Page 1 of 3 Next  Dr. Hrishikesh Samant Sr. Associate Editor (Honorary), GIS Development Reader, Department of Geology, St. Xavier's College, Mumbai, India hrishikesh@gisdevelopment.net It must have all started with the evolution of the first 'two eyed' life form on earth and were most probably the early trilobites who roamed the oceans 500 million years ago. They had a pair of compound eyes and must have had a rudimentary perception of the third dimension, giving them the necessary visual acuity to catch moving prey. 3D viewing or stereoscopy has been extensively used for aerial reconnaissance since the first world war and photo interpreters used the stereo coverage to break camouflage. The same stereo coverage has been used for measuring ground heights and generating height contours in hard to reach areas. Today, these once very tedious and labour intensive processes have been automated but the principles of calculation remain unchanged. What is "3D"? We live in a three-dimensional world and our appreciation of space around us is almost exclusively by our eyes. The spacing between our eyes, results in the lenses in them projecting two slightly different pictures of the same object onto the retina, which are then transformed by the brain into an image with the effect of depth. Using the same logic, by taking two lenses and simulating the eyes, we can create and see pictures in 3D. Historical background The quest for making two-dimensional objects like pictures (printed or painted) or photographs appear in 3D is also rather old. In 1838 Sir Charles Wheatstone described the theory of stereoscopic vision. It is surprising that the concept of stereoscopy predates photography. Though most associate Sir David Brewster with the invention of the stereoscope, it was Wheatstone who, in June 1838, in his address to the Royal Scottish Society described his instrument as "I...propose that it be called a Stereoscope, to indicate its property of representing solid figures." Wheatstone’s stereoscope consisted of two mirrors aligned such that the would reflect the left and right images to the human eyes simultaneously. Wheatstone's actual stereoscope is preserved at the Science Museum in London. Almost contemporaneously, Brewster designed the 'lenticular stereoscope' which is used even today as a portable pocket stereoscope for viewing stereograms. The principle of viewing is different from the mirror stereoscope. If two images of the same object made with a slight difference in the 'point of view' or in Brewsters words 'half images' are placed at a short distance from the viewer the eyes will be converged and the images will not fuse. Brewster eliminated this difficulty by placing a bi-convex lens one focal length in front of the half images resulting in parallel convergence of the eyes and the images appear fused. Having laid the scientific base for the understanding of 'stereo vision', Wheatstone and Brewster paved the way for others to find other novel ways of '3d viewing'. Other Methods Filter separation Stereoscopes Anaglyph This technique for stereoscopic presentation uses filters to separate the two eyes' images. It is commonly used for black-and-white 3-D movies and printed cartoon strips in magazines. In the anaglyph method the two half images are printed in roughly complementary hues and then physically superimposed. The common colours are red and blue, though Green and Red have also been used. These pictures are then viewed using the corresponding colour filters over the eyes which allows each eye to see just that half image corresponding to the colour of the filter. This very simple, cheap and easy method has two disadvantages - the images are not coloured and there is often a lot of "leakage" between the filters resulting in part of the other (complementary) half image being partially seen resulting in dilution of the 3D effect . It also results in tremendous eye strain for the user.
Page 1 of 3
Next |