Since the video frames are usually captured in a 728x568 
pixels resolution, a display resolution of 800x600 pixels is good 
enough in order to display vertically the two subfields 
interlaced, plus the required blank interval between them. The 
images are loaded from the hard disk, placed to the system 
RAM memory and then moved on to the display memory. 
From 22 to 30 stereoscopic views per second should be 
displayed on screen in order to achieve smooth animation. 
This is rather impossible for most PC based systems, because 
of the large amount of digital data required. About 110 MB of 
image data should be loaded from disk, vertically compressed, 
interlaced and moved to the display memory in order to be 
able to produce 5 seconds of 3-D animation at this frame rate. 
At present all the above described operations allow for about 
5-6 views per second, which results to a slow-motion 3-D 
animation (Figure 3). A significant improvement can be done 
by suitably programming the multimedia graphics accelerators 
which are present in some video boards. In this way, image 
scaling and bit block transfers between system RAM and 
display memory will be undertaken by the accelerator, 
independently of the system processor. 
  
  
  
Figure 3 
2.4 Interactive 3-D coordinate masurements 
3-D coordinate measurements may be performed interactively 
on a displayed stereoscopic view by using a pointing device 
able to indicate positions in a virtual 3-D world. In this study a 
regular mouse was used and a 3-D cursor simulation was 
created with the help of suitable software. 
A 3-D mouse cursor, or a 3-D photogrammetric floating mark 
in this case, consists of two separate cursors, i.e. floating 
mark images, one for the left and one for the right eye. Since 
the stereo pair has been rectified to the normal case, the two 
cursors must have the same y coordinate when the system is 
passed in stereo mode. A difference in x coordinates 
introducing an x-parallax may exist between the two cursors. 
When the mouse is used normally, the left and right motions 
correspond to the x coordinate and the forward and backward 
motions to the y coordinate. If the x-parallax is always the 
same, the 3D cursor is moved on the same plane (Lipton 
1991). If the x-parallax increases or decreases interactively, 
the cursor is moved on different virtual planes. In the system 
under development, pressing the rigat mouse button indicates 
to the software that the distance in x direction of the two 
cursors should be increased, while pressing the middle button 
indicates that the distance should be decreased. In this way 
virtual motion in z is achieved. 
Since the use of 3-D cursors is not directly supported by most 
Microsoft compatible mouse drivers, specialised software 
ought to be developed in order to simulate a 3-D cursor. The 
default mouse cursor, provided in graphics modes, is initially 
disabled. However, although the cursor is not visible, the 
mouse is working and generating interrupts to the host CPU. 
A new mouse cursor can be created by reprogramming the 
service routine which is executed when a mouse movement 
interrupt occurs (Gradecki 1994). The horizontal and vertical 
limits of the mouse movements are set equal to the left image 
viewport in order to ensure that the screen coordinates 
returned by the mouse will not be falling outside of the area 
used for the left image. The left cursor is mapped at a position 
X,, y, taken from the mouse movement and the right cursor is 
mapped at a position xg, ya which is calculated according to 
the following equations (Akka 1991b): 
Xp = X, + Xparallax 
(7) 
Ya^JL* YOFFSET 
where Xparallax is the current parallax in x direction and 
YOFFSET the vertical distance between the two cursors in 
order to have the same y coordinate in stereo mode. The 
distance YOFFSET depends on the current screen resolution 
XWIDTH, YHEIGHT and on the blank interval YBLANK 
inserted between the left and right image (Figure 2): 
"ues YBLANK | YBLANK 
YOFFSET - (8) 
The exact location of the two cursors can be estimated by 
counting the pixels determining the Xparallax. Additional 
information, taken from the absolute orientation of the images, 
is used in order to translate x-parallax measurements to 
accurate Z positions in space. 
The minimum configuration for the smooth operation of this 
system is a PC with at least a 486/100MHz processor, a 1GB 
hard disk drive, a mouse, a monitor capable of at least 120 Hz 
refresh rate and the Stereographics Corp. CrystalEyes 
hardware. 
3. SYSTEM APPLICATION 
The described system was developed for the needs of a major 
European Union research project, whose main objective is to 
examine the bahaviour of ancient monuments to earthquakes. 
This is a significant contribution to the protection of cultural 
heritage from natural hazards, especially in countries prone to 
earthquakes like Greece. 
114 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996 
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