1. Field of the Invention
This invention is directed to the area of optical reconstruction filters for color mosaic (matrix) displays in general and for flat panel liquid crystal displays in particular. The invention is directed to optically filter flat panel images, i.e., particularly to interpolating sampled image data shown on color mosaic displays using a phase diffraction grating in order to present a higher quality image to the viewer.
2. Background
The use of flat panel color matrix displays is increasing rapidly. These displays have regular structures of color pixels, as for example is shown in FIG. 1, which are used to create the color image. An existing problem is that the dotted and discontinuous appearance of images shown on color mosaic displays is not desirable and needs improvement. That is, the underlying grid structure results in objectionable visual artifacts commonly referred to as sampling noise. Examples of sampling noise are pixel edges and gaps. These artifacts cause flat panel color matrix displays to have noticeably lower image quality than CRTs, rendering them inadequate for many situations.
This problem of visible dot structure in color matrix displays can be viewed as a two-dimensional image processing situation, which can be understood more easily by comparing it to its one-dimensional analog as shown in FIG. 3. Segment 3a of FIG. 3 shows an ideal signal (image) which is to be processed. An initial filter, the anti-aliasing filter, 3b, is used at the outset to limit the bandwidth of the ideal signal to frequencies the processing system can handle. Frequencies that are too high result in spurious noise and moire patterns.
The cutoff frequency of the anti-aliasing filter is determined, by sampling theory, to be at one-half of the frequency the system uses to sample the incoming signal. The system in the two-dimensional case consists of an image generator and color matrix display device. This cutoff frequency is commonly referred to as the Nyquist frequency. The output of the anti-aliasing filter is the actual signal (image) to be entered into the system, as shown at section 3c. The signal (image) is then digitized through an A/D converter (image generator), shown at section 3d, and is ready to be transferred to the rest of the system.
At the other end of the system, the digital signal (image) passes through a D/A converter, shown at section 3e. The output waveform of the D/A, shown at section 3f, is a signal (image), with undesirable high frequency noise present. The noise is due to the underlying sampling grid and results from an incomplete reconstruction process. To complete the process, the signal is passed through another filter, the reconstruction filter, shown at section 3g, with its cutoff again determined by the Nyquist criterion. At this point, assuming ideal filtration has been accomplished, the output shape shown at section 3h, is identical to the system input at section 3c.
In the image processing case, just as for the one-dimensional signal, a reconstruction filter is needed to make the output identical to the system input. This invention is an optical reconstruction filter made by using a diffraction grating. The diffraction grating reconstruction filter is placed between the flat panel and the eye, as shown in FIGS. 4a and 4b. This filter interpolates between data points of like color and acts as a reconstruction filter for images to be displayed on the color matrix display. The cutoff frequencies of the filter are determined by the color matrix display sampling structure of the color matrix display. The result of applying this reconstruction filter is an output image identical to the system input, free of sampling grid artifacts.
This invention solves problems evident in the prior art. All color matrix displays, intentionally or not, have relied on one of two types of optical reconstruction filters; 1) the eye itself with its associated low pass filter characteristics, or 2) a diffuse, or scattering, optical surface.
The eye as a reconstruction filter does not work satisfactorily for current flat panel display resolutions. For example, present color matrix displays typically have pixels 6 to 8 mils across. Human factors experiments have determined these pixel sizes result in sampling grids all too easily seen by the eye. The frequency content of the color matrix display structure, the display sampling grid, is clearly well within the bandpass characteristics of human vision. The eye cannot filter out spatial frequencies this low at typical viewing distances. The resolution of color matrix displays must increase significantly before the eye alone will be a sufficient low pass filter. This, however, is the reconstruction filter most often used for color matrix display applications.
Some color matrix display applications have used a diffuse scattering surface to eliminate sampling grid artifacts. A diffuse surface scatters the light, giving it optical low pass filter characteristics. The more scattering the surface accomplishes, the more diffuse the filter, and the more it smooths the image. A common example is the diffuse picture glass frequently placed over photographs to reduce specular reflections. Some optical low pass filtering results as well. Sudden luminance changes are attenuated giving the image a softer, smoother look. But, while eliminating specular reflection and while softening the image, these filters exhibit strong diffuse reflections of ambient light. The more a filter diffuses, the more light is reflected over a wider range of viewing angles.
In display applications, even small amounts of reflected ambient light are objectionable. In higher ambients the diffuse reflections wash out the image altogether, rendering it unviewable. To get the amount of diffusion needed to eliminate the sampling noise of present color matrix display technology, the reflections become unacceptable, especially for cockpit display applications.
Another drawback of diffuse filters is that their passband characteristics are not tailorable over direction. The cutoff frequency is the same in all directions. For typical color matrix displays, whose underlying grid structure is not circularly symmetric, a filter with passband characteristics tailorable over direction is extremely desirable. Otherwise, the full frequency capability of the color matrix display is not taken to full advantage. Too much filtering will be exerted in some directions and/or too little will be exerted in others. Ideally, the low pass profile will exhibit characteristics determined directly by the color matrix display's own two-dimensional frequency capability.