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Image Dependent Gamut Mapping

IP.com Disclosure Number: IPCOM000010882D
Publication Date: 2003-Jan-29

Publishing Venue

The IP.com Prior Art Database

Abstract

The ColourKit utilities allow scanned images to be rendered using a 'Best Facsimile' mode. The aim of this mode is to produce, within the limits of the rendering process, a visual match to the original being scanned. The 'Best Facsimile' algorithm is described in detail in Ref.1. In summary the scanned image is transformed to absolute colorimetric values using the information in the ICC profile for the scanner, and tone curve data for the image and the profile. The rendering process may not be able to reproduce the full colour gamut of the original, and an L* mapping is applied to ensure that the L* values of the black and white points of the image lie within the range which can be reproduced on the output medium. The L* mapping is determined by examining the ICC profile for the output process. The image is then transformed to output device space using the ICC output profile. (In practice the input profile, L* mapping, and output profile may be applied to the image as one concatenated transform.)

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Image Dependent Gamut Mapping.

Part1: Image and Print Gamut Characterisation and Comparison

Version:                � � � � � � � � � � � � � � � � DRAFT

Author:                � � � � � � � � � � � � � � � � � J.Deane

Document No.        � � � � � R 329        � � � � �

Issue Date.        � � � � � � � � � � � 22nd February 2002

� � � � � � � � � � � � � � � Copyright FUJIFILM Electronic Imaging Ltd,� 2002

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1. Introduction

The ColourKit utilities allow scanned images to be rendered using a 'Best Facsimile' mode. The aim of this mode is to produce, within the limits of the rendering process, a visual match to the original being scanned. The 'Best Facsimile' algorithm is described in detail in Ref.1. In summary the scanned image is transformed to absolute colorimetric values using the information in the ICC profile for the scanner, and tone curve data for the image and the profile. The rendering process may not be able to reproduce the full colour gamut of the original, and an L* mapping is applied to ensure that the L* values of the black and white points of the image lie within the range which can be reproduced on the output medium. The L* mapping is determined by examining the ICC profile for the output process. The image is then transformed to output device space using the ICC output profile. (In practice the input profile, L* mapping, and output profile may be applied to the image as one concatenated transform.)

        � � � � � � � � � � � � � � � The problem with this process is that although some effort is made to ensure the range of the image is adjusted to lie within that of the output process, no effort is made to ensure that, after the L* mapping is applied, the full gamut of colours in the image can be reproduced on the output medium. If the rendering process is not capable of reproducing the full gamut of the image, the way in which they are rendered will be dependent on the way in which out of gamut colours have been handled in the output profile. In practice this will result in colours being clipped to those at the edge of the output gamut loosing modelling in the subject. These effects have been seen in� 'Best Facsimile' reproductions of fruit giving very unnatural looking results. The results are often unnaturally saturated as much of the image is reproduced at the edge of the output process gamut.

        � � � � � � � � � � � � � � � The purpose of this project is to introduce an image and output process dependent edit to follow the current L* mapping. This is to adjust the colorimetric values of the image to maintain the facsimile appearance of the majority of the image, whilst modifying the values in areas of the colour gamut which will otherwise be clipped. This should maintain the modelling in the final result. The first stage of this...