Correction of shading effects in video images

Details Correction of shading effects in video images Correction of shading effects in video images

1985-02-01

1987-09-22

Groody, James J.

Facsimile and static presentation processing

Facsimile

Specific signal processing circuitry

358166, 358284, 382 51, H04N 514, H04N 5228, G06K 938

Patent

active

046958843

ABSTRACT:
Video images are corrected for shading effects by generating a histogram of picture element (pel) gray scale intensity values and calculating the median and the black and white extremes thereof. A range of permitted background gray scale values is empirically selected such as by calculating the distance between the median and the closest color extreme and setting the range as twice that distance centered at the median. A background image is then created by sampling the intensity values of the original image pels and using a look-up table to remap the image. In one embodiment, pel intensity values are compared with the median value and only corrections lying within the range are carried out. In another embodiment the original image pels are remapped from 8 bits to 5 bits with all values outside the permitted range set to that of the nearest color extreme. A duplicate image is created from the remapped image and the intensity values of a number of pels in a predetermined region about a first pel in this image are sampled and the value of the corresponding pel in the background image is changed to the largest value among those of the first pel and the sampled pels. This sampling and changing is continued using a look-up table with a 7 bit output until all of the pels in the background image have been changed accordingly. The duplicate image is then changed to conform to the changed background image and the sampling procedure is carried out over a larger surrounding region, or neighborhood, and a larger number of pels with the corresponding changes being made in the background image. The foregoing conforming, sampling, and changing operations may be repeated until a background image with the desired level of shading correction is achieved.

REFERENCES:
patent: 3621129 (1971-11-01), Fisher
patent: 3800078 (1974-03-01), Cochran et al.
patent: 3979555 (1976-09-01), Opittek et al.
patent: 4232400 (1980-11-01), Yamamoto et al.
patent: 4493106 (1985-01-01), Farhangi et al.
patent: 4504864 (1985-03-01), Anastassiou et al.
patent: 4504972 (1985-03-01), Scherl et al.
patent: 4578712 (1986-03-01), Matsunawa

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