Topic: StereoscopyBy

T.SrinivasaRao

Relief displacement

from the above 2 equations,d = rxh/H or rxh/(A-E).

Definition

Stereoscopy (also called stereoscopics) is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision.(source: Wikipedia)

➔ The science of stereoscopy deals with the stereoscopic effects and techniques which produces stereoscopic effects.

➔ The effect of stereoscopy is the stereoscopic vision which enables the observer to view an object in a pair of aerial photographs in the form of a 3 dimensional model or stereoscopic model.

➔ Stereoscopic vision is more conveniently produced with the aid of an optical instrument called stereoscope.

➔ In practice stereoscopic vision is produced by viewing 2 photographs of same area taken from different angles.

➔ This results in 3 D model due to mental superposition and fusion of 2 images.

Human Eye and Stereoscopic vision

➔ Eye ball is globular in form and consists of pupil which controls the intensity of light ray.

➔ cornea and lens forms the image on the retina

➔ retina consists of millions of cones and rods which are sensitive to light.

➔ when light falls on retina, a photochemical change occurs within them, which in turn, stimulates the optical nerves and sends electrical signals to the brain.

➔ Since, 2 eyes are present depth perception is possible.

➔ The monocular vision permits the perception and determination of distances and position of objects in their relative direction in the field of view.

➔ A false depth perception is produced due to varying object sizes with varying scales at varying distances and overlapping of rear objects with the front objects.

➔ It is impossible to distinguish objects in depth, if the difference of angle of convergence is less than 20 seconds of an arc.

➔ Depending upon interpupillary distance of a person and for angle of 20 seconds the max.distance limit of depth perception varies from 1700 to 2450 ft.

The limiting distance is called as radius of stereoscopic perception.

Stereoscopy - observations till now

- Deals with the use of binocular vision for achieving 3-D effects.

- It enables us to view an object simultaneously from 2 different perspectives.

- 2 aerial photographs taken from 2 different points are normally used in RS.

Stereoscopic pair➔ Stereoscopic pair of photographs consists of 2 adjacent,

overlapping photos in the same flight line.➔ Stereoscopic view is observed only in the overlapped

area.➔ A minimum overlapping of 50% is required for complete

stereoscopic coverage.➔ As a safety factor additional 10% is considered usually.

Stereogram

➔ In a stereopair, if we cut common area (overlapped) in each photograph, and then properly orient and place them side by side a stereogram will be formed.

Stereogram

Stereoscope

➔ To achieve stereoscopic vision the binocular optical instrument used is called as stereoscope.◆ lens stereoscope◆ mirror stereoscope◆ scanning stereoscope◆ zoom stereoscope

Geometry of Stereoscopy

➔ In order to stereoscopic vision, it is important to have properly oriented photographs that are aligned in the geometry when the images were taken.◆ coordinate axes◆ absolute parallax◆ flight line location

Coordinate axes

➔ Geometry of overlapping photographs is different from single vertical photograph.

➔ In single vertical photograph x, y axes are defined by fiducial marks.

➔ For 2 successive photos in the flight line x-axis is determined by the line joining principal point and its conjugate principle point.

cont..➔ y-axis is the line that passes through the

principal point and is perpendicular to the x-axis.

➔ This is called as the flight line system of the of coordinates for stereoscopic pairs of photographs compared to the fiducial system used in single vertical photographs.

cont...

➔ If we take a stereo triplicate, the center photograph will have 2 sets of coordinate systems if the aircraft does not fly in the straight line.

Absolute Parallax

➔ Stereoscopy is possible because of the displacement of objects of different elevations differently in the successive photographs along the x-axis.

➔ This difference in displacement is called as absolute parallax (dP).

cont...

➔ The absolute parallax of a point on a pair of overlapping vertical photographs is equal to the x-coordinate of the point measured on the left hand photograph minus the x-coordinate of the same point measured in the right photograph.

cont...

Definition: The absolute parallax is the algebraic difference, measured parallel to the line of flight (x-axis) from the corresponding y-axis to the 2 images of the point on a stereoscopic pair of aerial photographs assuming they are taken at the same altitude.

(source: David P.Paine)

Flight line location

➔ flight line passes through the principal point

➔ for stereoscopic vision minimum 50% overlap is required.

➔ principle points of adjacent photos should be imaged on each photograph.

cont...

➔ Mark principal point and conjugate principle points on each vertical photograph.

➔ By drawing a line between the principal point and conjugate principal point we can mark the flight line for that vertical photograph.

Theory of stereoscopy (ToS)

Single eye alone cannot perceive depth accurately.perception is because of association unlike the formation of stereoscopic model in binocular vision.

Accommodation and convergence

➔ The relationship of accommodation (change of focus of the eye for distance) and convergence is very important in stereoscopic vision.

➔ when our eyes focus on a nearby object they also converge, hence lines of sight from eyes intersect at the object.

➔ This is a problem in case of stereo pairs.➔ For getting stereovision, lines of sights

should be parallel.➔ This is achieved our eyes on an object at

infinity.➔ Many stereoscopes works by making use

of this principle.

Depth perception

Floating Dot Principle

Vertical Exaggeration (V.E)

➔ V.E is an important factor which makes the topography of an area look more steeper and higher.

➔ It is caused due to the differences in the ratio of the base height with that of the viewing height while viewing stereo photographs.

➔ V.E increases with the ratio of the distance (air base) between exposure stations over the flying height above the ground.

➔ V.E can be calculated by multiplying the air base by flying height ratio with inverse of eye base to apparent viewing distance ratio.

AVD is estimated as 17 inches, and EB as 2.625 inches for calculations.

1. Two ground points A and B appear on a pair of overlapping photographs, which have been taken from a height of 3650 m above MSL. The base lines as measured on the two photographs are 89.5 and 90.5 mm respectively. The mean parallax bar reading from A and B are 29.32 mm and 30.82 mm respectively. If the elevation of A above MSL is 230.35 m, compute the elevation B.

2. In the above problem if the lengths of base lines are not known and the absolute parallax of A is measured to be 89.80 mm, compute the elevation of B. Also, find the height of another point C whose parallax bar reading is 32.32 mm.