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S-CIELAB简介
1、Measuring perceptual colordifference
Like the CIELAB Delta E metric,the S-CIELAB metric is a "perceptual color fidelity" metric. Itmeasures how accurate the reproduction of a color is to theoriginal when viewed by a human observer. The CIELAB metric issuitable for measuring color difference of large uniform colortargets. The S-CIELAB metric extends the CIELAB Delta E metric tocolor images.
Color discrimination is determined by a lotof factors, such as the cone sensitivity of the eye, the ambientillumination, the spatial pattern of the targets, etc. The effectsof some of these factors on color sensitivity are better understoodtha n others. To learn the basics (and advances) about color scienceand vision in general, check out the book Foundations of Vision,written by Brian Wandell.
2、CIE L*a*b* Delta Emetric
One of the most widely usedperceptual color fidelity metric is the Delta E metric, given aspart of the CIE L*a*b* standard color space specification. Tomeasure perceptual difference between two lights using this metric,the spectral power distribution of the two lights are firstconverted to XYZ representations, which reflect (within a lineartransformation) the spectral power sensitivities of the three coneson the human retina. Then, the XYZ values are transformed into anL*a*b* space, in which equal distance is supposed to correspond toequal perceptual difference (a "perceptually uniform" space). Then,the perceptual difference between the two targets can be calculatedby taking the Euclidean distance of the two in this L*a*b* space.The difference is expressed in "Delta E" units. One Delta E unitrepresents approximately the threshold detection level of the colordifference.
The CIE L*a*b* Delta E metric is intended tobe used on large uniform color targets (at least 2° visual angle insize).
3、Color difference of patterned targets
Color discrimination and appearance is afunction of spatial pattern. In general, as the spatial frequencyof the target goes up (finer variations in space), colordifferences become harder to see, especially differences along theblue-yellow color direction. This result came from many studies,including Poirson & Wandell (1993, 1996), Kelly(1983), etc.
So, if we want to apply the CIE L*a*b* Delta E metric to colorimages, the spatial patterns of the image have to be taken intoaccount. The goal of the S-CIELAB metric is to add a spatialpre-processing step to the standard CIELAB Delta E metric toaccount for the spatial-color sensitivity of the human eye.
4、Structure of S-CIELAB: A spatial extension toCIELAB
The S-CIELAB calculation is illustrated below.The key components are the color transformation and the spatialfiltering steps before the standard CIELAB Delta E calculations.This is a "Pattern-Color Separable" architecture. For moreinformation, please check out some related papershere.
