…the following is from a
three-part series on Landscape Rendering appearing in the “Beyond Mapping”
Over the past three decades, cutting-edge GIS has evolved from computer mapping to spatial database management, and more recently, to map analysis and modeling. The era of a sequestered GIS specialist has given way to mass marketed applications, such as MapQuest’s geo-queries, On-Star’s vehicular telematics and a multitude of other Internet-served maps.
The transition of GIS from an emerging industry to a fabric of society has radically changed traditional perspectives of map form, content and applications. Like a butterfly emerging from a cocoon, contemporary maps are almost indistinguishable from their predecessors. While underlying geographic principles remain intact, outward appearances of modern maps are dramatically different.
This evolution is most apparent in multimedia GIS. Traditional maps graphically portray map features and conditions as static, 2-D abstractions composed of pastel colors, shadings, line types and symbols. Modern maps, on the other hand, drapes spatial information on 3-D surfaces and provides interactive query of the mapped data that underlies the pictorial expression. Draped remote sensing imagery enables a user to pan, zoom and rotate the encapsulated a picture of actual conditions. Map features can be hyperlinked to text, tables, charts, audio, still images and even streaming video. Time series data can be sequenced to animate changes and enhance movement of in both time and space.
While these visualizations are dramatic, none of the multimedia GIS procedures shake the cartographic heritage of mapping as much as virtual reality. This topic was introduced in a feature article in GeoWorld a few years ago (Visualize Realistic Landscapes, GeoWorld, August, 1998, pages 42-47). This and the next couple of columns will go behind the scenes to better understand how 3-D renderings are constructed and investigate some of the approaches important considerations and impacts.
Since discovery of herbal dyes, the color pallet has been a dominant part of mapping. A traditional map of forest types, for example, associates various colors with different tree species—red for ponderosa pine, blue for Douglas fir, etc. Cross-hatching or other cartographic techniques can be used to indicate the relative density of trees within each forest polygon. A map’s legend relates the abstract colors and symbols to a set of labels identifying the inventoried conditions. Both the map and the text description are designed to conjure up a vision of actual conditions and the resulting spatial patterns.
The map has long served as an abstract summary while the landscape artist’s canvas has served as a more realistic rendering of a scene. With the advent of computer maps and virtual reality techniques the two perspectives are merging. In short, color pallets are being replaced by rendering pallets.
Like the artist’s painting, complete objects are grouped into patterns rather than a homogenous color applied to large areas. Object types, size and density reflect actual conditions. A sense of depth is induced by plotting the objects in perspective. In effect, a virtual reality GIS “sees” the actual conditions of forest parcels through its forest inventory data— species type, mixture, age, height and stocking density for each parcel. A composite scene is formed by translating the data into realistic objects that characterize trees, houses, roads and other features then combined with suitable textures to typify sky, clouds, soil, brush and grasses.
Fundamental to the process is the ability to design realistic objects. An effective approach, termed geometric modeling, utilizes an interface (figure 1) similar to a 3-D computer-aided drafting system to construct individual scene elements. A series of sliders and buttons are used to set the size, shape, orientation and color of each element comprising an object. For example, a tree is built by specifying a series of levels representing the trunk, branches, and leaves. Level one forms the trunk that is interactively sized until the designer is satisfied with the representation. Level two establishes the pattern of the major branches. Subsequent levels identify secondary branching and eventually the leaves themselves.
<![if !vml]><![endif]>Figure 1. Designing tree objects.
The basic factors that define each level include 1) linear positioning, 2) angular positioning, 3) orientation, 4) sizing and 5) representation. Linear positioning determines how often and where branches occur. In fig. 1, the major branching occurs part way up the trunk and is fairly evenly spaced.
The angular positioning, sets how often branches occur around the trunk or branch to which it is attached. The branches at the third level in the figure form a fan instead of being equally distributed around the branch. Orientation refers to how the branches are tilting. Note that the lower branches droop down from the trunk, while the top branches are more skyward looking. The third-order branches tend show a similar drooping effect in the lower branches.
Sizing defines the length and taper a particular branch. In the figure, the lower branches are considerably smaller than the mid-level branches. Representation covers a lot of factors identifying how a branch will appear when it is displayed, such as its composition (a stick, leaf or textured crown), degree of randomness, and 24-bit RGB color. In figure 1, needle color and shading was changed for the tree on the right to simulate a light dusting of snow. Other effects such as fall coloration, leaf-off for deciduous trees, disease dieback, or pest infestations can be simulated.
<![if !vml]><![endif]><![if !vml]><![endif]>Figure 2. The inset on the left shows various branching patterns. The inset on the right depicts the sequencing of four branching levels.
Figure 2 illustrates some branching patterns and levels used to construct tree-objects. The tree designer interface at first might seem like overkill—sort of a glorified “painting by the numbers.” While it’s useful for the artistically-challenged, it is critical for effective 3-D rendering of virtual landscapes.
