| 摘要 |
The present invention is a dynamic thresholding means for distinguishing dark and light portions of a scanned image, particularly useful where background noise is present obscuring the image. A three dimensional low pass filter applied to the output of optical sensor elements reading the original document, is used as the basis for establishing a three dimensional threshold curve above which data is designated as dark pixels of the image. One means for generating the threshold curve is by raising the output voltage of the low pass filter by a constant voltage for each pixel in the image. Another means for generating the threshold curve is by varying the increment of the low pass filter's output, based on the actual voltage of the low pass filter output for each pixel in the image producing a variable threshold curve. Another means for generating the threshold curve is by producing a slowly changing curve following the peaks of the output voltages of the optical sensors, and when used in conjunction with the low pass filter curve, the threshold voltage is established by taking an intermediate value of both curves for each pixel, to produce another variable threshold curve.
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