Correction of shading effects in video images

Claim 3

A method according to claim 1, further comprising the step of: determining image polarity from said intermediate value and said first value and said second value.

Claim 4

A method according to claim 1, further comprising the step of: comparing said correction value to said range of background gray scale variation to determine if said correction value is within said range.

Claim 5

A method according to claim 1, further comprising the step of: averaging said sampling gray scale intensity values to obtain a correction value. 5. A method according to claim 1, further comprising the step of° selecting a maximum color value from said sampled gray scale intensity values to obtain a correction value.

Claim 6

A method according to claim 5, further comprising the steps of: testing said maximum color value to determine if said maximum color value is within said range of acceptable background values; sampling gray scale values over a predetermined expanded area of said mage if said maximum color value is outside said range; repeating said steps of testing and sampling until a maximum color value s found within said area or a predetermined number of iterations of said testing and sampling steps has been executed.

Claim 7

Apparatus for correcting shading effects in video images having a plurality of pels, wherein each said pel is represented by a gray scale intensity signal having a plurality of bits, comprising: a. means for generating a histogram of distribution of gray scale intensity levels of pels in said image; b. means for calculating from said histogram a-first gray scale intensity value, a second gray scale intensity value and a gray scale value intermediate said first value and said second value in said image; c. means for determining a range of background gray scale variation from said steps of calculating and determining above; d. means for sampling gray scale intensity values of pels in a region around a current pel; e. means for calculating a correction value for said current pel by comparing said current pel gray scale value with said range of background gray scale variations; f. means for setting said correction value to a predetermined constant if said current pel gray scale value is outside said range of background gray scale variations; g. means for correcting said current pel gray scale value by subtracting said correction value therefrom to achieve a shading corrected value for said current pel; h. means for executing each of the above steps for each pel in said image to correct for undesired shading effects in said image.

Claim 8

Apparatus for correcting shading effects in video images having a plurality of pels, wherein each said pel is represented by a gray scale intensity signal having a plurality of bits, comprising: a. means for generating a histogram of distribution of gray scale intensity levels of pels in said image; b. means for calculating from said histogram a-first gray scale intensity value, a second gray scale intensity value and a gray scale value intermediate said first value and said second value in said image; c. means for determining a range of background gray scale variation from said steps of calculating and determining above; d. means for sampling gray scale intensity values of pels in a region around a current pel; e. means for calculating a correction value for said current pel by comparing said current pel gray scale value with said range of background gray scale variations; f. means for setting said correction value to a predetermined constant if said current pel gray scale value is outside said range of background gray scale variations; g. means for correcting said current pel gray scale value by subtracting said correction value therefrom to achieve a shading corrected value for said current pel; h. means for executing each of the above steps for each pel in said image to correct for undesired shading effects in said image.