What follows is a sub-program written in BASIC 2.0 on the Macintosh, to calculate the intersection of two lines:
In fig. 2, segments and contribute to the escape and GH x; segments FI and GL to y, FH and EL z. Find the triangle of the joints, it is time to check the accuracy, calculating the orthocentre, which is the foot of the optical axis, ray departing from the eye and normal to the plane of projection (Fig. 3).
The following subroutine calculates the orthocentre a triangle
The distance between the center of the frame, P, and should be orthocentre found nothing, since the two coincident points, measuring the distance between the two, therefore, gives us the heels of the orientation of the system to find the real one, that the error rate exists.
As with the graphical methods, analytical procedures with the restitution of perspective is not to be free from error factors, which may result from imprecision in the reading of the coordinates on the plate or the approximation in the calculations. Various methods can be used to compensate for errors or eliminate its effects. As for the leak detection and error dell'ortocentro, the compensation method adopted is based on the verification of the horizontal plane, if it is tilted beyond a certain threshold, you can force it to horizontal, moving one of the three points escape, according to a criterion for selecting an arbitrary operator.
The following subroutine, given two vanishing points, calculates the coordinates of the third flight, based on the point P the center of the frame.
this system, there may seem to lack a theoretical point of view, appears to be powerful in practice as it gives the possibility of finding a vanishing point for which there is no sufficient geometric elements can be derived from the image. This happens very often: for example, photographs of building facades, shooting in adverse conditions in perspective, as, for example, within very narrow streets, in which case it is no longer the possibility of eliminating the effect of so-called "falling lines" with optical methods, such as rectifiers, objectives overhead and more. Assuming, however, the flight of the lines normal to the plane of the listing that interests us, we can work on it analytically, bypassing many of the problems of photographic optics.
The distance between the eye and the projection plane can be obtained using the two measures B and C, taken in absolute value. From geometry we know that the square of the height of a right triangle has the same area of \u200b\u200bthe rectangle whose sides the projections of the "legs on the hypotenuse, ie
2 D = C * B
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