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High quantization image processing within a quantization-constrained image path, with applications to Imaging Based Controls

IP.com Disclosure Number: IPCOM000129245D
Publication Date: 2005-Oct-03
Document File: 3 page(s) / 48K

Publishing Venue

The IP.com Prior Art Database

Abstract

The present invention is a signal processing method that performs high quantization processing within a quantization constrained signal path (image path). It operates by using local spatial dithering to encode information beyond the conventional capacity of the image path. A key example of the general method is to receive a image at low quantization (e.g., 8 bits/pixel), pad the pixel values to high quantization (e.g.,10 bits/pixel), apply a Tone Reproduction Curve (TRC) from a user control or feedback control, and perform N-level error diffusion or N-level halftoning to return the image to low quantization (e.g., 8 bits/pixel). Increased quantization resolution will be increasingly important TRCs are applied to neighboring pixel columns for streak compensation. Very small color differences can be perceived when they occur along a straight vertical or horizontal edge within otherwise uniform subject matter.

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High quantization image processing within a quantization-constrained image path, with applications to Imaging Based Controls

Manipulation of gray scale image values at 8 bits/pixel can lead to gray level defects in a printed or displayed image.  For example, applying a strong tone reproduction curve (TRC) from a user control or from automatic feedback control can lead to defects such as false contouring and streaks.  More generally, the accuracy of many signal-processing algorithms can suffer for fixed low quantization resolution.

The present invention is a signal processing method that performs high quantization processing within a quantization constrained signal path (image path).  It operates by using local spatial dithering to encode information beyond the conventional capacity of the image path.  A key example of the general method is to receive a image at low quantization (e.g., 8 bits/pixel), pad the pixel values to high quantization (e.g.,10 bits/pixel), apply a TRC from a user control or feedback control, and perform N-level error diffusion or N-level halftoning to return the image to low quantization (e.g., 8 bits/pixel).  Increased quantization resolution will be increasingly important TRCs are applied to neighboring pixel columns for streak compensation.  Very small color differences can be perceived when they occur along a straight vertical or horizontal edge within otherwise uniform subject matter.

Background

Streak compensation via spatially indexed TRCs (

US

6,760,056) has the potential to introduce undesirable spatial structure.  To compensate for streaks of arbitrary width, a TRC-based streak compensation method will apply different TRCs to neighboring pixel columns, which could result in small color differences along a straight vertical or horizontal edge within otherwise uniform subject matter.  The human visual system is very sensitive to such image structure and 8 bit quantization can be too coarse to completely avoid this problem.  For example, consider an L* range of 90 and 8 bit quantization.  A single gray level step is roughly 0.35 L*, which may be perceivable when it is a vertical edge within a uniform field on a low noise printer.  We wish to increase the quantization resolution of this operation without incurring a significant cost to the image path. 

These concepts are quite general and could be applied to operations beyond TRCs and applications beyond printing. 

Invention

The inve...