Method for image skew detection
A method for calculating a skew angle of an original image, executed at least in part on a computer system stores image data for the original image in an electronic memory, then forms an energy-normalized image according to the relative contrast amplitude of image features over each of a plurality of local image regions within the stored image data. A partitioned image is formed by partitioning the energy-normalized image into a number of sub-regions. A summed region is formed as a combination of image pixel data from the sub-regions. A Fourier magnitude spectrum is obtained by performing a Fourier transform on the summed region. The skew angle is calculated according to the peak value of a radial line integration function that is formed by integrating the Fourier magnitude spectrum along each of a plurality of lines of constant radial angles. An output signal indicates the calculated skew angle.
1. A method for calculating a skew angle of an original image, the method executed at least in part on a computer system and comprising:
storing image data for the original image in an electronic memory;
forming an energy-normalized image according to the relative contrast amplitude of image features over each of a plurality of local image regions within the stored image data;
forming a partitioned image by partitioning the energy-normalized image into a plurality of sub-regions;
forming a summed region as a combination of image pixel data from the plurality of sub-regions;
obtaining a Fourier magnitude spectrum by performing a Fourier transform on the summed region;
calculating the skew angle according to a peak value of a radial line integration function that is formed by integrating the Fourier magnitude spectrum along each of a plurality of lines of constant radial angles; and
providing an output signal that is indicative of the calculated skew angle.
2. The method of claim 1 further comprising rotating the original image according to the calculated skew angle.
3. The method of claim 1 further comprising preprocessing the stored image data to reduce image resolution.
4. The method of claim 1 further comprising preprocessing the stored image data by averaging pixel values over one or more regions of the image.
5. The method of claim 1 wherein providing the output signal comprises providing an encoded digital value.
6. The method of claim 1 wherein forming the summed region comprises adding corresponding pixel data values from each of the plurality of sub-regions.
7. The method of claim 1 further comprising providing a confidence-level signal that is indicative of a confidence level related to the calculated skew angle.
8. The method of claim 1 wherein forming the energy-normalized image comprises normalizing the mean and variance over the local image regions.
9. The method of claim 1 wherein forming the energy-normalized image comprises normalizing the minimum-to-maximum pixel contrast over each of the plurality of image regions.
10. The method of claim 1 wherein forming the partitioned image comprises forming the plurality of sub-regions of pixels, the sub-regions of size P×P, wherein P is an integer.
11. The method of claim 10 further comprising multiplying pixel data values for one or more sub-regions by a scalar.
12. The method of claim 10 wherein P is a power of 2.