Multiviewangle Effects

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Multiview-angle effects are everywhere. Consider a newly mowed lawn or harvested crop field. Distinct stripes are visible both on the ground and from the aerial vantage (Fig. 4-3). These stripes reflect passage of the mower back and forth across the field such that the grass or crop stubble is bent at opposed angles for alternate stripes. As this example demonstrates, multiangular reflectance is a basic property of the natural world, and this has many implications for small-format aerial photography. Directional reflectance anisotropy represents the sum of differential reflection by objects that have complicated three-dimensional geometry (Lucht, 2004). Any scene is composed of certain geometrical properties in terms of the sizes, shapes, angles, and positions of reflective elements, such as tree and grass leaves, water surfaces, roof tiles, beach pebbles, glacier ice crystals, and so on. Furthermore, each of these

Small-Format Aerial Photography

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FIGURE 4-1 Chalk buttes surrounded by short-grass prairie in the Smoky Hill River valley of western Kansas, United States. (A) Low-oblique view showing cluster of buttes with an arch. Individual buttes stand 10-15 m high; road to lower right is ~6 m wide. (B) Vertical view of same butte cluster. Note shadows and light coming through the arch in the butte wall. Shadow of a person is visible just above the arrow (A). Kite aerial photographs (Aber and Aber, 2009, fig. 35).

FIGURE 4-1 Chalk buttes surrounded by short-grass prairie in the Smoky Hill River valley of western Kansas, United States. (A) Low-oblique view showing cluster of buttes with an arch. Individual buttes stand 10-15 m high; road to lower right is ~6 m wide. (B) Vertical view of same butte cluster. Note shadows and light coming through the arch in the butte wall. Shadow of a person is visible just above the arrow (A). Kite aerial photographs (Aber and Aber, 2009, fig. 35).

geometric elements reflects certain wavelengths of light more strongly than others (see Section 4.4 below).

Vegetation has particularly complex reflectance geometry. Consider, for example, a forest. Some sunlight is reflected directly from the leaves in the crown of the canopy, some is reflected from middle levels, and some light penetrates to the forest floor and is reflected upward through the canopy. Volumetric scattering and transmission by leaves send some radiation off in other directions within the canopy, where further scattering, transmission, or absorption may take place.

Another strong influence on reflected radiation is shadow casting. Trees, grass, soil clumps, pebbles, or other irregularities of the surface cast shadows. Each discrete object has an illuminated (bright) side and a shadowed (dark) side. The sizes, shapes, and spatial arrangement of shadows have a strong influence on overall scene brightness, and depend on the angle of viewing in relation to the sun

FIGURE 4-2 Diagram of aerial photography and typical BRDF. Amount of reflectivity in the solar plane is indicated by the black oval. Maximum reflectivity occurs directly back toward the sun. Illustration not to scale; adapted from Ranson et al. (1994, fig. 1).

position. These factors give rise to two special lighting effects—sun glint and the hot spot, both of which are observed in the solar plane.

Sun glint is direct, specular, forward reflection from an optically smooth (mirror) surface, such as water, glass, or metal, in which the angle of incident sunlight equals the angle of reflection (Fig. 4-4). This phenomenon is also called the sunspot or solar flaring (Teng et al., 1997). The most common reflective materials involve water bodies and man-made metal structures. Water surfaces with ripples or waves may produce a variant called sun glitter, in which each wave surface forms a small reflecting facet (Fig. 4-5). Sun glint is quite common in oblique views taken toward the sun; it is seen less often in vertical views.

Sun glint and glitter are observed in small-format aerial photographs frequently compared to traditional large-format images, because of the wider field of view and

Lawn Aerial View Vertical
FIGURE 4-3 Vertical photograph of lawn with mowed grass. Note linear mowing pattern. The large ''birdie'' is an outdoor sculpture on the campus of the Nelson-Atkins Museum of Art, Kansas City, Missouri, United States. Helium blimp aerial photograph by JSA, April 2005.
Fov Sun Glint

FIGURE 4-4 Geometric elements for specular reflection of sun light from a horizontal surface in a wide-angle vertical photograph. The angle at the focal point (Ssp) is equal to the solar zenith angle ($z), which is the complement of the sun angle at the specular point (SP). If this angle (Ssp) is less than half the angular field of view (FOV), sun glint could appear in the image. Modified from Mount (2005, fig. 2).

