet works showed that not all complexity need be geometric.

      • Optimal particle on object hybrid space technique


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Prior Work

  • NPR

    • Built on earlier efforts at interactive frame rates. More than one style.


Method l.jpg
Method

  • System Framework

    • Use OpenGL to render polyhedral models.

    • Models are divided into surface regions (patches).

    • Each patch has one or more procedural texture.


Method15 l.jpg
Method

  • System Framework

    • Reference Images: they are off-screen renders of scene that are rendered into textures

      • Use 2:

        • Color Reference image

        • ID reference image.


Method16 l.jpg
Method

  • System Framework

    • Color Reference Image

      • An active texture of each patch will render into this image in the appropriate manner. (How to draw and where to draw tufts, grass…)


Method17 l.jpg
Method

  • System Framework

    • ID Reference Image

      • Triangles or edges are each rendered with a color that uniquely identifies that triangle or edge.

      • Then these are edges or triangles are stored in the patches that contains that edge or triangle.


Method18 l.jpg
Method

  • Graftal Textures

    • Procedurally place fur, leaves, grass or other elements.

    • Two Rules:

      • Must be placed with controlled density

      • Seem stick to the surface.


Method19 l.jpg
Method

  • Placing Graftals

    • Use Difference Image Algorithm where a blurred image of the stroke is subtracted from a difference image.

    • At each subsequent step, they search the pixel most in need of darkening.

    • This handles the controled density for graftal placement.


Method20 l.jpg
Method

  • Graftal Placement

    • To control density each graftal texture draws its patch into the color reference image so that darker tones correspond to denser graftal areas.

    • Darker in silhouettes in this image.


Method21 l.jpg
Method

  • Graftal Placement

    • Use the ID Reference image to convert 2D screen position to 2D position on a surface. (Constant time)


Method22 l.jpg
Method

  • Graftal Placement

    • Frame to frame

      • In first frame graftals are place according to the Difference Image Algorithm

      • Each successive frame try to use prior graftal texture

      • If the old can not be used, the Difference Image Algorithm is used to place new graftals


Method23 l.jpg
Method

  • Graftal Placement

    • A graftal can be not selected for two reasons

      • It is not visible, it is zoomed too far out.

      • Insufficient desire for it to be placed in the new image.

    • Use a bucket sort to find greatest desire (Constant Time)


Method24 l.jpg
Method

  • Subtracting Blurred Image

    • This determines the desire of a graftal.

      • Graftals subtract a blurred image of themselves from the difference image. (Gaussian Dot)

      • Pixels in the desire image are coded with values from zero to one. This is associated with its screen space area based on their volume.


Method25 l.jpg
Method

  • Subtracting Blurred Image

    • Graftals can scale their geometry and volume to maintain desired density and size.


Method26 l.jpg
Method

  • Computing Scale Factors

    • Convert Object space length L to screen space length s every frame.

    • Uses scale factor r composed of 2 users specified variables

      • d: the screen space length

      • Vo: corresponding volume


Method27 l.jpg
Method

  • Computing Scaling Factors

    • Volume per frame is calculated


Method28 l.jpg
Method

  • Computing Gaussian Dot of Graftal

    • Calculate the gaussion dot using this equation.

    • The Pixels are then subtracted from the desired image. The desire should equal the volume. (Optimal)


Method29 l.jpg
Method

  • Displaying Graftals

    • If the desire is less than the volume of a graftal, the LOD of the graftal is reduced. This prevents popping.


Method30 l.jpg
Method

  • Drawing the Tuft

    • They use a tapering shape guided by a central spine. The tapered values are stored in an array.

    • To orient the tufts, the compute the dot product of the view vector and the normal. This determines the LOD.

    • The tufts are drawn in an almost orthogonal to the view and pointed down or clockwise.


Results conclusions l.jpg
Results / Conclusions

  • They were able to produce scenes with simple geometry models.

  • They were able to produce interactive scenes on higher-end PCs.

  • Problem

    • Its still easy to create cluttered graftals from the DIA at a frame to frame.


