John McCann, Carinna Parraman& Alessandro Rizzi

McCann Imaging, Belmont, MA, USA

Centre for Fine Print Research, University of the West of England, UK

Dipartimento di Informatica

48Edges and Gradients

in real 3-D color scenes

Color appearance can be analyzed either by an object's reflectance and scene illumination, or by edges and gradients.

• In one framework, appearance should correlate with reflectance, because humans might discount the spectra and the nonuniformity of illumination. • In the other, both reflectance and illumination make equal contributions to scene radiances. Edges generate large changes in appearance, while gradient are much less visible.

Experiments using painted blocks (3-D Mondrians) measured the appearance of objects with constant reflectance in complex real-world illumination.

We matched the appearances of identical surface reflectances in variable illumination.

Abstract

Matches of appearance correlate with the spatial configuration of

edges and gradients.

They do not correlate with reflectance and discounted illumination.

Summary

Experiment• Two identical sets of blocks [11 paints]

• Same room - same time

• Two different illuminations: HDR, LDR

• Compare the appearances of identical surface reflectances

Measurements• A. Magnitude estimate matches [Munsell Book]

• B. Painted reproduction of both LDR & HDR

• Rendering intent - match appearance

• Measure reflectance of watercolor painting

Illumination on gray blocksLDR HDR

Only 11 paints

!

Do identical reflectances look the same?

LDR illumination HDR illumination

Illuminationmakes edges

• A. Magnitude estimate matches [Munsell Book]

Low Dynamic Range Illumination (LDR)

GroundTruth

• A. Magnitude estimate matches [Munsell Book]

High Dynamic Range Illumination (HDR)

Average of 11 observers

Changes from Illumination

B. Measure LDR & HDR appearancesby painting with watercolors

LDR HDR

Carinna Parraman’s Watercolor Painting

Departures from “perfect constancy”are the signature of the

color constancy mechanism.

J. McCann, (2004) "Mechanism of color constancy," IS&T/SID 12th Color Imaging Conference: 29-36 358.

J. McCann, (2005) "Do humans discount the illuminant?,"HVEI, SPIE Proc, vol. 5666, 9-16.

LDR Painting Reflectance

0

20

40

60

80

100

400 500 600 700

Wavelength (nm)

Lig

htn

ess

(L*

)

30 LDR 32 LDR 33 LDR

18 LDR 9LDR

HDR Painting Reflectance

0

20

40

60

80

100

400 500 600 700Wavelength (nm)

Lig

htn

ess

(L*

)

30 HDR 32 HDR 33 HDR

18 HDR 9 LDR

LDR HDR

Appearance = Reflectance ?Planar LDR HDR

Yes Mostly Rarely Spatial, Cone

McCann JJ, McKee, SP.,& Taylor, T.

Vision Research, 16, 445–458, (1976).

<http://mccannimaging.com/Retinex/3-D_Mondrians.html>

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