# US 8,571,313 B2 · Methods and systems for estimation of document skew in an image

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- Number: US 8,571,313 B2
- Document ID: US8571313B2
- Status: Granted
- Kind: B2
- Application number: 12/729,128
- Filed: 2010-03-22
- Published: 2011-09-22
- Granted: 2013-10-29
- Earliest claimed: 2010-03-22
- Assignees: Sharp Laboratories of America, Inc.
- Applicants: John E. Dolan
- Inventors: John E. Dolan
- Web page: https://patentgrep.com/patent/US8571313B2

## CPC

- H04N 1/3878: {Skew detection or correction}
- G06V 30/1478: {of characters or characters lines}
- H04N: PICTORIAL COMMUNICATION, e.g. TELEVISION
- G06V: IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING

## Abstract

Aspects of the present invention are related to systems and methods for determining a skew angle associated with a document image. According to a first aspect of the present invention, a rotation vector may be estimated for at least one layer in a vertical-edge buffer and a horizontal-edge buffer. According to a second aspect of the present invention, a rotation vector may be estimated directly from the vertical-edge buffer and the horizontal-edge buffer using a fixed-sized, progressively constrained histogram.

## Claims

### Claim 1

A system for determining a skew angle associated with a scanned document page, said system comprising:
a) an edge-buffer generator for generating, from a normalized image associated with a scanned document page, a vertical-edge buffer of vertical edges and a horizontal-edge buffer of horizontal edges; and
b) a constrained-histogram generator for forming a plurality of constrained histograms, wherein said plurality of constrained histograms comprises:
i) a first constrained histogram of deltas associated with said vertical edges and said horizontal edges at a first baseline length; and
ii) a second constrained histogram of deltas associated with said vertical edges and said horizontal edges at a second baseline length, wherein:
(1) said first baseline length is less than said second baseline length;
(2) said first constrained histogram and said second constrained histogram are the same size; and
(3) said second constrained histogram is centered around a first delta determined from said first constrained histogram.

### Claim 2 (depends on 1)

A system as described in claim 1 further comprising:
a) an image receiver for receiving an input image associated with said scanned document page; and
b) an image preprocessor for generating said normalized image from said received input image.

### Claim 3 (depends on 2)

A system as described in claim 2, wherein said image preprocessor comprises:
a) a down-sampler; and
b) a color-to-grayscale converter.

### Claim 4 (depends on 1)

A system as described in claim 1, wherein said edge-buffer generator comprises:
a) a vertical-strip summary-measure calculator for calculating a summary measure associated with each vertical strip in a plurality of vertical strips in said normalized image, thereby producing a vertical-strip buffer;
b) a horizontal-strip summary-measure calculator for calculating a summary measure associated with each horizontal strip in a plurality of horizontal strips in said normalized image, thereby producing a horizontal-strip buffer;
c) a vertical-edge detector for identifying a first plurality of edges in said vertical-strip buffer, thereby producing said vertical-edge buffer; and
d) a horizontal-edge detector for identifying a second plurality of edges in said horizontal-strip buffer, thereby producing said horizontal-edge buffer.

### Claim 5 (depends on 1)

A system as described in claim 1, wherein said first delta corresponds to a peak in said first histogram.

### Claim 6 (depends on 1)

A system as described in claim 1, wherein said system resides in a device selected from the group consisting of an imaging device and a computing device.

### Claim 7 (depends on 1)

A system as described in claim 1, wherein said first constrained histogram and said second constrained histogram are stored in the same memory locations.