Results conclusions32 l.jpg
Results / Conclusions

  • Future Work

    • Use of fading and alpha blending to fade out graftals.

      • Depends highly on style being used.

      • Silhouettes seem to be missing some graftals because they are so faded.

      • Use another call to the DIA for a back-faced and front-faced graftals. This would reduce the popping of graftals along the silhouettes.


Results conclusions33 l.jpg
Results / Conclusions

  • Future Work

    • Static graftal placement

      • Draw in a view dependent manner with lower detail the farther a graftal is away from the silhouette.

      • Problem is that you can not zoom in and out too far.

      • A fix they have planned to priority values from 0 to 2. They will do work on the graftals with the lowest priority value first using the DIA. Then the next are drawn.


Results conclusions34 l.jpg
Results / Conclusions

  • Future Work

    • Static Graftal Placement

      • Successful for single instances, but has not been tested for landscapes yet.


Questions l.jpg
Questions???

  • ??????


References l.jpg
References

  • [1] Norman I. Badler and Andrew S. Glassner. 3D object modeling.

  • In SIGGRAPH 97 Introduction to Computer Graphics Course Notes.

  • ACM SIGGRAPH, August 1997.

  • [2] OpenGL Architecture Review Board. OpenGL Reference Manual,

  • 2nd Edition. Addison-Wesley Developers Press, 1996.

  • [3] J. D. Foley, A. van Dam, S. K. Feiner, and J. F. Hughes. Computer

  • Graphics: Principles and Practice. Addison-Wesley, Reading, MA,

  • 2nd edition, 1992.

  • [4] Dr. Seuss (Theodor Geisel). The Lorax. Random House, New York,

  • 1971.

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  • York, 1988.

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  • lighting model for automatic technical illustration. In

  • SIGGRAPH 98 Conference Proceedings, pp. 447–452. ACM SIGGRAPH,

  • July 1998.

  • [7] Geoffrey Hayes. Patrick and Ted. Scholastic, Inc., New York, 1984.

  • [8] Lee Markosian. Art-based Modeling and Rendering for Computer

  • Graphics. PhD thesis, Brown University, November 1999 (expected

  • completion).

  • [9] LeeMarkosian, JonathanM. Cohen, Thomas Crulli, and JohnHughes.

  • Skin: A constructive approach to modeling free-form shapes. In

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  • Bourdev, Daniel Goldstein, and John F. Hughes. Real-time nonphotorealistic

  • rendering. In SIGGRAPH 97 Conference Proceedings, pp.

  • 415–420.ACM SIGGRAPH, August 1997.

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  • 96 Conference Proceedings, pp. 477–484.ACMSIGGRAPH, August

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  • fuzzy objects. ACM Trans. Graphics, 2:91–108,April 1983.

  • [13] William T. Reeves and Ricki Blau. Approximate and probabilistic algorithms

  • for shading and rendering structured particle systems. In

  • SIGGRAPH 85 Conference Proceedings, pp. 313–322. ACM SIGGRAPH,

  • July 1985.

  • [14] Michael P. Salisbury,Michael T.Wong, John F. Hughes, and David H.

  • Salesin. Orientable textures for image-based pen-and-ink illustration.

  • In SIGGRAPH 97 Conference Proceedings, pp. 401–406. ACM SIGGRAPH,

  • August 1997.

  • [15] Alvy Ray Smith. Plants, fractals and formal languages. In SIGGRAPH

  • 84 Conference Proceedings, pp. 1–10. ACMSIGGRAPH, July 1984.

  • [16] T. Strothotte, B. Preim, A. Raab, J. Schumann, andD. R. Forsey.How

  • to render frames and influence people. In Computer Graphics Forum,

  • volume 13, pp. 455–466. Eurographics, Basil Blackwell Ltd, 1994.

  • Eurographics ’94 Conference issue.

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  • pen–and–ink illustration. In SIGGRAPH 94 Conference Proceedings,

  • pp. 91–100.ACM SIGGRAPH, July 1994.

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