The mathematical expression of an object allows the computer to generate a series of “digital photographs” of a representative tree under a variety of look-angles and sun-lighting conditions. The effect is similar to flying around the tree in a helicopter and taking pictures from different perspectives as the sun moves across the sky. The background of each bitmap is made transparent and the set is added to the library of trees. The result is a bunch of snapshots that are used to display a host of trees, bushes and shrubs under different viewing conditions.
The object-rendering process results in a “palette” of objects analogous to the color palette used in conventional GIS systems. When displaying a map, the GIS relates a palette number with information about a forest parcel stored in a database. In the case of 3-D rendering, however, the palette is composed of a multitude of tree-objects. The effect is like color-filling polygons, except realistic trees are poured onto the landscape based on the tree types, sizing and densities stored in the GIS. How this scene rendering process works is reserved for next month.
Author's Note: the Tree designer module of the Virtual Forest software package by Pacific Meridian Resources was used for the figures in this column. See http://www.innovativegis.com/products/vforest/ for more examples and discussion.
The previous column described how 3-dimensional objects, such as trees, are built for use in generating realistic landscape renderings. The drafting process uses an interface that enables a user to interactively adjust the trunk’s size and shape then add branches and leaves with various angles and positioning. The graphic result is similar to an artist’s rendering of an individual tree.
The digital representation, however, is radically different. Because it is a mathematically defined object, the computer can generate a series of “digital photographs” of the tree under a variety of look-angles and sun-lighting conditions. The effect is similar to flying around the tree in a helicopter and taking pictures from different perspectives as the sun moves across the sky.
The background of each of these snapshots is made transparent and the set is added to a vast library of tree symbols. The result is a set of pictures that are used to display a host of trees, bushes and shrubs under different viewing conditions. A virtual reality scene of a landscape is constructed by pasting thousands of these objects in accordance with forest inventory data stored in a GIS.
Figure 1. Basic steps in constructing a virtual reality scene.
There are six steps in constructing a fully rendered scene (see Figure 1). A digital terrain surface provides the lay of the landscape. The GIS establishes the forest stand boundaries as geo-registered polygons with attribute data describing stand make-up and condition.
The link between the GIS data and the graphic software is critical. For each polygon, the data identifies the types of trees present, their relative occurrence (termed stocking density) and maturity (age, height). In a mixed stand, such as spruce, fir and interspersed aspen, several tree symbols will be used. Tree stocking identifies the number of trees per acre for each of the species present. This information is used to determine the number tree objects to “plant” and cross-link to the appropriate tree symbols in 3-D tree object library. The relative positioning of the polygon on the terrain surface with respect to the viewpoint determines which snapshot of the tree provides the best view and sun angle representation.
Finally, information on
percent maturity establishes the baseline height of the tree. In a detailed tree library several different
tree objects are generated to represent the continuum from immature, mature and
old growth forms. Figure 2 shows the
tree exam files for two polygons identified in the adjacent graphic. The first column of values identifies the
tree symbol (library ID#). Polygon 1573
has 21 distinct tree types including snags (dead trees). Polygon 1658 is much smaller and only
contains 16 different types. The second
column indicates the percent maturity while the third defines the number of
trees. These data shown are for an
extremely detailed U.S. Forest Service research area in
<![if !vml]><![endif]>Figure 2. Forest inventory data establishes tree types, stocking density and maturity.
Once the appropriate tree symbol and number of trees are identified the computer can begin “planting” them. This step involves determining a specific location within the polygon and sizing the snapshot based on the tree’s distance from the viewpoint. Most often trees are randomly placed however clumping and compaction factors can be used to create clustered patterns if appropriate.
<![if !vml]><![endif]>Figure 3. Tree symbols are “planted” then sized depending on their distance from the viewpoint.
Tree sizing is similar pasting and resizing an image in a word document. The base of the tree symbol is positioned at the specific location then enlarged or reduced depending on how far the tree is from the viewing position. Figure 3 shows a series of resized tree symbols “planted” along a slope—big trees in front and progressively smaller ones in the distance.
The process of rendering a scene is surprisingly similar to that of landscape artist. The terrain is painted and landscape features added. In the artist’s world it can take hours or days to paint a scene. In virtual reality the process is completed in a minute or two as hundreds of trees are selected, positioned and resized each second.
Since each tree is embedded on a transparent canvas they obscure what is behind them—textured terrain and/or other trees, depending on forest stand and viewing conditions. Terrain locations that are outside of the viewing window or hidden behind ridges are simply ignored. The multitude of issues and extended considerations surrounding virtual reality’s expression of GIS data, however, cannot be ignored. That discussion is reserved for next month.
Author's Note: the Landscape Viewer module of the Virtual Forest software package by Pacific Meridian Resources was used for the figures in this column. See http://www.innovativegis.com/products/vforest/ for more examples and discussion.
