Figure 5.1 Sunlight diminished by clouds and haze encircling distant mountains gives rise to strong silhouettes of trees in foreground, in a landscape outside of Athens, Greece.
5.2.1 Sunlight
5.2.2 Moonlight
5.2.3 Night Lighting
5.6.1 Haze and Fog
5.6.2 Rain
5.8.1 Solar Motion
5.8.2 Wind
Figure 5.3 a. The sun in the sky, rendered in VRML.
Figure 5.10 a. Clouds seen form overhead, from an airplane. b. Clouds at sunset.
Landscape modeling and visualization requires attention, beyond the constituent elements (landform, plants, and water) to the two essential ingredients that shape how the landscape looks and feels: the atmosphere, and the light that passes through it. Sunlight, moonlight, or streetlights (or the lack of them) have everything to do with how we see the landscape and its elements. Horizontal rays of golden sunlight on waving fields of grain; leaden gray clouds over rolling hills of heath and moss, enveloped in hazy gray fog; moonlight reflected beneath a stone bridge over a still pond; fluffy white clouds in a clear blue sky - these are all images whose metaphorical "atmosphere" depends on the literal atmospheric and lighting conditions which produce them.
...(Note: This Website contains abbreviated text. For
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The overriding factor in any computer rendering - as in painting, or photography - is use and control of lighting. In the natural landscape, most lighting, most of the time, is provided by the sun, modulated by clouds and filtered by vegetation. Light, or its absence, creates or prevents shadows in the landscape, which constitute an essential depth-cue and textural element in any scene. In built landscapes, including urban scenes, natural light may well be augmented by artificial lights, spotlights and floodlights, streetlights and reflected neon signs, which create emphasis and color. Moonlight has different qualities yet - colder, bluer, less intense, because of its essential reflected nature. Cloud conditions vary from none, in completely clear skies, with their attendant crisp shadows and high-contrast lighting conditions, through a wide range of cloud forms and textures, casting moving shadows and dappled sunlight on the landscape below, to completely overcast conditions, giving rise to shadowless, low-contrast, almost monochromatic landscape conditions. Lights in the landscape sparkle and reflect around water, gleam and disappear through the branches and foliage of trees and shrubs, disappear altogether into deep fog on the seashore or in river valleys.
Figure 5.1 Sunlight diminished by clouds and haze encircling distant mountains gives rise to strong silhouettes of trees in foreground, in a landscape outside of Athens, Greece.
...(Note: This Website contains abbreviated text. For
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book.)
Figure 5.2 View of University Commons landscape produced
with: a. Ambient light only, no directional light; b. Direct light, with shadows
(note that steps and other features become visible with directional light).
The primary source of light for most landscape images is Sol, the earth's sun. Often naively drawn as a yellow ball, the light from the sun is naturally white, a combination of all colors, which are revealed when broken up by a prism into a rainbow, or spectrum. The sun is sufficiently far away from the earth that its light rays are essentially parallel when striking the landscape. The sun's position in the sky (to the south, in the northern hemisphere, and to the north, in the southern hemisphere) is determined by the viewer's location on the planet (in latitude and longitude), by the season, due to the annual revolution of the earth around the sun, and by the hour of day, due to the daily rotation of the earth on its axis.
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Figure 5.3 a. The sun in the sky, rendered in VRML.
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Technical note The actual position of the sun in the sky, and hence the direction of its parallel rays on a landscape scene, is controlled by celestial mechanics and the combination of daily rotation, annual revolution, and tilted axes of the earth. Given the latitude and longitude of the landscape project, and the date and time of day, the sun's position can be simply calculated in terms of azimuth (compass degrees, from 0¡or 360¡ north through 180¡ south) and elevation (in degrees above the horizon, from 0¡ at sunset or sunrise, to 90¡, at noon on the equator). These values can be found in printed tables, computed by simple trigonometric formulae or special software, and are often built-in to modern rendering software packages. (See also the note in the next section, on Shadows.) |
Absent the sun, the landscape may be illuminated - on some nights each month, anyway - by light from Luna, the earth's moon. This is just sunlight, reflected by the reflective surface of the moon, and is most visible and effective in the landscape when the moon is full or very nearly full. Then it casts light strong enough to cast shadows, and bright enough to see by. The light is much cooler, and bluer, than sunlight, because of scattering of the long wave (reddish) frequencies of the light. (In fact, the moon's surface is a slightly brown color, but that is only revealed through a telescope.) The full moon on a clear night provides only about 1 millionth as much light as the sun on a day (less than 1 lux - lumens per square meter - for the moon, versus over 100,000 lux for sunlight), and there is far less ambient light at night as well. (It's remarkable testimony to the sensitivity of the human eye that you can see well enough to read by over a range of one million to one!)
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Figure 5.4 a. Image of the full moon, from the Lick
Observatory. b. Rendering of a 3D model of the moon, reflected in a pool of
water under a bridge.
