7.3 Softcopy-Based Systems 215

Figure 7-12 Softcopy photogrammetric workstation. Note special cursor for 3-Dmeasuring mark motion. and active stereo glasses. Courtesy of Z/I Imaging.

The softcopy equivalent of point marking during multi-image triangulation issaving and labeling a small subimage around the pass point or tie point of interest.Single image displays can present enhanced capabilities in a number of ways. If theimage displayed is rectified or orthorectified, then ground point planimetric positionscan be displayed and recorded directly. This setup can be used to implement a heads-up digitizing system, in which one extracts point, line, or area features directly froma digital orthophoto. An equivalent functionality can be implemented using a mono-plotter approach, in which one views raw imagery, but employs the image orientationparameters to track the intersection of the ray defined by the cunent cursor positionwith a digital elevation model. thereby obtaining rigorous 3-D ground coordinatesfrom a single image. Intersecting a ray with a complex 3-D surface can be slow. In-tersecting a ray with a 2.5-0 surface (a OEM) is faster, but can still present someproblems if occlusions are not handled properly (Section 5.4).

7.3.2 Stereo Environment

One of the difficult aspects of classical analog and analytical stereo plotters has al-ways been the training of new operators. Hardcopy stereoplotter viewing optics areconfigured for a single individual, and adding a second set of training optics is ex-pensive and inconvenient, if possible at all. This is an area where softcopy stereo

216 Chapter 7 Photogrammetric Instruments

C •

IPn>jo<:I:""'.fI'i

1-:'-RIgllt: z_'.•••9tao1Olg~-9W1lng~-

J"l\1eW1d

IUnIts:-., Ie-a: 19R

1"HI'- ..

.J

'l. '

Figure 7-13 Screen print from softcopy photogrammetric workstation. Courtesy of LH Systems.

has a distinct advantage. Three of the four technologies for presenting softcopy ste-reo, which will be discussed later, permit multiple viewers to observe the stereomodel simultaneously. This opportunity for discussion and collaboration whileviewing the same 3-D model is a significant value of the softcopy stereo environ-ment. Training new operators in the use of stereo, in particular, benefits from thispossibility. Other ambiguous measurement and interpretation tasks also benefit inthe same way.

Analog and analytical stereoplotters with oriented imagery and image rotationoptics present parallax-free views of the model in the vicinity of the two projectedfloating half-marks. Softcopy stereo viewers can either pan and scroll the imageryover fixed floating marks, or pan and scroll the floating marks over fixed imagery.Real-time image rotation is not usually provided by image display hardware.Therefore some preprocessing is usually necessary to align the photo base with theviewing setup. Softcopy systems use the method of image normalization or pair-wise rectification, which guarantees that conjugate points have zero y-parallax.This permits the left and right images to be presented with only the height compo-nent, or x-parallax, unresolved. Images that are taken with nearly parallel view di-rections can be pairwise rectified or normalized. This process is also calledepipolar resampling. Figure 7-14 illustrates the geometry of the image normaliza-tion process. For clarity the sketch shows photographs that are not nadir looking,but the mathematical approach is general.

In Fig. 7-14, PI and P2 represent the original photograph pair and NI and N2 rep-resent the normalized photograph pair. The transformation between the two will be

7.3 Soflcopy- Based Systems 217

-)- ----__ - -- - ~_ _ ~=-- - - - B~

.•.. -----

\\\\\

Y,. Z"

XI'Object spacecoordinate

system

Figure 7-14 Geometry of the image normalization process.

derived. The rotation matrices that relate the object space (axes parallel to Xl" Yl., Zl')and the two original photographs are M] and M 2' First rotate X" to lie in the vel1icalplane through the base.

Then make the once rotated X; parallel to the base.

(7-7)

(7-8)8, = tan-I (J ~ B~ 2)Bx + By

Make the twice rotated Z:.' close to the original direction of view (this step is notunique).

WI + W2() = ----x 2 (7-9)

(7-11 )

(7-12)

218 Chapter 7 Photogrammetric Instruments

The w terms in this equation are interpreted as teI1iary rather than primary x-rotationsin the sequential formation of M) aradM2.

The rotation matrices corresponding to the (J,. 8,. and ()~rotations just derived areMr, M.", and M~, respectively. Their product is

M8 = Ai; My ~( (7-10)

The matrix M 8 relates the object space coordinate system and the system parallelto the normalized image coordinate system. The composite rotation matrices be-tween the original photographs and the normalized photographs are

MN =M8MTI

MN2=M8MJ

Coordinates (xp, Yp) in the original photographs are transformed into their normalizedcounterparts (xN, YN) by

mN xp+mN Yp+mN (-f)- f II 12 I)XN - - -----------=-m N x p + m N ,Y p + mN, ( - f)31 3_ .)

_ fmN21xp + mN22Yp + mN2J( -f)YN - - -----------

mN31xp + mN32Yp + mNJ3( -f)

Note that for simplicity we are assuming that (xo, Yo) == (0, 0). Coordinates in the nor-malized photographs are transformed into their counterparts in the original photo-graphs by

( ,

I····

(7-13)

Equations 7-12 can be used to find the limits of coverage in the normalized system.A grid of appropriate spacing is then defined in the normalized system and the re-sampling is carried out using Eq. 7-13 to actually create the normalized image.Fractional values of xp and YP will necessitate interpolation in the original image toobtain a gray value or color components.

