Journal of the Society of Motion Picture Engineers (1930-1949)

which we can work back through the projection and production processes to the camera which is to be controlled on the studio floor. Psychological Viewing Factors In the spectator's mind two altogether different sets of impulses are at work. The binocular faculty attempts to place objects in space by methods which are still imperfectly understood in their entirety, but which may for simplicity be likened to the working of a rangefinder. At the same time, other departments of the mind are busy observing all sorts of other clues to depth and position in space. There may well arise conditions when these two sets of data will conflict, leading to an ambiguity in the image which different people will resolve differently — much as two people may sit down before a Picasso canvas in the Museum of Modern Art and come to wholly different conclusions as to what it is all about. Even more serious difficulties will arise if the conflict is so fundamental that the spectator cannot bring himself to believe in the stereoscopic data. A scene may be brought forward to a certain plane in space, but will not in fact appear to be there because the audience cannot accept the fact that a dining room table or a ballet dancer is poised in space over the front rows of the stalls. This effect has been known for many years, and is well analyzed in a classic paper by Professor J. T. T does The planning of our stereoscopic films of course take account of these and many other psychological factors; and hope, if interest in the 3-D film conactive, to discuss in a later paper a mber of new ways of bridging the remaining gap between audience and space film. In the present paper we shall confine ourselves to considering the physical elements in the stereoscopic transmission system, since these have been the subject of much fruitless debate, which it is time to try and replace with an agreed nomen clature and method of mathematical approach. The Mechanics of Viewing The elements of a piano-stereoscopic projection system, with image separation at the spectators' eyes, are sketched in Fig. 1. A generalized spectator is shown, placed at a distance, V, from the screen, onto which have been thrown left and right-eye images. It is convenient to consider these images as consisting of a multitude of separate points, much as is often done in discussions of film resolving power. In general, to each point on the left-eye image there will be some corresponding point in the right-eye image, both image points having the characteristic that they represent the same object point in the original scene.* These image points are sometimes called homologous points, and they are represented in Fig. 1 by L and .ft.f The eyes are shown as having a separation, t, this letter also being used in our nomenclature to denote the lateral separation of optical axes, suitable subscripts being used to distinguish the camera and projector. Through their selecting viewers, the eyes regard separately the left and right members of each pair of homologous points on the screen, whose horizontal separation is known as parallax. Parallaxes are always denoted by * Note that the original object may be imaginary, as in 3-D abstract and cartoon films. f It is noteworthy that, in a projection system such as we are discussing, the eyes are able to prompt the mind without any additional clues as to which pairs of points are to be considered homologous; occasional errors — as in the fusion of wire mesh and wallpaper patterns — occur also in binocular vision and are of negligible importance in practice. On the other hand, some types of integral screen, which dispense with viewers for seeing 3-D films, require the transmission of information as to which points are homologous, and are therefore "information-consuming" and wasteful. Spottiswoode, Spottiswoode and Smith: 3-D Photography 251