Art-Based Rendering of Fur, Grass, and Trees

1. Art-Based Rendering of Fur, Grass, and Trees Michael A. Kowalski, Lee Markosian, J.D. Northrup, Lubomir Bourdev, Ronen Barzel

2. Overview • Introduction • Prior Work • Method • Results / Conclusion

3. 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.”

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

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

6. 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.

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

8. 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

9. 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.

10. 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.

11. 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.

12. 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

13. Prior Work • NPR • Built on earlier efforts at interactive frame rates. More than one style.

14. Method • System Framework • Use OpenGL to render polyhedral models. • Models are divided into surface regions (patches). • Each patch has one or more procedural texture.

15. 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.

16. 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…)

17. 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.

18. Method • Graftal Textures • Procedurally place fur, leaves, grass or other elements. • Two Rules: • Must be placed with controlled density • Seem stick to the surface.

19. 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.

20. 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.

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

22. 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

23. 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)

24. 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.

25. Method • Subtracting Blurred Image • Graftals can scale their geometry and volume to maintain desired density and size.

26. 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

27. Method • Computing Scaling Factors • Volume per frame is calculated

28. 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)

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

30. 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.

31. 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.

32. 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.

33. 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.

34. Results / Conclusions • Future Work • Static Graftal Placement • Successful for single instances, but has not been tested for landscapes yet.

35. Questions??? • ??????

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