"Non-Geometric" Plant Modeling: Image-Based Landscape Modeling and General Texture Problems with Maya - Examples and Limitations

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1 "Non-Geometric" Plant Modeling: Image-Based Landscape Modeling and General Texture Problems with Maya - Examples and Limitations Peter OEHMICHEN 1 Introduction When starting a new visualization project, one has to think about the demands of the project and the possibilities and resources one already has at hand. Before starting to develop something completely new, it is quite often useful to consider the available computer program features that are not often used, specifically plug-ins or combinations of programs. Instructions and tips from other program sources are often adaptable without too much effort and are relatively simple and effective to use. And while not everyone is an experienced programmer, it can be a good idea to call in outside help. This can lengthen the whole process, but there are many dealing with the same kind of problems and sometimes they have the time (and get paid) to develop the perfect or almost perfect solution. One of the programs with features that are helpful for a large landscape visualization is Maya with Paint Effects. What does Paint Effects of Maya have that could be of interest, one might well ask. The author believes it offers a solution to fill the gap that sometimes arises between geometric and non-geometric plant representations. And it is a built-in feature of a standard package for visualization purposes. To create different kinds of visualization without using a very specific landscape visualization software product, one can use one of the largely available tools such as Max, Cinema4D, LightWave or even Maya. Like the other tools, Maya is usable for all kinds of standard visualization and animation tasks and like all the others it has specific advantages and weaknesses or limitations. The author, as well as other users, always concentrates on the advantages and specific features of the software already being used because that is the reason it was selected. 2 Vegetation Modeling The main topic of this article is vegetation modeling. The non-geometric in the title does not mean there is no geometric information, but that the focus is on additional data and information needed to create or show a visual representation of a plant. This could be texture information (image representation) or a procedural instruction to automatically create a plant species. Highly differentiated and therefore difficult to compute objects with a large number of triangles/faces should be avoided.

2 2 P. Oehmichen 2.1 Scale Dependencies The kind of visual representation to be chosen depends on the distance to the object to be represented. The triangle, as the simplest geometric form, is suitable to present a texture. With more complicated texture/shape outlines, it is possible to choose a quadrate. Normally it is best suited for large numbers of plant objects in the distance. As you come closer to the object, more details can be seen and have to be added. Conventionally it is done by a greater geometric differentiation and smaller pieces of texture (despite the option to enhance the texture resolution). The number of faces necessary to represent the plant/tree rises from the minimum of 1 to tens of thousands with a great influence on render times while the textures differ only slightly in size. Paint Effects plants are positioned only in space with the help of curves and do not contain any geometric faces. They can have a different degree of refinement and therefore require more or less rendering time but less than true 3D-models 2.2 Maya Paint Effects What is Behind Paint Effects Paint Effects is procedural determined (a kind of L-system). It is a particle trace with defined splitting points. The particle is then replaced by a so called splat (airbrush stamps) with different color settings and written into a special memory buffer. Finally, everything is rendered as a post process after standard scanline rendering. This results in low memory consumption and fast rendering speeds. It offers many of possibilities to achieve a variety of different visual effects, not only in vegetation models, but also in things such as clouds, fire and smoke, liquids and so on The Procedural Structure In Maya s node-based structure one has 3 nodes for the particular Paint Effect result: a general transform node, a stroke shape node and a brush node. There is also a curve node for the positioning of the Paint Effect in World Space coordinates. The most interesting node for the effect itself (and the plant of course) is the brush node. Important attribute settings will be highlighted as examples. Fig. 1: Hypergraph window with node relationship

3 "Non-Geometric" Plant Modeling: Image-based Landscape Modeling 3 Fig. 2: Transform node (stroke example): Contains the basic information about positioning, rotating and scaling. The general node for the positioning etc. in relation to the World Coordinate System or hierarchical higher levels is the Transform node. More important than the Transform node is the Shape node. It contains information about the Display Quality in the standard view ports, Sample Density (how many pieces per unit), Seed (shape variations under the same stroke settings), Surface Offset, End Bounds (used parts of the curve), Normal Direction, Pressure Mapping (variations in size, elevation etc. by a pressure curve derived from stylus pressure or edited curve points) and more. Fig. 3: Shape node (strokeshapeexample) The most important node for the creation of the desired plant species, as mentioned above, is the brush node. It contains about 250 settings for the final look of the plant or effect, but it is not necessary to use all of them. Brush Type is for Paint (Effects), and Global Scale is

