|| In this thesis, we focus on solving the problem of uneven light effect on document images. A method based on EMD theorem is proposed to estimate and rectify the light-field. Our proposed method does not need to extract the IMF signal and only need to consider the max-envelope instead of all envelopes and residue function in EMD to obtain the light-field distributions of images. Our system consists of three parts including max-envelope calculation, light-field distribution adjustment, and signal scale adjustment. |
In the first part, we first use fast method, least-squares approximation, to find max-envelope form input signal scale via one dimensional way. In the second part, we observe the distributions of max-envelope and signal to find the max differential point between light-fields as the cut points. Then, segment the signal by the cut points and calculate local max-envelopes to obtain closer light-field distribution of original signal. In the third part, we will set the contrast value depending on light-field intensity and enhance the gray scale signal. The contrast value will be fixed if it is an over-exposure signal. Last, enhance gray scale signal by contrast value and finally adjust the enhanced gray scale.
Experimental results show that our method exhibits good outcome about removing over-exposure, shadow, reflection and back-light effects on document images. Besides, the execution time is much faster than EMD and the recognition result can achieve the same efficiency of EMD method.
As to the color image uneven light-field effect, we calculate the max-envelope by “Triangle Linear Interpolation”, and adjust the value-element (V of HSV color system) to conserve the property of continuous color information. Experimental results demonstrate that our proposed method performs well on color images too.
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