Macbeth ColorChecker的RGB坐标解析

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"这篇文档揭示了Macbeth ColorChecker色卡在不同RGB色彩空间中的坐标值，包括8位和16位格式，涉及Adobe、Apple、ProPhoto和sRGB四个常见RGB色彩空间。文档还介绍了计算这些数据的方法和方程式，并对比了制造商提供的参考数据与用户测量的值。主要关键词包括ColorChecker、RGB坐标、RGB空间、色彩空间和色彩转换。" 在摄影和视频制作中，Macbeth ColorChecker色卡被广泛用作色彩校正的标准参照物，用于快速评估成像设备的颜色再现准确性。色卡由24个色彩补丁组成，这些补丁模拟了常见的自然色彩，如肤色和植物叶色，以帮助确保色彩在不同场景下的一致性和准确性。 RGB坐标是描述颜色在特定色彩空间中的位置的关键参数，它们由红色(R)、绿色(G)和蓝色(B)三个基色的强度组合而成。在不同的RGB色彩空间中，这些基色的定义（如主色、光照条件和伽马响应）会有所不同，导致相同颜色在不同空间内的坐标值也会变化。例如，Adobe RGB和Apple RGB空间广泛应用于图像编辑软件，而ProPhoto RGB具有更宽的色域，能捕捉更多的颜色，sRGB则是互联网和许多显示器的默认色彩空间。文档中提供的RGB坐标数据对于色彩管理至关重要，摄影师和后期制作人员可以使用这些数据来校准相机、显示器或打印机，确保色彩在不同设备间的准确传递。此外，通过比较制造商提供的参考数据和实际测量值，可以评估设备的色彩还原能力，识别并修正色彩偏差。计算RGB坐标通常涉及到色彩空间转换的数学过程，包括线性化、色彩空间定义的主色转换以及光照条件的考虑。例如，从线性RGB（未经伽马校正）转换到非线性RGB（如标准sRGB）时，需要用到伽马函数，以符合人类视觉对亮度的感知。这份文档提供了深入的色彩科学知识，包括色度学原理，它不仅对专业人士，也对那些对色彩管理有兴趣的业余爱好者非常有价值。通过理解并应用这些RGB坐标和色彩空间转换方法，用户可以提高其作品的色彩精度和一致性，从而提升整体的视觉效果。

2006-06-01

The first comparison that comes to mind is the one between

the two “official” tristimulus data sets, published in 1976

(“ColorChecker 1976”) and 2005 (“ColorChecker 2005”),

shown in Table 1a. In order to compare the two sets on the

same basis, we have chosen to convert the xyY Ill-C (1976)

coordinates to the L*a*b* D50 color space used for the most

recent reference. From xyY, one can readily determine XYZ

values, then use a Chromatic Adaptation Transform (CAT), in

this case the Bradford matrix discussed in Section 5.2, to

convert the XYZ coordinates between Illuminant C and

Illuminant D50, and finally compute the proper L*a*b*

values. The use of a CAT is required since we do not have the

spectral data corresponding to the xyY coordinates. While

using a CAT can introduce an error, this error has less of an

effect than if it was simply added to the inherent difference

between the data sets; see Section 6 for more information.

The second comparison, shown in Table 1b, is between the

“ColorChecker 2005” data set and the “BabelColor Avg.”.

The third comparison, Table 1c, is between the

“ColorChecker 2005” set and tristimulus data derived from a

spectral reference file of the ColorChecker (ProfileMaker

2004). This file is provided by GretagMacbeth as part of their

ProfileMaker software package; the measurement date shown

in the file is “2/5/2004”. The fourth comparison, Table 1d, is

between the “BabelColor Avg.” and “ProfileMaker 2004”

data sets.