FIGURE 4-4 Geometric elements for specular reflection of sun light from a horizontal surface in a wide-angle vertical photograph. The angle at the focal point (Ssp) is equal to the solar zenith angle ($z), which is the complement of the sun angle at the specular point (SP). If this angle (Ssp) is less than half the angular field of view (FOV), sun glint could appear in the image. Modified from Mount (2005, fig. 2).

rectangular-format cameras often employed for SFAP (Mount, 2005). The occurrence of sun glint in vertical views increases for photographs taken in late spring or early summer when the sun is highest in the sky or in imagery from low latitudes where the sun is always high overhead. In extreme cases, views taken toward the sun may display internal lens reflections, if sunlight strikes the lens directly (Fig. 4-6). This can happen in high-oblique views when the sun is low in the sky.

The hot spot is the position on the ground in direct alignment with the sun and camera. The hot spot is located at the antisolar point, which is the point on the ground opposite the sun in relation to the camera (Lynch and Livingston, 1995; Teng et al., 1997). The hot spot appears brighter than its surroundings, as if that position is overexposed in the image (Fig. 4-7). The hot spot is also known as the opposition effect or shadow point, because the shadow of the platform that carried the camera may appear at the center of the hot spot (Fig. 4-8; Murtha et al., 1997).

The hot spot phenomenon has received a great deal of investigation in recent years, and most agree that its primary cause is a result of shadow hiding (Hapke et al., 1996; Leroy and Breon, 1996; Lucht, 2004). The absence of visible shadows causes the spot to display substantially brighter than the surroundings in which shadows appear (Fig. 4-9). This effect derives from shadows cast by objects of all sizes, from sand grains on a beach to forest trees. Furthermore, the color of the hot spot differs from that of the surroundings (Lynch and Livingston, 1995). This is because scattered

FIGURE 4-5 Sun glint and glitter. (A) High-oblique view of sun glint from lake surface in left background and from metal roof in right foreground. Lake Kahola, Kansas, March 1997. (B) Low-oblique view of sun glint from smooth water (*); sun glitter from ripple and wave surfaces elsewhere in scene. South Padre Island, Texas, October 2005. (C) Vertical view of sun glitter from rippled stream surface upper left; sun glint from steel track of railroad (A). Palisades State Park, South Dakota, July 1998. Kite aerial photographs by JSA and SWA, United States.

FIGURE 4-5 Sun glint and glitter. (A) High-oblique view of sun glint from lake surface in left background and from metal roof in right foreground. Lake Kahola, Kansas, March 1997. (B) Low-oblique view of sun glint from smooth water (*); sun glitter from ripple and wave surfaces elsewhere in scene. South Padre Island, Texas, October 2005. (C) Vertical view of sun glitter from rippled stream surface upper left; sun glint from steel track of railroad (A). Palisades State Park, South Dakota, July 1998. Kite aerial photographs by JSA and SWA, United States.

FIGURE 4-6 The streak of bright octogons was created by internal reflections in the lens from direct sunlight. Winter image taken in late afternoon looking toward the sun. Kite aerial photograph by SWA and JSA; Lake Kahola, Kansas, United States, December 2002.
FIGURE 4-7 Hot spot displayed in a harvested agricultural field. Note bright spot next to arrow (>). Kite aerial photograph by D. Galazka and JSA; Mlawa, Poland, September 1998.
FIGURE 4-8 Antisolar point marked by shadow of a small helium blimp in lower-right portion of this vertical view of a formal rose garden. Image taken by JSA; Loose Park, Kansas City, Missouri, United States, June 2006.

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FIGURE 4-9 Schematic illustration showing the vertical view of shadows cast by objects around the antisolar point at scene center. The lack of visible shadows at the antisolar point renders the scene brighter at the hot spot. The position of each shadow is a function of relief displacement (see Chapter 2.2.3). Illustration adapted from Lynch and Livingston (1995, fig. 1-5B).

