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THE SOLID IMAGE: AN EASY AND COMPLETE WAY 
TO DESCRIBE 3D OBJECTS 
L. Bornaz (*), S. Dequal (*) 
(*) Politecnico di Torino - Dipartimento di Georisorse e Territorio 
C.so Duca degli Abruzzi, 24 — 10129 Torino 
Tel. +39.011.564.7687 / 7601 Fax. +39.011.564.7699 
Email: leandro.bornaz@polito.it; sergio.dequal@polito.it 
Commission V, WG V/2 
KEY WORDS: Digital photogrammetry, Analysis, Image, Application, Architectural Heritage, 3D image, laser scanning. 
ABSTRACT: 
A digital photo image can be considered, with a good approximation, as a central perspective of the acquired object. If the internal 
and external orientation parameters of the camera are known, it is possible to establish the direction in the space of each point of the 
object which is represented by a pixel in the image. 
Using a dense digital surface model (DDSM) of the acquired object each pixel (and therefore each direction in the space) can be 
associated to the value of distance between the centre of perspective and the point of the object that is represented by the pixel itself. 
In this way each pixel can be made to the 3D position of the corresponding point of the object in an absolute reference system. 
The DDSM can be derived from an existing map or by using surveying instruments and procedures. A series of new instruments, 
based on laser technology, have recently been introduced onto the market. These instruments offer the possibility of obtaining a 
DDSM in a quick and cheap way. 
The integration of the DDSM geometric data with the image radiometric data allows a new concept of the Solid Image to be 
obtained. 
Solid images can be used in architectural, archaeological and land surveying applications to determine, in real time, the position of 
any point in a 3D reference system, using a common PC or to carry out correct three-dimensional measurements (lines, areas, 
volumes, angles, ...), by just selecting some points on the image. 
In addition, with the Solid Image even unskilled users can easily plot profiles, sections and plans using simple drawing functions, 
generate stereo models and realistic 3D models. 
The developed software, which is able to generate and manage the solid image, is described and some examples are shown. 
1. THE SOLID IMAGE If the coordinates of an image point i are measured, the 
horizontal and vertical angles 3 and a can easily be 
1.1 Definition of a Solid Image determined as: 
Any image can, with a good approximation, be considered a £ 
central prospective of the photographed object. Ó — arctan— 
If the orientation and calibration parameters of a digital C (1) 
image are known, it is possible to establish a direction in t. 7]; 
space in the x, y and z reference system, for each pixel of the e c orem S Lr 
image. (see figure 1). etu 
Where: 9 = the horizontal angle 
a = the vertical angle 
c = the principal distance 
&, M = the coordinates of the image point i 
One image is not sufficient to determine the 3D position of an 
object point. If a single image is used it is possible, at the 
most, to establish its direction in the space. In order to 
J reconstruct objects in three dimensions, it is necessary to use 
at least 2 images according to the classical principles of 
photogrammetry. , 
  
A valid alternative to the well known photogrammetric 
restitution techniques is offered by the knowledge of the 
distance d between the centre of prospective of an image and 
the object itself for each direction in the space. In this way it 
Z