EXAMPLE 7-1 Normalization Computations

Two photographs have the following exterior orientation parameters:

(Xv Yv ZL) I = (5000, 5000, 610) meters

(w, 4>, K), = (1.4145001, 1.414070, 44.982543) degrees

(Xv Yv Z,J2 = (5260, 5260, 630) meters

(w, 4>, K)2 = (-0.707143, -0.707089, 44.995636) degrees

The cumera pummetcD arc (x(), Yo' f) - (0, 0, 152.4) 111111.A gruuIIll puint 11UScOOI'llinales(5080, 5180, 50) meters.

Compute the transformation to normalize each of the photographs. Then verify that it iscorrect by computing the photo coordinates of the given point in each photograph, trans-forming each one to the normalized plane, and then showing that the normalized y coordinatesare equal.

7.3 Softcopy-Based Systems 219

SOLUTION

0.7071070.706676-0.024678

0.7071070.7069990.012341

0.03~899l0.999391J

-0.0~ 7452l

0.999848 J

rBxj [X L, - X.L] [2601B = Y - Y = 260Y L, L,

Bz ZL, - ZL, 20

Extract the wangles from the two rotation matrices under the assumption that the rotation or-der is K (primary), ¢ (secondary), w (tertiary). Note that the usual assumed order in this textis just the reverse of this. The extraction is done by expressing the matrix symbolically andsolving for the unknown angles.

Therefore, WI = 2.0 degrees, w2 = -1.0 degrees. Next, obtain the angles for the base matrixfrom Eqs. 7-7 through 7-9.

0_ = 45.0°

0" = - 3.113412°

Ox = 0.5°

Compute the base matrix and the two transformation matrices using Eqs. 7-10 and 7-11.

[

0.706063MB = M,(Ox)M,(O,)M:(O) = -0.707415

-0.032233

[

0.998524MN, = -0.000474

-0.054310

[

0.998524MN, = -0.000474

-0.054310

0.0018950.9996570.026125

-0.0009480.999657

-0.026151

0.706063 0.054312~0.706745 0.008714

-0.044574 0.998486

0.0542791-0.0261900.998182

0.05430410,0261640,998181

,f

The original photo coordinates of the given ground point are (49.843, 13.860) and (-48.418,21.285). The normalized coordinates, from Eq. 7-12, are (40.968,17.585) and (-57.530,17.585). Note that the .,.coordinates are equal as desired.

Just as in the case of the optical projection plotter. channel separation for stereoviewing within a computer environment can be achieved in several basic ways.

220 Chapter 7 Photogrammetric Instruments

1. The anaglyph approach displays the left image in blue and the right image inred (or vice versa). and the viewer uses a corresponding set of glasses. A dis-advantage of this approach is that color imagery cannot be presented since thecolor component is used for the separation. Multiple viewers are possible.

2. The split-screen approach puts the left image on the left side of the computerscreen and the right image on the right side of the computer screen and forcesthe viewer to look through an optical stereoscope to perceive stereo. This is alow-cost solution, and allows color imagery, but the field of view is cut inhalf, and, most importantly. it restricts the viewer's head motion and pre-cludes having multiple simultaneous viewers.

3. Polarization can be used, just as in the case of the optical projection plotter. Aliquid crystal panel in front of the monitor alternates between two polarizationstates and the viewer wears a pair of spectacles with cOITesponding polar-izations. Each image of the stereo pair cOITesponds to one of the polarizationstates; the images are alternately displayed on the monitor and the polariza-tion changed accordingly. In this case the active element is the screen and thepassive element is the viewer glasses. Color imagery can be used and multiplesimultaneous viewers can be accommodated.

4. High-frequency flicker is very similar to the image alternator approach used inthe early optical projection instruments. In this case a high-frequency (120 Hz)flicker between left and right images is presented on the monitor. Synchro-nized spectacles worn by the viewer function as the active viewing element, al-ternately making the left and right lens opaque, thus effecting the desiredchannel separation. Because of the synchronization required, the spectaclesmust communicate with the video driver, either via a tether cable, radio, or in-frared optical communication. This system allows color imagery and permitsmultiple viewers.

7.3.3 General Photogrammetric Workstation Capabilities

Another advantage of the softcopy stereo environment is the ease of including whatused to be called superimposition. It is often desirable to display collected vector fea-ture information overlaid on the source imagery, in stereo if possible. Such a capabil-ity was available only at the cost of enormous complexity in the optical/mechanical!analytical plotter environment, yet comes almost for free in the softcopy environment.This is very important for checking accuracy and completeness in a mapping project.

The most important capabilities of the digital photogrammetric workstation arefor the performance of tasks that were not possible on earlier platforms. Furthermore,these disparate tasks can be done in a manner such that they fit into a unified projectworkflow, and can interact with each other. Figure 7-15 shows a block diagram of themajor components of a softcopy stereo workstation. Figure 7-16 shows an outline ofthe major functional capabilites of such a workstation. Although there are differencesbetween vendors, many of these elements will be present in some form in all photo-gram metric workstations. Following is a general discussion of these capabilities.

7.3.3.1 Import

The import function of photogrammetric software must be able to read and interpretimages from a variety of sensors, and in a variety of image formats. Sensor options al-ways include frame, and may include others such as pushbroom, scanner, and SAR.