measureColor

Measure color reproduction using test chart

Syntax

colorTable = measureColor(chart)

[colorTable,colorCorrectionMatrix] = measureColor(chart)

Description

colorTable = measureColor(chart) measures the color values at all color regions of interest (ROIs) of an Imatest^® eSFR chart [1] or a Calibrite ColorChecker^® Classic chart [2].

[colorTable,colorCorrectionMatrix] = measureColor(chart) also returns a color correction matrix computed using a linear least squares fit.

Examples

Measure Color Accuracy of eSFR Chart

Read an image of an eSFR chart into the workspace.

I = imread("eSFRTestImage.jpg");

Create an esfrChart object, then display the chart with ROI annotations. The 16 color patch ROIs are labeled with white numbers.

chart = esfrChart(I);
displayChart(chart,displayEdgeROIs=false, ...
    displayGrayROIs=false,displayRegistrationPoints=false)

Measure the color in all color patch ROIs.

colorTable = measureColor(chart)

colorTable=16×8 table
    ROI    Measured_R    Measured_G    Measured_B    Reference_L    Reference_a    Reference_b    Delta_E
    ___    __________    __________    __________    ___________    ___________    ___________    _______

     1         67            57            58          38.586           7.541         7.0812       15.15 
     2        156           127           122          62.182          13.225         13.826      9.8745 
     3         73            95           152          49.369        -0.51463        -20.062      18.455 
     4         62            79            58          43.926         -6.8587         17.278      14.849 
     5        104           109           171          53.415           9.457        -22.822       12.99 
     6        118           175           187           69.95         -20.889       -0.21752      13.123 
     7        214           192            69          78.643          1.8052         67.091      9.2399 
     8        154            73           138          46.853          41.998        -17.056      5.1282 
     9         62           120           182           51.05         -15.166        -22.416      22.813 
    10         55            80           185          40.811          8.7346        -44.265      22.782 
    11         79           135            72          55.716         -23.419         28.839      9.4826 
    12        152            53            77          42.759          44.167         7.9536      5.4168 
    13        169            91            52          58.211           27.58         47.578      15.988 
    14        142            63            87          47.012           39.15         8.5453      11.651 
    15         91            67           102          40.591          17.951         -9.525      10.897 
    16        152           183            80          70.505         -16.318         49.811      10.253 

Display the color accuracy measurements. Each square color patch is the measured color, and the thick surrounding border is the reference color for that ROI. Each color accuracy measurement is displayed as Delta_E, the Euclidean distance between measured and reference colors in the CIE 1976 L*a*b* color space. More accurate colors have a smaller Delta_E.

figure
displayColorPatch(colorTable)

For an alternative representation of the color accuracy measurements, plot the measured and reference colors in the CIE 1976 L*a*b* color space on a chromaticity diagram. Red circles indicate the reference color. Green circles indicate the measured color of each color patch. The chromaticity diagram does not portray the brightness of color.

figure
plotChromaticity(colorTable)

ROIs with a shorter distance between the reference and measurement points have smaller differences in chromaticity, which can contribute to a smaller value of Delta_E. However, brightness also contributes to the value of Delta_E. For example, even though the reference and measurement points for ROI 13 are near each other on the chromaticity diagram, they have a large Delta_E because of their large difference in brightness.

Measure Color of ColorChecker Chart

Read an image of a ColorChecker® chart into the workspace.

I = imread("colorCheckerTestImage.jpg");

Create a colorChecker object, then display the chart with ROI annotations.

chart = colorChecker(I);
displayChart(chart)

Measure the color in each color patch ROI.

colorTable = measureColor(chart)

colorTable=24×9 table
    ROI         Color          Measured_R    Measured_G    Measured_B    Reference_L    Reference_a    Reference_b    Delta_E
    ___    ________________    __________    __________    __________    ___________    ___________    ___________    _______

     1     {'DarkSkin'    }       160           129           120           37.54          14.37          14.92       20.193 
     2     {'LightSkin'   }       229           200           191           64.66          19.27           17.5       22.587 
     3     {'BlueSky'     }       146           191           241           49.32          -3.82         -22.54       27.312 
     4     {'Foliage'     }       130           161           117           43.46         -12.74          22.72       20.404 
     5     {'BlueFlower'  }       175           187           248           54.94           9.61         -24.79       23.073 
     6     {'BluishGreen' }       155           232           226           70.48         -32.26          -0.37       18.284 
     7     {'Orange'      }       255           161            99           62.73          35.83           56.5       16.114 
     8     {'PurplishBlue'}       130           164           254           39.43          10.75         -45.17       28.889 
     9     {'ModerateRed' }       252           146           160           50.57          48.64          16.67       23.601 
    10     {'Purple'      }       139           118           175            30.1          22.54         -20.87       24.672 
    11     {'YellowGreen' }       187           226           110           71.77         -24.13          58.19        15.21 
    12     {'OrangeYellow'}       241           194            76           71.51          18.24          67.37       14.148 
    13     {'Blue'        }        96           131           255           28.37          15.42          -49.8        33.34 
    14     {'Green'       }       118           209           130           54.38         -39.72          32.27       22.461 
    15     {'Red'         }       234           116           114           42.43          51.05          28.62        21.87 
    16     {'Yellow'      }       241           227           105            81.8           2.67          80.41       23.495 
      ⋮

Input Arguments

chart — Test chart

esfrChart object | colorChecker object

Test chart, specified as an esfrChart object or a colorChecker object.

Output Arguments

colorTable — Color values

p-by-8 table

Color values in each color patch, returned as a p-by-8 table, where p is the number of color patches on the test chart, chart.

The eight columns represent these variables:

Variable	Description
ROI	Index of the sampled ROI. The value of ROI is an integer in the range [1, 16]. The indices match the ROI numbers displayed by displayChart.
Measured_R	Mean value of red channel pixels in the ROI. Measured_R is a scalar of the same data type as chart.Image, which can be of type single, double, uint8, or uint16.
Measured_G	Mean value of green channel pixels in the ROI. Measured_G is a scalar of the same data type as chart.Image.
Measured_B	Mean value of blue channel pixels in the ROI. Measured_B is a scalar of the same data type as chart.Image.
Reference_L	Reference L* value of the ROI. Reference_L is a scalar of type double.
Reference_a	Reference a* value of the ROI. Reference_a is a scalar of type double.
Reference_b	Reference b* value of the ROI. Reference_b is a scalar of type double.
Delta_E	Euclidean color distance between the measured and reference color values in the L*a*b* color space, as outlined in CIE 1976. Delta_E is a scalar of type double.

colorCorrectionMatrix — Color correction coefficients

4-by-3 matrix

Color correction coefficients, returned as a 4-by-3 matrix. colorCorrectionMatrix represents an affine transformation that you can use to color-correct images that are captured under similar lighting conditions as the test chart image.

Data Types: double

Tips

• The measureColor function assumes that measured RGB values are in the sRGB color space. The measureColor converts the values to the L*a*b* color space to calculate the color error, Delta_E.

• When the chart is an esfrChart object, the white point of the reference L*a*b* values is the CIE standard illuminant D65.

• When the chart is a colorChecker object, the white point of the reference L*a*b* values is the CIE standard illuminant D50. The reference L*a*b* values are for the "After November 2014" version of the ColorChecker chart.

References

[1] Imatest. "Esfr". https://www.imatest.com/mathworks/esfr/.

[2] Calibrite. "ColorChecker Classic". https://calibrite.com/us/product/colorchecker-classic/.

Version History

Introduced in R2017b