FIGURE 4-9 Schematic illustration showing the vertical view of shadows cast by objects around the antisolar point at scene center. The lack of visible shadows at the antisolar point renders the scene brighter at the hot spot. The position of each shadow is a function of relief displacement (see Chapter 2.2.3). Illustration adapted from Lynch and Livingston (1995, fig. 1-5B).

blue light, which normally illuminates shadowed zones, is absent from the hot spot vicinity. Thus, the hot spot typically appears more yellow (lacking in blue). For example, dark green forest appears light yellowish green at the hot spot (Fig. 4-10).

In addition to shadow hiding, other factors may contribute to the hot spot (Lynch and Livingston, 1995). Small rounded grains (sand and pebbles) may act as minute reflectors that collect light and send it back toward the sun.

Likewise, mineral crystals within rocks may function as internal corner reflectors, and crystal faces may act as tiny mirrors. Light may be back reflected from tiny liquid droplets, such as dew or tree sap. Finally, coherent back-scatter may contribute to the opposition effect. The combination of these factors with shadow hiding creates marked hot spots in many situations for small-format aerial photography.

The hot spot is commonly observed in oblique views taken opposite the sun; it is evident less often in vertical views. As with sun glint, the presence of the hot spot in vertical views increases with use of a wide-angle lens, for late spring or early summer imagery, or from low latitudes. In the authors' experience, hot spots are most noticeable for terrain in which the ground cover is relatively homogeneous, such as forest or prairie canopy, agricultural fields, and fallow or bare ground. In these cases, the color and brightness of the subject are more-or-less uniform, so the hot spot is conspicuous. For terrain with more complex land cover, the hot spot may not be so obvious. This applies often to urban scenes that contain large variations in the intrinsic colors and brightness of objects.

The foregoing discussion suggests that the appearance of the landscape changes dramatically with different viewing directions relative to sun position. This leads to a general assessment of the visual quality of oblique or wide-angle vertical images acquired with small-format aerial photography (Aber et al., 2002). In general, better oblique images are acquired in the azimuth range 50°-160° relative to the sun position (Fig. 4-11). This viewing range represents a balance of shadows and highlights with more-or-less uniform brightness levels. Views toward the sun (<50° azimuth) suffer from excessive shadowing, high contrast, and poor depiction of color; furthermore, sun glint may be present. On the other hand, views directly opposite

FIGURE 4-11 Schematic azimuthal (plan) diagram of lighting conditions for oblique small-format aerial photography relative to the sun position. The indication of image quality according to direction is a subjective visual interpretation. Adapted from Aber et al. (2002, fig. 5).

FIGURE 4-10 Hot spot at scene center in canopy of pine-spruce forest. The hot spot has a pale yellowish-green color compared to the normal dark-green color of the surrounding forest. Kite aerial photograph by SWA and JSA; coastal Vormsi, Estonia, August 2000.

FIGURE 4-11 Schematic azimuthal (plan) diagram of lighting conditions for oblique small-format aerial photography relative to the sun position. The indication of image quality according to direction is a subjective visual interpretation. Adapted from Aber et al. (2002, fig. 5).

the sun may include the hot spot, wherein details are hardly visible due to lack of shadows. Figure 4-12 demonstrates visible changes in image quality with different viewing directions over a coastal setting in western Denmark.

Multiviewangle Effect

FIGURE 4-12 Effect of viewing direction on oblique image quality relative to azimuth of the sun. (A) View toward ~ 160°; note shadow of lighthouse. (B) View toward ~120°; note shadow of tractor to left. (C) View toward ~40°; notice sun glint along right edge. A and B depict good color with moderate brightness contrast. C is heavily shadowed and displays excessive contrast. Kite aerial photographs by IM, SWA, and JSA; Bov-bjerg, North Sea coast of Denmark, September 2005.

FIGURE 4-12 Effect of viewing direction on oblique image quality relative to azimuth of the sun. (A) View toward ~ 160°; note shadow of lighthouse. (B) View toward ~120°; note shadow of tractor to left. (C) View toward ~40°; notice sun glint along right edge. A and B depict good color with moderate brightness contrast. C is heavily shadowed and displays excessive contrast. Kite aerial photographs by IM, SWA, and JSA; Bov-bjerg, North Sea coast of Denmark, September 2005.

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