Previous columns (GEOWorld, ?? and ??) described the steps in rendering a virtual landscape. The process begins with a 3D drafting program used to construct mathematical representations of individual scene elements similar to a painter’s sketches of the different tree types that occur within an area. The tree library is linked to GIS data describing the composition of each forest parcel. These data are used to position the polygon on the terrain, select the proper understory texture (“laying the carpet”) and paste the appropriate types and number of trees within each polygon (“pouring the trees”).
The result is a strikingly lifelike rendering of the landscape instead of a traditional map. While maps use colors and abstract symbols to represent forest conditions, the virtual forest uses realistic scene elements to reproduce the composition and structure of the forest inventory data. This lifelike 3D characterization of spatial conditions extends the boundaries of mapping from dry and often confusing drawings to more familiar graphical perspectives.
<![if !vml]><![endif]>Figure 1. Changes in the landscape can be visualized by modifying the forest inventory data.
The baseline rendering for a data set can be modified to reflect changes on the landscape. For example, the top two inserts in figure 1 depict a natural thinning and succession after a severe insect infestation. The winter effects were introduced by rendering with a snow texture and an atmospheric haze.
The lower pair of inserts show the before and after views of a proposed harvest block. Note the linear texture features in the clearcut that identify the logging road. Alternative harvest plans can be rendered and their relative visual impact assessed. In addition, a temporal sequence can be generated that tracks the ‘green-up” through forest growth models as a replanted parcel grows. In a sense, the baseline GIS information shows you “what is,” while the rendering of the output from a simulation model shows you “what could be.”
While GIS modeling can walk through time, movement to different viewpoints provides a walk through the landscape. The viewer position can be easily changed to generate views from a set of locations, such as sensitive viewpoints along a road or trail. Figure 2 shows the construction of a ‘fly-by” movie. The helicopter flight path at 200 meters above the terrain was digitized then resampled every twenty meters (large red dots in the figure). A full 3D rendering was made for each of the viewpoints (nearly 900 in all) and, when viewed at 30 frames per second, forms a twenty-eight second flight through the GIS database (see author’s note).
Admittedly, real-time “fly-bys” of GIS databases are a bit futuristic. With each scene requiring three to four minutes to fully render on a PC-level computer, a 30 second movie requires about 45 hours of processing time. The Lucas Films machines would reduce the time to a few minutes but it will take a few years to get that processing power on most desktops. In the interim, the transition from traditional maps to fully rendered scenes is operationally constrained to a few vanguard software systems.
<![if !vml]><![endif]>Figure 2. A “fly-by” movie is constructed by generating a sequence of renderings then viewing them in rapid succession.
There are several concerns about converting GIS data into realistic landscape renderings. Tree placement is critical. Recall that “stocking” (#trees per acre) is the forest inventory statistic used to determine the number of trees to paste within a polygon. While this value indicates the overall density it assumes the trees are randomly distributed in geographic space.
While trees off in the distance form a modeled texture, placement differences of a couple of feet for trees in the foreground can significantly alter a scene. For key viewpoints GPS positioning of specific trees within a few feet of the viewer is required. Also, in sequential rendering the trees are statistically placed for the first scene then that “tree map” is used for all of the additional scenes. Many species, such as aspen, tend to group and statistical methods are needed to account for “clumping” (number of seed trees) and “compaction” (distance function from seed tree).
<![if !vml]><![endif]>Figure 3. Strikingly real snapshots of forest data can be generated from either limited or robust GIS data.
Realistic trees, proper placement, appropriate understory textures and shaded relief combine to produce strikingly real snapshots of a landscape. In robust, forest inventory data the rendering closely matches reality. However, equally striking results can be generated from limited data. For example, the “green” portions on topographic maps indicate forested areas, but offer no information about species mixture, age/maturity or stocking. Within a GIS, a “best guess” (er… expert opinion) can be substituted for the field data and one would be hard-pressed to tell the differences in rendered scenes.
That brings up an important point as map display evolves to virtual reality—how accurate is the portrayal? Our cartographic legacy has wrestled with spatial and thematic accuracy, but “rendering fidelity” is an entirely new concept. Since you can’t tell by looking, standards must be developed and integrated with the metadata accompanying a rendering. Interactive links between the underlying data and the snapshot are needed. Without these safeguards, it’s “viewer beware” and opens a whole new avenue for lying with maps.
While Michael Creighton’s emersion into a virtual representation of a database (the novel Disclosure) might be decades off, virtual renderings of GIS data is a quantum leap forward. The pastel colors and abstract symbols of traditional maps are becoming endangered cartographic procedures. When your grandchild conjures up a 3D landscape with real-time navigation on a wrist-PC, you’ll fondly recall the bumpy transition from our paper-map paradigm.
Author's Note: the Virtual Forest software package by Pacific Meridian Resources was used for the figures in this column. See http://www.innovativegis.com/products/vforest/, select “Flybys” to access the simulated helicopter flight described as well as numerous other examples of 3D rendering.