4 4 P. Oehmichen for proportional adjustments of height and width. The main sections are: Brush Profile for the width of the effects including some twisting and stamp density (the splats), Shading for the color and texture settings, Illumination for the light setting (real or faked single direction) and some translucence and specular shading, Shadow Effects (faked 2D or 3D), Glow for light effects, Tubes for the procedural growth of the object (explained more in depth later), Gaps for any open spaces in the sequence of splats and Flow Animation for some animated growth like effects. Fig. 4: Brush node (overview) Tubes gives the possibility to extend the stroke into the 3 rd dimension. With Tube Completion activated, the plant at the end of the stroke grows to its full extent. The Creation section gives all necessary information for the first tube (or the trunk) and with the Growth section it can be divided into finer structures. The Behavior section gives the opportunity to add some irregular movement over time and reaction to applied forces. Fig. 5: Tubes section In the Creation section one can set the density, initial number, size (length and width), number of segments, direction, elevation and azimuth of the tubes. Together with the Shading and Texturing section, it forms the base for the Branch and Twigs creation in the Growth section.

5 "Non-Geometric" Plant Modeling: Image-based Landscape Modeling 5 Fig. 6: Creation section In the Growth section one can decide to have all or part of the possible elements: Branches, Twigs, Leaves, Flowers and Buds, with each again having its own collection of attributes to adjust in order to achieve the desired look. Fig. 7: Growth section The Branches section will be highlighted as a principal example of the Growth section. Start Branches sets the number of branches rising from the ground. Num Branches gives the number of branches at each splitting point, while Split Max Depth is the total of splits and Branch Dropout removes some branch positions. All of the Split attributes organize the splitting itself: position, angle and twist.

6 6 P. Oehmichen Fig. 8: Branches section as example The Behavior section is a powerful tool for additionally influencing the final look and automatic animation possibilities for the entire plant. For example, one can add gravity and directional force influences as well as some stiffness in the Forces section or let the tree or grass swing in the wind in an animation with the help of the Turbulence section. And all of the attributes can be animated, like nearly everything in Maya. Fig. 9: Behavior section with Turbulence as example Finally, if one is satisfied with the look of the plant (or brush), one can use it throughout the scene or save it to disk for repeated use elsewhere Usage There are two general possibilities to use the Paint Effects brush in Maya. First is the 3D use inside of the scene it was based on in the creation section of the procedural structure introduction. Here, one can paint as many brush strokes on the ground or any NURBSsurfaces as desired, attach the brush to existing curves or mix different brush settings. But one can also let the program flood entire surfaces with one type of brush strokes to create evenly distributed masses of vegetation. Second, but no less interesting is the 2D use to paint on a canvas in a given texture resolution. There one can create ground covers or other (seamless) textures of green (and other) masses and save color, alpha and depth information for further use. And of course there are built-in brushes in Maya, additional brushes on the homepage of the manufacturer and other sites of Maya enthusiasts in the Internet.

7 "Non-Geometric" Plant Modeling: Image-based Landscape Modeling Limitations The main limitation is the lack of precision in the distribution of the splats which is the cause for a loss of realism in very close-up shots. Furthermore, it is impossible to classically animate individual parts of the plant. And due to the specific internal structure of Paint Effects, there are some other limitations as well. It can also not be used with ray tracing as it is a post effect. Therefore it is not possible to have any reflections, refractions or ray traced shadows directly. You need to use depth map shadows, reflection maps and sometimes a number of render passes to achieve these effects. Even if the technology behind Paint Effects is optimized for speed and memory usage, things can slow down if one has large numbers of tiny elements that are rarely seen. Partial or general texture use (for tubes or leaves) can help to solve that problem. As with all other renderings, artifacts and unwanted effects can appear and one has to look for workarounds. But again, one can get help from the online help or documentation, the hotline, in user groups or on web pages Examples A few examples should give a better impression on the general ways of using Paint Effects plants. First is the ground cover painted in 2D and then applied as texture to a plane. In the foreground, a 3D grass can be added to improve the look. For further refinement even more different plants can be added. Fig. 10: Color, Alpha and Depth (Bump) file Fig. 11: Perspective view of ground cover and with additional 3D grass