The color differences in Table 1 are computed using both

CIELAB and CIEDE2000. CIEDE2000 is the most recent

color difference formula recommended by the Commission

Internationale de l'Éclairage (CIE). Like the CIE94 and CMC

color difference formulas which came after CIELAB, it

strives to improve the match between the perceived color

difference and the computed difference values. CIEDE2000,

similarly to the CIE94 and CMC formulas, includes weighting

functions for lightness, chroma and hue. However, it

introduces an extra term which combines chroma and hue

with the goal of improving the performance for blue colors

(for hue angles – the h* in the L*C*h* presentation format –

around 275 degrees). It also associates a scaling factor to a*

for low chroma colors, to improve the formula performance

near the illuminant. Many users have confirmed that

CIEDE2000, while still not perfect, does achieve its goal of

improving the match between computed difference numbers

and perceived difference

In Table 1a, we see a noticeable difference between the

“ColorChecker 1976” and “ColorChecker 2005” data sets,

whereas the difference is quite small when comparing the

2005 data with either the “BabelColor Avg.” or the

“ProfileMaker 2004” sets in Tables 1b and 1c. The 1976 data

may have been deemed sufficiently precise at a time where the

chart was mostly used to visually judge the quality of silver-

based films, and not used to make precise digital

measurements as we do now.

As per GretagMacbeth Web site, the 2005 ColorChecker data

“is intended to be an average measurement of all ColorChecker Charts”.

The fact that, on average, this data set cannot be visually

differentiated from either the “ProfileMaker 2004” or the

“BabelColor Avg.” data sets makes it difficult to select the

best one. There is no detailed information on where the 2005

data comes from; it may be an average from one, or from

many production lots. There is even less information on the

origin of the ProfileMaker reference file but its good match to

the other data sets indicates it is also an average of some sorts.

As for the data compiled by BabelColor, the match to the

other two data sets is quite good, especially considering the

mix of experimental conditions imposed by many users using

different instruments. Overall, the similarity of the three data

sets points to some outstanding long term production

consistency.

Readers interested in seeing spectral graphs for each patch, as

well as information on spectral and L*a*b* variance, can

download the “ColorChecker_RGB_and_spectra.xls”

spreadsheet from the BabelColor Web site (see Ref. 3).

3. RGB coordinates of the ColorChecker

The R’G’B’ values of the ColorChecker for four common

RGB spaces, Adobe, Apple, ProPhoto and sRGB, are shown

in 8-bit format in Table 2, and in 16-bit format in Table 3.

Table 3 is a more precise version of Table 2, with more

significant digits per value. The 16-bit values can be used

mainly in programming environments, such as MATLAB,

since there is no color picker that yet offers 16-bit resolution.

You should be aware that, for computing efficiency reasons,

Photoshop processes 16-bit file as if 15-bit and resaves the

file as 16-bit; the displayed color numbers are thus divided by

two from the 16-bit values.

In Tables 2 and 3, the tables labeled “ColorChecker 2005”

show the L*a*b* D50 values provided by GretagMacbeth.

You will notice two columns with sRGB in their title in

Table 2; the one labeled “sRGB (GMB)” contains the values

provided by GretagMacbeth, while the “sRGB” column was

derived from L*a*b* D50 using the procedure presented in

Section 5. The other R’G’B’ values of the “ColorChecker

2005” table were derived in a similar manner. It should be

emphasized that for ProPhoto, a D50 based RGB space, there

is no need to perform a chromatic adaptation transform when

starting with L*a*b* D50 and that there is minimal

“conversion process-induced” errors (see Section 6).

All R’G’B’ values of the “BabelColor Avg.” tables were

obtained with the spectral reflectance average of 20 charts, the

space Illuminant spectral distribution, and the 2-degrees

Standard Observer. In other words, they were not obtained

using a chromatic adaptation transform, and do not comprise

the errors this transform may introduce.

It is interesting to note in Table 2 that the “sRGB (GMB)”

cyan patch is measured to be within the sRGB gamut, with an

R’ value of 8, while this coordinate is clipped to zero when

derived from the L*a*b* data (as can be seen in the “sRGB”

column of the “ColorChecker 2005” table). The cyan is

similarly clipped in the “BabelColor Avg.” tables.

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Macbeth ColorChecker的RGB坐标解析

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