Browse Prior Art Database

Crosshatch Detection and Removal Algorithm

IP.com Disclosure Number: IPCOM000005364D
Publication Date: 2001-Sep-06
Document File: 9 page(s) / 141K

Publishing Venue

The IP.com Prior Art Database

Abstract

When a copy of a digital print is made by a digital print station, an artifact is observed in the copy that has a crosshatch appearance. The Crosshatch Detection and Removal Algorithm detects the presence of the crosshatch artifact. If the artifact is detected it is then removed from the image by the application of a notch filter. This report explains how the algorithm works and results are reported on the performance of the algorithm for a test set of over 100 images.

This text was extracted from a Microsoft Word 97 document.
This is the abbreviated version, containing approximately 35% of the total text.

Crosshatch Detection and Removal Algorithm

ABSTRACT:

When a copy of a digital print is made by a digital print station, an artifact is observed in the copy that has a crosshatch appearance. The Crosshatch Detection and Removal Algorithm detects the presence of the crosshatch artifact. If the artifact is detected it is then removed from the image by the application of a notch filter. This report explains how the algorithm works and results are reported on the performance of the algorithm for a test set of over 100 images.

Crosshatch Detection and Removal Algorithm

Introduction 3

Principles Of Operation 3

Artifact Detection 3

Artifact Removal 7

Performance 8

Summary 9

Introduction

The Crosshatch Detection and Removal Algorithm is designed to detect and remove artifacts that occur when a print made by a digital printer is copied and enlarged by a digital print station, such as found in a photo kiosk. Specifically, two problems are encountered:

The digital print has invisible banding that becomes visible in an enlarged copy.

A raster pattern in the digital print aliases to a lower frequency pattern in a copy that may be visible.

The goal of the detection stage is to determine if a print contains one or both of the artifacts mentioned above. In the removal stage, the artifacts are removed from the print without degrading scene content.

Principles Of Operation

Artifact Detection

The purpose of the detection phase is to determine if the digital image has a banding artifact. The banding artifact occurs in prints from a digital scanner, not in optical prints. Therefore, the banding artifact should be accompanied by an aliasing artifact that arises from the grid of dots produced by the digital printer.

The aliasing artifact is strongest in the fast direction of the printer. Whereas, the banding artifact only occurs in the slow direction of the printer. Therefore, in order to reduce the possibility of false detection, a print is classified as having banding only if a banding signature is accompanied by an orthogonal aliasing signature.

The signature detected is a peak in the power spectrum at the frequency that aliasing and banding is known to occur. The aliasing frequency is determined by the number of dots-per-inch used by the printer to produce the digital print and the resolution at which the print is scanned by a digital print station. The banding frequency has been found to be a fixed characteristic of the digital printer and is provided as a parameter to the detection algorithm.

A critical feature of the detection algorithm is its execution speed because it must be run on all prints if there is a possibility that a print may contain a banding artifact. For this reason, instead of looking for the aliasing and banding artifact throughout the print, the print is divided into small squares of nominal size 256x256. Artifact detection is only performed for a sub-sampling of the squares.

The algorithm attempts to detect horizontal banding, vertical banding, horizontal aliasing,...