Notes for Chapter 3, Principles of Stereoscopic Vision

 

3.1 Definitions

Stereoscopic Vision – the use of binocular vision to achieve three-dimensional effects.

Enables viewing an object simultaneously from two different perspectives i.e. two aerial photos taken from different camera positions

 

Stereoscopic pair – consists of two adjacent overlapping photos in the same flight line. Need a minimum of 50% overlap, safety factor of 60% often used

 

Stereogram – cut out the part of photos in a stereopair that show some point of interest and mount them side by side (Fig. 3.1)

 

Stereoscope – a binocular optical instrument that permits viewing tow properly oriented photos to obtained the mental impression of a three-dimensional model. Four main types

 

Lens Stereoscope – two mounted magnifying lenses mounted with a separation equal to the average interpupillary distance of the human eye. (Figure 3.2) Usually adjustable. May or may not have magnification. Cheapest most convenient. Overlap may require “peeling” photos to reach area of interest

 

Mirror Stereoscope – consists of a pair of small eyepiece mirrors and a pair of larger wing mirrors, each of which oriented at a 450 angle with respect to the plane of the photos. Magnifying lenses may be attached to the eyepieces. Photos can be completely separated for viewing (no overlap) (Fig. 3.3)

 

Scanning Mirror Stereoscope – A more advanced mirror stereoscope. Has two knobs that let the viewer scan the image without moving the stereoscope or the image (Fig. 3.4)

 

Zoom stereoscopy (Fig. 3.5) provides a continuously variable in-focus magnification from 2.5 to 20 power with a single set of eyepieces. Great for viewing randomly oriented uncut film. Lenses can be adjusted.

 

3.2 Geometry of Stereoscopy

3.2.1 The Coordinate Axes

Geometry of overlapping vertical aerial photos somewhat different than geometry of  single photos: X and Y axes not defined by the fiducial marks when crab exists

 

Flight-line system of Coordinates – The X axis is the line that passes through the PP and the CPP. The Y axis is the line that passes through the PP perpendicular to the X.

 

Fiducial mark system – uses just the PP to align the axes on a single photo

 

Only if both photos are free of tilt, drift and crab will the x and y axes pass through the side fiducial marks. (Figure 3.6)

 

In a stereo triplicate the center photo may have two different sets of x and y coordinates if  aircraft does not fly a perfect line. (Figure 3.7)

 

3.2.2 Absolute Parallax

 

Why is stereoscopy possible? Images of points lying at different elevations have been topographically displaced by different amounts along the x axis on successive photos

 

The displacement is known as difference in absolute parallax.

 

Absolute parallax of a point is the algebraic difference, measured parallel to the line of flight from the corresponding y axes to the two images of the point on a stereoscopic  pair of aerial photos. (Figure 3.8) Will be used later to measure heights from aerial photos.

 

3.2.3 Flight-Line Location

 

Flight line passes through the PP of each photo.

PP of one photo located on the successive point on the photo is the Conjugate Principal Point (CPP) (Figure 3.9)

 

Connecting the PP and CPP on a photo gives the flight line. Should use the nadir in less than vertical photos, but difficult to define.

 

3.3 Theory of Stereoscopy

 

Single eye can’t perceive depth (pencil demonstration). Need a second eye to see depth.

 

3.3.1 Accommodation and Convergence

Accommodation – the change of focus of the eye for distance

 

Convergence – when eyes focus on a nearby object they converge so that lines of sight join at the object.

 

Use the sausage exercise to make the lines of sight parallel (focus on infinity) and still see sharply. (Figure 3.10)

 

3.3.2 Depth Perception

Look at (Figure 3.11) If the left eye sees only the left image and the right eye sees only the right image, the object (Washington Monument) will appear in 3-D.

 

3.3.4 The Floating Dot Principle

 

See Figure 3.12 A dot can be made to  appear to move up and down. More when we look at object heights.

 

3.3.4 Vertical Exaggeration

Figure 3.13 Shows stereo triplicate with higher exaggeration on the right image than the left.

 

Vertical exaggeration increases with the ratio of the distance between exposures (the airbase) over the flying height above the ground Leave the math for now. Skip the slope estimator for now.

3.3.5 The Pseudoscopic Stereo Model

Accidentally reversing the two photos so that the right eye views the left and the left eye views the right. Reverses the relief of the landscape. (Figure 3.16) Sometimes done on purpose to better see drainages

 

3.4 Proper Orientation of a Stereo Model

  1. Make sure the photos are consecutively numbered and n the same flight line.
  2. Locate and mark the principal point on both photos
  3. Prick with a needle or sharp pin. Marking with a Sharpie works too.
  4. Locate and mark the CPP using stereoscopic viewing. Gives four points between the two photos
  5. Orient the photos so that all the four points are on a single straight line, separated by a comfortable  viewing distance.

 

Best if shadows fall towards you. If not rotate the photos by 180o

Once in place fasten the photos with drafting or masking tape.

 

May have to change which photo is on top if using a lens stereoscope

(Figure 3.17)

 

3.5 Stereoscopic Viewing without a Stereoscope

Skip for now.

3.6 Test for Stereoscopic Perception

(Figure 3.18)