HP - PURDUE CONFIDENTIAL Slide No. Fundamentals 1.

HP-PURDUE CONFIDENTIAL Slide No.

Fundamentals

1


Motivation

Issue

Text and lines are indispensable to print quality

Toner overdevelopment causes toner scatter

Toner scatter makes printed pages appear blurred near the edges of text or thin lines

Current solutions

Limit the color gamut of printers» Undesired color shifts and introduction of halftone texture

Compensate for toner overdevelopment by the coring algorithm» Little research due to new exclusive method

2


Toner Scatter

3

OPC Drum

Developer Roller

Charging Roller

Laser Beam

Cleaning Blade

Transfer RollerPressure Roller

Fuser Roller

Paper

Process Direction

EP process Effect of toner scatter


Overview of Coring

Technique of decreasing amount of colorant in the interior

of text or characters to diminish effect of toner scatter

Optimal coring value: minimum decrease of colorant

amount by percentage ∆ of nominal colorant level

4

1 pixel 2-pixel protection

Nominal colorant

level

Profile of cored line

No coring 18% coring 50% coring

Annulus with10-pixel widthPrinted at 600 dpi, scanned at 1200 dpi


Overview of Coring (cont.)

By utilizing both high and low resolution image data

5

Does pixelhave colorant levelto produce toner

scatter?

Is pixel located within 2-pixel

protected zone?

Estimate dimension of line

structure

Core pixelFetch next pixel

Image(high resolution)

Image(low resolution)

yes

yes

no

no

Coring algorithm


Goals

Reduce toner scatter in region of interest, while yielding

the appearance of sharper edge transition

Overcome disadvantages caused by limiting the color

gamut of printers

6

No coring Optimally cored color clipped

Printed at 600 dpi, scanned at 1200 dpi


Outline

Remote Print Defect Diagnosis

Print Quality Assessment

Fundamentals

Determination on Optimal Coring Values

Preference Experiment

Conclusions

Future Work

7


Determination onOptimal Coring Values

8


Subjective Assessment

Objectives

To evaluate threshold of coring amount by psychophysical experiments

To determine limits of the coring algorithm that may cause unacceptable results

Environment

Equipments» HP Color LaserJet 2605 at printing resolution of 600 dpi

» HP Color Laser Glossy Photo Paper 220 g/m2

Normal ambient lighting condition in controlled laboratory

9


Experimental Design

Pilot experiments

Test stimulus in the shape of annulus Line widths between 5/600 inch and 50/600 inch

Nominal colorant levels between 90% and 100% in steps of 2.5%» Only combinations of cyan (C) and magenta (M) with same amounts

10

7 colorant levels(100%

~ 70%

)

26 coring levels(Increasing order)

Pilot test 1

7 coring levels (Increasing order)


~ 90%

)

Pilot test 4 Pilot test 5



~ 90%

)

Pilot test 2


~ 80%

)


Pilot test 3

7 coring levels(Random order)

5 Colorant levels

(100% to 90%

)


Experimental Design (cont.)

11

Symmetric coring

More coring for C and M

Asymmetric coring

More coring for M

Test pattern Fixed nominal colorant

level and fixed line width

Five repetitive trials

Randomly distributed over sets of test pages



12

Test pattern

Calibration patches

Color planeregistration bars

Coring increases Coring of M increases

Coring of C

increases

Varying

nom

inal colo

rant level

(fixed line width

)

Skew mark

• Symmetric coring Decreasing same amountsof C and M colorants Perception of toner scatter

• Asymmetric coring Decreasing different amountsof C and M colorants Perception of toner scatter Color matching



Charge to subject

View at normal viewing distance of 10 to 15 inches

Use apparatus to protect test page and facilitate examination of one test pattern at a time

Choose one test stimulus» Symmetric leftmost one with no toner scatter

» Asymmetric one with no toner scatter and mostclosely matched color of thick border of solid colorant

Disregarded factors:

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Background noise Jaggedness Color plane misregistration

Apparatus for symmetric coring


Responses of Subjects

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Symmetric coring (25 subjects) Asymmetric coring (21 subjects)


Optimal Coring Values

More coring for high colorant levels and thin lines

Less coring for low nominal colorant levels and wide lines

More coring for colorant M than colorant C except nominal

colorant level 100% with thin line width

15


Comparison with Bernal et al.’s Model

Difference

Line width: very thin and wide line widths (5 and 50 pixels)

Nominal colorant level: low colorant level (90%)

Basis of data for Bernal’s model Monochrome softcopy experiments

using blur in edges and lines

Obtained from two subjects

16


Influence of Edge Directionon Toner Scatter

17

More coring for vertical than horizontal

Coring amounts for horizontal don’t depend

on line width

Front view

Developerroller

Process direction

Stealing toner

Latent image of page on OPC drum surface

Horizontalpattern

Verticalpattern

Duration of toner stealing for vertical is

longer than one for horizontal


Influence of Edge Directionon Toner Scatter (cont.)

18

Line width: 10 pixels, Nominal colorant level: 100%Printed at 600dpi, scanned at 1200dpi


Outline

Remote Print Defect Diagnosis

Print Quality Assessment

Fundamentals

Determination on Optimal Coring Values


Conclusions

Future Work

19



Objective

To evaluate whether human viewers prefer optimally cored prints over prints without coring in real-world images

Test material

Seven different page contents» Seven different nominal colorant levels (94%~100% in steps of 1%)

» Asymmetric coring

Comparison

Control group: 10 subjects who participated in previous experiments

Experimental group: 10 new subjects who don’t know about coring

Charge to subject

Choose a page they prefer over another in terms of color and blur

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Preference Experiment (cont.)

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Test set


Preference Experiment (cont.)

Results

Overall rate of preference to optimal coring over no coring: 98.57%» Group

Control group: 99.39% Experimental group: 97.76%

» Colorant level

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Colorant level (%)Number of chosen page

Preference rate (%)No coring Optimal coring

100 0 140 100

99 0 140 100

98 1 139 99.29

97 1 139 99.29

96 0 140 100

95 2 138 98.57

94 10 130 92.86

Total 14 966 98.57


Conclusions

Subjects preferred

Less coring as colorant level decreases or line width increases

More coring for colorant M than colorant C

Vertical edges of text or characters provoke toner scatter

easily than horizontal edges do

Confirmed that optimally cored prints were overwhelmingly

preferred over non-cored prints

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HP - PURDUE CONFIDENTIAL Slide No. Fundamentals 1.

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