Facsimile and static presentation processing – Static presentation processing – Attribute control
Patent
1991-10-22
1993-01-19
Groody, James J.
Facsimile and static presentation processing
Static presentation processing
Attribute control
358139, 358 51, H04N 1702, H04N 9093
Patent
active
051810980
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention concerns a procedure and a device for the automatic correction of skew, of color superimposition, and of image uniformity for television cameras.
DESCRIPTION OF RELATED ART
The use of a camera to obtain a faithful monochrome image presupposes the existence of a completely linear relationship between the "time" variable and the coordinates of any point whatever within the scene being scanned. Now, filming optics, the scanning tube, and its associated electronics produce distortions termed "skew," which prevent the electronic image of a completely periodic optical pattern from any longer exhibiting this appearance locally. In particular, filming optics produces distortions at the edge of the image in the form of pads or drums. The scanning tube and its associated electronics do not make it possible to obtain (and to measure) completely linear high-voltage ramps, especially since the reduction of scanning noise requires the greatest possible reduction of the pass-band of the amplifiers, and since obtaining a faithful color image requires, in addition to observing the linearity between the time variable and the position of each point of the scene being scanned, that the three electronic beams of the three tubes R, G, and B simultaneously scan the three data emanating from each single point of the source image. "Differential" distortions between the red and blue tubes and the green reference tube are added to the "common mode" distortions mentioned above. The optical system obviously plays a part in this distortion, because magnification depends on wavelength. The spacings separating tube position, the differential astigmatism between the tubes, and the electronics, through the dispersion of component characteristics, also contribute to these distortions.
Furthermore, the presentation of a uniformly white scene to a three-tube camera generally supplies video signals at the output of each tube, these signals exhibiting variations of level as a function of the positions of the points scanned.
This situation can be ascribed to the lens-vignetting phenomenon, to the phenomenon of diffusion of the loads in the filming tubes on the periphery of the unscanned screen areas, and to the variations in scanning speed, in particular along the vertical dimension of the image.
All of these phenomena contribute to the appearance of blanking spots in the white parts of the image.
Inversely, when the lens is completely closed, a non-uniformity of the level of black is produced which results principally from the non-uniformity of the shower lighting. For a color camera, the difference in uniformity between levels leads to the appearance of color changes.
The appearance of spots may also result from the compensation for defects of skew and of superimposition, which consist in the modulation of horizontal and vertical scanning speeds.
One conventional method for solving this problem consists in generating characteristic predistortion functions.
This method is based on the assumption that if distortions appear within the image, the necessary corrections must be able to be expressed as polynomials.
However, this method is limited by the order of the highest polynomial generated and by the optimization algorithm applied, which rapidly becomes complex when the number of coefficients increases. Furthermore, the generation of signals representative of the elementary products at x.sup.i y.sup.j of the polynomial must preserve adequate temperature and temporal stability, which is achieved only with difficulty.
SUMMARY OF THE INVENTION
The purpose of the invention is to overcome the difficulties cited above.
To this end, the object of the invention is a procedure for automatic correction of skew, of color superimposition, and of image uniformity for television cameras, by means of an optical grid placed in front of the camera and composed of predetermined patterns whose centers of rotation are located, in accordance with a matrix structure, at the points of intersection of evenly-spaced ro
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Guerin Jean-Claude
Morel Philippe
Groody James J.
Powell Mark R.
Thomson Consumer Electronics
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