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 “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.”

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

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

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.

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

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 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 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 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 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 Work NPR –Built on earlier efforts at interactive frame rates. More than one style.

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

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.

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…)

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.

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

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.

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.

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

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

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)

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.

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

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

Method Computing Scaling Factors –Volume per frame is calculated

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)

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

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

Questions??? ??????

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