The camera-view projection system introduced in this paper 
profits from CAD environment facilities like design cubes and 
virtual cameras, trying to match the shearing photograph model 
by changing the position and orientation of the virtual camera 
whilst the actual 3-D model is fixed. In this way a simulation of 
camera's position orientation and internal geometry is achieved 
(Streilein A., 1994). 
Elements in 3-D computer modeling must be drawn on the 
screen that is planar. Hence a projection is needed. The two 
principal methods of projection used in CAD systems are: 
parallel (or cylindrical) projection and perspective (or conical) 
projection. 
2.1 Parallel Projection 
In Parallel projection the rays or projectors are all parallel and 
intersect the image plane at the same angle. A parallel 
projection can be thought of as like a perspective with an 
infinitely distant eye-point. A special case of parallel projection 
is produced when the rays meet the image plane at right angles. 
This kind of parallel projection is known as orthographic 
projection and is used in 3-D computer modeling for 
engineering and architectural applications. The visual images 
generated by an orthographic projection are called multi-views. 
Examples of such as multi-views are the widely used in 
engineering Top, Left, Right, ISOmetric and Front views 
offered by well-known 3-D CAD systems (e.g. Bentley's 
MicroStation, Autodesk's AutoCAD). In our system the whole 
computer screen is divided into four viewports with three of 
these are used to show the Top, Right and ISOmetric multi- 
views of the facade. For a planar and vertical facade (the usual 
case) its image appears to be a horizontal line in Top view and a 
vertical one in Right/Left view. So, these multi-views serve as a 
control error-checking system to camera's approximating 
positioning procedure. The ISOmetric view is used for 
increasing visualisation purposes. The fourth viewport, which is 
the bigger, is devoted to the visual image of the facade 
generated by the camera view projection [Fig. 4, 5, and 6]. 
The main disadvantage of the parallel projection is that there 
is no perspective associated with objects. Parallel lines appear 
parallel on the screen and distant objects appear at the same 
scale as near ones of the same dimensions. In real life the thinks 
are different. The more distant objects appear smaller to viewer 
(Rooney J. et al., 1987). 
2.2 Perspective Projection (Camera-View Projection) 
The Perspective projection, known as well as Camera-View 
projection, enhances realism of modeling and mimics the way a 
conventional camera works. This projection system is used in 
current paper for facade-model visualisation. 
The geometry of photography is essentially equivalent with the 
perspective projection geometry (Fig. 1) In camera-view 
projection the position of the eye is being taken by virtual 
camera's lens center, and the plane of the image corresponds to 
camera's plate or film. The "difference" of perspective and 
camera-view projection is that for the late the 'eye-point' (lens 
center) is between the object (facade) and the image plane 
(film), and as a consequence the image is formed down-upside 
(negative case) instead of upside-down (diapositive case). 
Otherwise these two projections are geometrically similar 
(Baker P. et al., 1994). 
452 
  
  
Figure 1. The Basic Geometry of Perspective Projection. 
2.3 Projection Relationships between the Camera, the 
Photo and the 3-D Object (facade) 
Using the central projection logic, which is the base for the 
camera-view projection system, there is always just one 
particular perspective regarding the camera position, the 
exposed photo and the spatial 3-D object. 
In other words in central projection a 3-D object-image has been 
taken using a particular interior and exterior orientation for the 
camera used. 
Generally, there are as many as eight different settings of the 
camera used (according to Patias P. et al., 1995a) that could 
lead to a particular image. Four of these camera settings refer to 
transparent objects and can be avoided. The selection of the 
remaining four settings is a problem, that could be avoided if 
approximate values are taken empirically (see: the automatic 
procedure of the system in next chapter). 
The Camera-View Projection System used in this paper follows 
the next conventions: 
In a FRONT View 
We are looking into the design cube from the front. 
The XZ plane is parallel to our screen. 
X is positive from left to right (horizontally). 
Z is positive from bottom to top (vertically). 
Y is positive away from the viewer and perpendicular to the 
screen. 
In a TOP View 
We are looking down on the design cube from the top. 
The XY plane is parallel to our screen. 
X is positive from left to right (horizontally). 
Y is positive from bottom to top (vertically). 
Z is positive towards the viewer and perpendicular to the 
screen. 
In a RIGHT View 
We are looking into the design cube from the right. 
The YZ plane is parallel to our screen. 
Y is positive from left to right (horizontally). 
Z is positive from bottom to top (vertically). 
X is positive toward the viewer and perpendicular to the 
screen. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996 
  
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