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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

Art-Based Rendering of Fur, Grass, and Trees

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

  • Introduction
  • Prior Work
  • Method
  • Results / Conclusion
  • “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.”
  • The Problem
    • How can we use current 3D graphics to create the effectiveness and persuasiveness or an artist’s few stroke drawings?
  • Motivation
    • Expand 3D graphics by using techniques for depicting complexity from art and illustration.
  • 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.
  • Solution
    • To simulate making strokes on a 2D surface, they propose stroke-based textures.
prior work
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
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
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
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
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
Prior Work
  • NPR
    • Built on earlier efforts at interactive frame rates. More than one style.
  • System Framework
    • Use OpenGL to render polyhedral models.
    • Models are divided into surface regions (patches).
    • Each patch has one or more procedural texture.
  • System Framework
    • Reference Images: they are off-screen renders of scene that are rendered into textures
      • Use 2:
        • Color Reference image
        • ID reference image.
  • 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…)
  • 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.
  • Graftal Textures
    • Procedurally place fur, leaves, grass or other elements.
    • Two Rules:
      • Must be placed with controlled density
      • Seem stick to the surface.
  • 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.
  • 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.
  • Graftal Placement
    • Use the ID Reference image to convert 2D screen position to 2D position on a surface. (Constant time)
  • 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
  • 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)
  • 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.
  • Subtracting Blurred Image
    • Graftals can scale their geometry and volume to maintain desired density and size.
  • 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
  • Computing Scaling Factors
    • Volume per frame is calculated
  • 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)
  • Displaying Graftals
    • If the desire is less than the volume of a graftal, the LOD of the graftal is reduced. This prevents popping.
  • 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
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
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
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
Results / Conclusions
  • Future Work
    • Static Graftal Placement
      • Successful for single instances, but has not been tested for landscapes yet.
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