Landscapes are seen and experienced at night, even without moonlight, by the aid of artificial lighting. Accurate rendering of artificial light conditions, depending on the style of fixture(s), type of bulbs, down to manufacturer and model number, is possible only with suitably advanced and specialized illumination software. Most rendering systems are not so specific, nor so accurate, and instead may merely enable you to specify materials as self-illuminating, so that they appear like lamps. This can be quite satisfactory for some night lighting images; sometimes, you will want to insert small lights into these objects in the model, so that they actually cast light, and so can cast shadows.
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the complete text,
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Figure 5.5 Rendering of the University Commons landscape at night, with up-lighting in the trees at the apex of the spiral mound.
Shadows are really just the perception of unlit areas, in contrast with lit areas, from differential illumination and from one object obscuring another's light. A bright light source casts a shadow of an opaque object because a zone behind the object - called the umbra of the shadow - intersects some shadow-receiving surface. The outline of the shadow is the profile of the object, seen from looking back directly at the light source; the size of the shadow grows with distance between the shadow-casting object and the shadow-receiving surface. Technically, only bright point lights, like a small lightbulb, create pure umbras; distant light sources, like the sun, or continuous ones, like a bank of lights, create intermediate zones of partial darkness - called the penumbra - giving rise to shadows with soft, or fuzzy, edges.
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the complete text,
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Figure 5.6 Photograph of theferry quai, Rapperswil , Switzerland. The dramatic forms of the "coppiced" linden trees is emphasized by lake fog (giving bright white background) and direct sunlight, casting strong shadows on paving in foreground.
Figure 5.7 Landform rendered with a. sun from the
North; b. Sun from the South. To many people, the image on the left seems "correct",
in the image on the right the terrain appears inverted, like a plaster mold
of the real terrain.
Figure 5.8 University Commons landscape, perspective
views with plan superimposed in background, showing: a: Shadows cast in mid-afternoon;
b: Elongated shadows in late afternoon, emphasizing the rolling landform.
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Technical note In overhead plan views, shadows are often shown cast to the lower right corner, from an implied light source in the upper left hand corner. This is because the human visual system finds images most comfortable when shadows are "down" in the visual field, as happens in the real world. In fact, with the conventional "north up" on a plan drawing, shadows to the lower right are next to impossible, as the sun is only in the northwest late in the afternoon, and then usually low to the horizon, casting quite elongated shadows if at all. When the only purpose of shadows on a plan drawing is to give a sense of depth, this is acceptable practice, but when real lighting and shadow conditions are being portrayed, it is important to locate the sun correctly (typically in the southern sky, in the northern hemisphere) and to correctly portray the shadows cast by objects in and on the landscape. |
Just as every building is surrounded by landscape, so every landscape is surrounded by sky. Sometimes referred to - and modeled as - a bowl, or hemisphere, the sky is visible directly overhead, unless obscured by building or tree, and in 360¡ around, at eye level, unless blocked by landscape elements, such as landform, buildings, or vegetation. Although ordinarily thought of as "blue," the sky in fact is capable of many appearances, depending on atmospheric conditions. The blue of the sky, which arises because of differential refraction in the atmosphere, which scatters blue light more thoroughly than other frequencies, has gradations and mottlings, modified by the position of the sun, and the quantity and form of water vapor in the air.
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Figure 5.9 a. The clear blue sky, with some brightening
towards the horizon. b. A hemispherical dome used as the sky in a 3D landscape
model.
The appearance of the sky is in large part determined by the presence or absence of clouds. A clear blue cloudless sky, a blue sky with fluffy white clouds, a leaden gray sky with layered dark stratus clouds, or a bank of fog reducing visibility to tens of meters or less, are all variants that arise from clouds. In nature, clouds are aggregations of water vapor and ice crystals suspended in air, held (loosely) together by their humidity and the tendency of air masses to move as a unit. Their edges are ragged, where the water can move back into vapor in the air, but seen from afar, from the ground, may appear quite smooth, bulbous and fluffy.
... (Note: This Website contains abbreviated text. For
the complete text,
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book.)
Figure 5.10 a. Clouds seen form overhead, from an
airplane. b. Clouds at sunset.
Figure 5.11 Various photographic cloud images are
an essential element in the digital image collection for landscape modelers.
Figure 5.13 Procedurally generated clouds: a) Fluffy
cumulous cloud form; b) Orange-tinted alto-cumulous clouds. Courtesy of David
Ebert, University of Maryland.
Figure 5.14 Computer-generated clouds over a terrain texture-mapped with a satellite image. Courtesy of the Remote Sensing Laboratory, University of Zurich.
Models of landscape cannot really incorporate weather; and models of weather are orders of magnitude more complex than the most complex landscape. Nonetheless, the most visible artifacts of weather, fog and rain, haze and rainbows, can appear in the landscape and can be used to add depth and realism to renderings.