8 8 P. Oehmichen Fig. 12: Medium size flower added, Delphinium type and with Dandelion fruit stand The next example is a Birch tree from the built-in collection of brushes. It uses texture information for the bark but not for the leaves. As a 3D brush it is set on the surface of the underlying landscape. Rendered alone against the background it can be used for the Paint Effects or billboard texture. If one would like to have it animated like the original Paint Effects tree it is no problem to use an image sequence or movie file. The use for level of detail solutions (LODs) is not recommended because of popping effects due to the greater variations of the Paint Effects trees. Fig. 13: Birch tree as Paint Effects and as texture (no shadow) 2.3 Texture vs. Geometry vs. Paint Effects There is no single solution for every problem, but this overview provides some recommendations and is therefore to be viewed as a reference for each individual decision. As mentioned earlier, there are always possibilities to combine techniques to achieve the optimal solution for a specific problem. Even with one technique it is possible to use different quality settings. A rule of thumb is to start with textures (billboard or Paint Effects) and if it does not look good enough, then continue with Paint Effects rather than geometry. Only for individual animations of parts of a plant are geometric plants necessary.

9 "Non-Geometric" Plant Modeling: Image-based Landscape Modeling 9 Usage Texture Paint Effects Geometry Close-ups Only very high resolution Only with high quality settings Very good Foreground Only high resolution Very good Good Middle-distance Good Good Simplified geometry only Far-distance Very good Simplified settings Not good Diversity General animation Individual animation (parts of the plant) Shadows Memory need (average) Render speed (average) Not with single texture With image sequences Good Included No Standard animation techniques only No No Yes Raytrace accurate, depth map mostly bad No raytrace, shadow map OK Raytrace and shadow map OK Low Low to medium High High Medium Low 3 Landscape (Terrain) Modeling and Texture Issues And last, a very short hint to terrain modeling. A lot of standard programs feature displacement mapping which can be used with any type of greyscale DTM texture maps where the resulting displacement can be converted to geometry. More important to mention is the right use of textures on that geometry. Resolution is very important and instead of using very large texture maps, one should concentrate on seamless tileable textures. The way to achieve that or to improve existing textures is to use (if built in) wrap functionality in your paint program to paint over the border edges of the texture file and/or to shift the image halfway vertically or horizontally and improve the look of the seam (for example in Photoshop: Filters other Filters displacement effect [sorry for that menu translation]). Homogeneity of the texture itself should be well known.

10 10 P. Oehmichen Fig. 14: USGS data, elevation map as texture and generated landscape 4 Conclusions & Outlook The importance of non-geometric based plant modeling is a fact. What the author wanted to show is the added possibility to use, change or create Maya Paint Effects plants and to use them with a high degree of interactivity even in large numbers. More ways to use them are still open, for example as a texture based transfer of geographic information regarding vegetation and the use of that texture to create different particle emissions. These particles can then be instanced with different Paint Effects brushes leading to an automatic generation of highly diverse and detailed vegetation on the landscape. 5 References Fleming, B. (2000): Mastering 3D graphics: digital botany and creepy insects. Wiley Computer Publishing, New York Kundert-Gibbs, J. & P. Lee (2001): Mastering Maya 3. Sybex, San Francisco Watkins, A. (2003): The Maya 4.5 Handbook. Charles River Media Inc., Hingham Schönherr, M. (2000): Maya 3. Addison Wesley Verlag, München Schönherr, M. (2001): Maya 4 Sketches. Addison Wesley Verlag, München