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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

  • Introduction

  • Prior Work

  • Method

  • Results / Conclusion

Introduction l.jpg

  • “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

  • 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

  • Motivation

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

Introduction6 l.jpg

  • 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

  • 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

Prior work13 l.jpg
Prior Work

  • NPR

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

Method l.jpg

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • Graftal Placement

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

Method22 l.jpg

  • 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

  • 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

  • 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

  • Subtracting Blurred Image

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

Method26 l.jpg

  • 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

  • Computing Scaling Factors

    • Volume per frame is calculated

Method28 l.jpg

  • 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

  • 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

  • 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

  • ??????

References l.jpg

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