Fig 1.5: Screen parallax
In the real world, our eyes focus (accommodate) and converge at a single point. When viewing 3D, our eyes always focus on the screen but may converge anywhere along the Z-axis. Extreme convergence can cause the eyes to turn excessively inward as a result the audience will not be able to fuse the 3D image. Infinity Deviation is the amount of background divergence. If this divergence exceeds the human interocular distance, it can cause the eyes to turn outward, which is both unnatural and extremely uncomfortable – as a result the audience will not be able to fuse the 3D image because of this unnatural eye movement and will quickly experience eye-strain and fatigue. The Fig 1.6 represents the convergence and divergence of the eyes.
Fig 1.6: convergence and divergence
While viewing 3 D on the display, the audience uses 3 D glasses for the 3 D effect and the different type of 3 D glasses are as follows.
a) Circular Polarization (Passive glasses)
Each image is circularly polarized by the display and shown together. The left eye is polarized clockwise and the right eye is polarized anticlockwise. The glasses also have a circular polarizing filter for each eye in which the left lens filters or blocks out the right eye image and the right lens filters or blocks out the left eye image. They are referred to as ‘passive’ because the glasses do not require any power to operate them. Passive glasses (Fig 1.7) are very cheap to manufacture and buy.
Fig 1.7: Passive glasses
b) Shutter (Active glasses)
Active glasses contain LCD lenses that alternately ‘black-out’ each eye depending on whether the right or left image is being displayed on the screen. The shuttering occurs in complete synchronization with the images, and happens so rapidly that we don’t notice the shutter-effect. They are referred to as ‘active’ because they require a battery to operate the synchronization sensor and LCD lenses. Active shutter glasses (Fig 1.8) are relatively expensive to manufacture and buy.
Fig 1.8: Active glasses
The two images are individually coloured (typically red and cyan) and then superimposed as a single image. Through the use of similarly coloured filters in the glasses, each eye sees only its correct image. Fig 1.9 shows the anaglyph glasses.
Fig 1.9: Anaglyph glasses
The 3 D effect is generated as discussed above and this is the basic method followed in all 3 D projections and so forms the basis for understanding the 3 D in microscopy and endoscopy which is discussed in the later chapters.