Halftone screen frequency and magnitude estimation for digital descreening of documents
View Patent ↗A highly efficient method and system for eliminating halftone screens from scanned documents while preserving the quality and sharpness of text and line-art is disclosed. The method and system utilizes a single channel screen frequency estimator module, which generates a screen frequency estimate for image data. The module generates a signal based on the highly filtered image signal at low contrast levels, and based on a reliable estimate to the halftone frequency at higher contrast levels. The single channel screen estimate module has adequate performance in resolution ranges from 300 to 600 dpi.
1. A single channel method for estimating a halftone screen frequency from image data, comprising:
multiplying a frequency measurement signal by a factor;
adding the multiplied frequency measurement signal to an image data signal to produce an output signal;
adjusting the factor multiplied to the frequency measurement signal based on a control signal, wherein the control signal is based on a characteristic of the image data;
interpolating the output signal to produce the halftone screen frequency estimate; and
subtracting a frequency signal from the image data signal, to produce the frequency measurement signal.
2. The method of claim 1 , further comprising:
measuring a contrast within a window of the image data to produce the control signal.
3. The method of claim 1 , further comprising:
filtering the image data using a low-pass filter to produce the image data signal.
4. The method of claim 1 , further comprising:
sub-sampling the image data to produce the image data signal.
5. The method of claim 1 , further comprising:
outputting the output signal which is an estimate of the halftone screen frequency, to a de-screening device.
6. An apparatus for estimating a halftone screen frequency, comprising:
a multiplier which multiplies a frequency measurement signal by a factor;
a combiner which combines the multiplied frequency measurement signal with an image data signal to produce an output signal;
an adjuster which adjusts the factor multiplied to the frequency measurement signal based on a control signal, wherein the control signal is based on a characteristic of the image data; and
an interpolator for interpolating the output signal to produce the halftone screen frequency estimate; and
a subtracting module for subtracting a frequency measurement from the image data signal, to produce the frequency measurement signal.
7. The apparatus of claim 6 , further comprising:
a contrast measuring device which measures contrast within a window of the image data to produce the control signal.
8. The apparatus of claim 6 , further comprising:
a low-pass filter for filtering the image data to produce the image data signal.
9. The apparatus of claim 6 , further comprising:
a sub-sampling filter for sub-sampling the image data to produce the image data signal.
10. The apparatus of claim 6 , further comprising:
an output device for outputting to a de-screening device the output signal which is an estimate of the halftone screen frequency.
11. A single channel apparatus for estimating a halftone screen frequency, comprising:
means for measuring a contrast of image data;
means for adjusting a factor multiplied to a frequency measurement signal based on the measured contrast;
means for combining the multiplied frequency measurement signal with an image data signal to produce an output signal;
means for interpolating the halftone screen frequency; and
means for subtracting a frequency measurement from the image data signal, to produce the frequency measurement signal.
12. A tangible computer-readable medium that stores computer-executable instruction which, when executed by a computer, causes the computer to perform the method of claim 1 .