2 THE DEVELOPMENT 
The development of the system progressed along the lines indicated by the 
feasibility study, with the main work areas defined as follows: 
* Imaging Devices; 
* Transmission Systems; 
* Image Processing System; 
* Stereo Viewing System; 
* Photogrammetric Software; 
e User Interface Software. 
The concepts have been recorded by Turner, Yule and Zanre 10 
Test marketing of the component technologies as they were developed indi- 
cated that the success of the system would depend on the imaging devices, 
which would limit the accuracy, convenience and cost of the system, and the 
user interface software which would determine who could use the system and 
so the size of the market. 
2.1 Imaging Devices 
The use of electronic visual imaging devices gives an immediate advantage 
over photographic devices: the image is available for analysis immediately 
after data capture without chemical or other processing. This comes with a 
disadvantage in that image resolution is generally lower. However, the market 
requirement placed the priority on speed rather than accuracy. 
Electronic visual imaging devices come in many forms. Scanned forms are 
only suitable when the camera and the subject are stationary. They can be used 
to transfer the image data from a photograph, for example, into an electronic 
form. The forms that are immediately of interest within any real time photo- 
grammetry system are video and digital stills cameras. Both can input into the 
Non-Contact Measurement System. 
2.1.1 Video Cameras 
Video cameras have the following benefits: 
* They have good time resolution, with the video framing rate 25 or 30 
frames (50 or 60 fields) per second, depending on the International 
Standard. This is useful when tracking a moving target or when viewing 
from a moving platform, to allow the image to be captured at the precise 
moment when the subject content is correct. 
* They are inexpensive, and so provide an immediate cost benefit. 
* They can provide colour images, which allow better interpretation of some 
subjects. 
They can provide intensified images, and so can be used in conditions of 
low ambient illumination without artificial lighting. 
* They are easy to install as they use commonly avail- 
able transmission systems. 
They can be used for real time stereo video. 
They do however, have the following disadvantages: 
* They have a poor space resolution, commonly about 
400 horizontal lines. 
The main applications for video input are: 
* When a cost effective system is required that is not expected to give 
extremely high precision measurements. 
«When there is a requirement for stereo viewing in real time. 
* For the supervisory control of robotic manipulators, where time resolution 
is more important that spatial resolution. 
2.1.2 Digital Cameras 
Video cameras were not the choice when more precise measurements were 
required. In this case digital stills cameras were specified. The most important 
aspect of this choice is that the camera is truly digital, so that the quality of the 
image is unchanged from camera, through transmission, storage and process- 
ing. With full error correction routines, any transmission interference can be 
eliminated with the image in a digital form. 
To correctly specify such a camera it is important to understand the difference 
between an analogue image and a true digital image. 
Ananalogue image consists of a stream of voltage amplitudes which represent 
the grey levels or colours of each line of the image, together with synchronising 
pulses which indicate the start of each line of the image. Such a data stream 
is illustrated in figure 3. 
A digital image is made up of individual bytes of data, each a number of bits 
deep. Each byte of data can be equated to a point on the image. The greater 
the number of bytes the greater the resolution of the image. The greater the 
number of bits, the more accurately the intensity of the illumination can be 
recorded, and hence reproduced. Such a data stream is illustrated in figure 4. 
While it is possible to "grab" the image from a video camera to convert it to a 
digital form, it is not possible by this process to identify the individual imaging 
element (pixel) on the charge coupled device (CCD) of the video camera that 
produced the byte within the grabbed image. (The problem is made much 
worse with tubed cameras to the extent that they are only used as a last resort 
within the Non-Contact Measurement System.) 
The importance of this to the photogrammetry process is that it is difficult to 
accurately calibrate an electronic camera that captures or processes or trans- 
Figure 1. A typical application of conventional photogrammetry within the 
Offshore Oil and Gas Industry. The drawing shows the result of a survey of 
tubular steel structural members prior to the installation of a clamp. 
  
  
  
  
* They have a low grey scale resolution of 64 levels. 
* They are analogue devices with their overall perfor- 
mance dependent on the value of electronic compo- 
nents which can change with time and temperature. 
Their images are hence not exactly reproducible, 
which makes it difficult to calibrate the cameras pre- 
cisely. The images can also be affected in trans- 
mission and by external interference making 
calibration of the system, rather than just the cameras, 
important. 
  
  
  
  
  
  
  
  
  
  
  
  
  
    
    
  
    
   
  
  
  
  
  
     
   
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
   
  
   
    
  
   
  
  
  
   
   
  
  
   
  
    
    
    
    
    
      
  
    
    
    
    
    
    
    
    
    
    
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