The real atmosphere is full of aerosol particles, including dust, ice crystals, water vapor, and air pollution, which under varying conditions give rise to a range of visual effects that affect the perception of the landscape. Perhaps the most common is atmospheric haze: the apparent blueing and fading of objects further from the viewer. This aerial perspective is seen most vividly in long distance landscape views over several miles, or more. In nature, it is due to the effect of airlight - sunlight that is scattered, and made bluer, by water molecules in the air - through which the distant mountains are seen. Additional particles, such as fog, or smoke, add additional opacity, although usually of a whiter, or grayer (or yellow-brown, etc.) color.
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the complete text,
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book.)
Figure 5.15 A mountainous terrain, rendered with fog and haze Constant fog fades to white in the background; "volumetric" fog appears in mid-ground and settled in the valley in center.
Landscapes are often represented under bright blue, sunny skies, but there are both times and places for which that is not the most appropriate. All living landscapes need some rain - even deserts - and some landscapes are the more beautiful for it. Rain tends to be associated with overcast skies and gray clouds, naturally enough, but it can also bring a luminous quality to the landscape, because of the increased reflectivity of wet surfaces.
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the complete text,
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book.)
Figure 5.16 Simulation of rain by particle system producing "droplets" from overhead; also graying of sky and thickening of fog.
Landscapes, and landscape representations including digital renderings as well as sketches, photographs, and paintings, can range in their coloration from monochromatic and muted to polychrome and exuberant. Choosing colors in a model is a matter of considering the purposes and uses of the renderings to be produced. If "realism" is important, then a careful selection of colors, usually from accurate photographic sources, is essential. If "expression" is the goal, or "atmosphere," then choosing colors is at first a subjective exercise, followed perhaps by some consideration of basic color or perceptual theory, choosing harmonizing, or contrasting colors and color combinations.
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the complete text,
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book.)
Figure 5.17 a. Colors of fall foliage in a New England
wetland landscape. b. The same image in grayscale.
Figure 5.18 A synthetic snowy, mountainous landscape
under varying lighting and atmospheric conditions: a. Clear blue, no shadows;
b.Clear blue, directional shadows; c. Overcast and hazy; d. Overcast, red/orange
sunset sky color. Modeled and rendered in Natural Scene Designer.
Like the rest of the landscape, the atmosphere is dynamic, constantly changing, on time scales from second-to-second, through diurnal, to yearly and longer. Modeling these changes in any realistic physical way is a complex task, well beyond the scope of most computer systems, and this book. The interactions of molecules of air and water are far too numerous and complex to begin to simulate without enormous computational resources and scientific knowledge.
... (Note: This Website contains abbreviated text. For
the complete text,
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The sun's position in the sky is changing all day long, along a mathematically controlled circular arc, giving rise to long shadows from the east in the morning, small shadows directly underneath at noon, and long shadows again at sunset, from the west. Sometimes it can be useful to animate the shadow conditions over the course of a day, to study their impact on playspaces, gardens, etc. This can be done in any system which provides parametric control of the sun (light) position; otherwise, you might have to manually set up a series of positions (keyframes) along the arc, and have the system linearly interpolate between them.
Click here to see the example animation of shadow motion.
Figure 5.19 The arc of the sun shown in the summer time (higher) and winter time (lower) over a draped terrain model.
An important contributor to apparent motion in the landscape is the motion of the atmosphere itself, in the form of wind. Not visible by itself, the wind's effects are most apparent on vegetation and foliage, causing leaves to sway and rustle, and larger limbs and trunks to move visibly, rhythmically, when the wind is strong enough. Snow and rain may also reveal the motion of wind.
... (Note: This Website contains abbreviated text. For
the complete text,
click here to order the Landscape Modeling
book.)
Figure 5.20 The right hand portion of this image illustrates
the parametric tree under a severe wind condtion compared to the left hand protion
of the image which shows the parametric tree in an idle state. (Note: The wind
condition is exaggerated for the purposes of the illustration. The animation
on the Website mimics a gentle breeze.)
Atmospheric effects - lighting, sky and clouds, wind motion - can give life and a sense of realism to landscape models and renderings. Getting them physically, environmentally, and meteorologically correct is too hard for most systems, but getting them convincing, and expressive, is just a matter of taking time to adjust parameters. Often observing carefully in the real landscape is the best way to get ideas and visual precedents for modeling and rendering the atmosphere.
(Note: This Website contains abbreviated text. For the
complete text,
click here to order the Landscape Modeling
book.)
Birn, Jeremy. [digital] Lighting & Rendering. Indiana: New Riders Publishing, 2000.
Lynch, David K., and William Livingston. Color and Light in Nature. Cambridge, England: Cambridge University Press, 1995.
Figure 5.21 Four images of an artificial landscape
under different lighting and atmospheric conditions. Modeled and rendered in
RealSoft3D, courtesy of Damian Sainsbury.
Figure 5.22 "Moon Lake" created with World Construction Set. Courtesy of Adam Hauldren, N-Light Ltd., Copyright 2000.