28 
to- 
ion 
29 
1) projecting the plane raster from a point P onto the object, 
a deformed and spatial raster is obtained whose nodes re- 
present a discretization of the object (it is assumed that 
the raster is permanently printed on the object); 
2) taking the object from the same point P with a camera whose 
optical axis is oriented as that of the projector, an un- 
deformed image of the raster is obtained. 
It is obvious that a projector having the same characteristics 
of the camera has to be used. It must have fiducial marks to 
locate the principal point and, the principal distance must be 
known. It must also be constructed with the same precision of 
the camera. We define such a projector a metric projector (4). 
In this way the classical stereoscopic pair of photograms can be 
replaced, without losing any information, by the original raster 
and by one single photogram taken from a point different from 
that of the projection. The advantage is obvious, the plate co- 
ordinates on the pseudophotogram are predetermined and are there- 
fore known with great precision. 
Thus, while in the case of two Stereoscopic photograms it will 
be necessary to measure the plate coordinates of corresponding 
points on two photograms, in our case only the plate coordinates 
of the imageries of the node of the raster on the single photo- 
gram are to be measured. The measurements have to follow an 
exact order, so as to allow a correct matching with the nodes of 
the pseudophotogram. 
In the following paragraphs, the term "raster", with reference 
to the processing of the photogram mentioned above, will mean 
the deformed image on the photogram of the original raster pro- 
jected on the object. 
3. Automatic measurement 
In this section we describe the procedure which measures the 
rectangular coordinates of the nodes of the raster in a coordi- 
nate system of the image plane (5). Figure 1 gives a block dia- 
gram of this procedure. 
The ordering criterion adopted for a correct matching consists 
in locating the nodes of the photogram sequentially from ieft 
towards right along the successive horizontal lines of the 
raster (which appear in general deformed) scanned from top down- 
wards. 
a) Digitizing 
The analog information contained in the photogram has to be di- 
gitized (6),(7) for processing on a digital computer. An image 
Scanner samples and quantizes the values of photometric transpa- 
rency or density of the image in grey levels, which are subse- 
quentiy stored in a matrix A(i,j) (Table 1).