measureColor
Measure color reproduction using test chart
Syntax
Description
esfrChart or colorChecker Object
Use an esfrChart or a colorChecker object when
you want to automatically detect the color regions of interest (ROIs) of a test
chart. The esfrChart object supports the Enhanced or Extended
version of the Imatest® eSFR test chart [1]. The
esfrChart object supports the Calibrite ColorChecker® Classic test chart [2].
colorValues = measureColor(chart)
[
also returns a color correction matrix computed using a linear least squares
fit.colorValues,colorCorrectionMatrix] = measureColor(chart)
Test Chart Image (since R2024a)
Use a test chart image for other types of test charts that are not supported by
the esfrChart or colorChecker objects. You must
identify the positions of the color ROIs.
colorValues = measureColor(im,roiPositions,refLAB)roiPositions for test chart image
im. The reference L*a*b* values of each ROI are given
by refLAB.
colorValues = measureColor(im,roiPositions,refLAB,Name=Value)
[
also returns a color correction matrix computed using a linear least squares
fit, using any combination of input arguments from previous syntaxes.colorValues,colorCorrectionMatrix] = measureColor(___)
Examples
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.
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
⋮
Read and display an image of a custom test chart with color patch ROIs. This example simulates a custom test chart image by cropping an Imatest eSFR test chart.
I = imread("RGBColorPatches.jpg");
imshow(I)
Draw ROIs for the red, green, and blue color patches.
numROIs = 3; roiPos = zeros(numROIs,4); for cnt = 1:numROIs hrect = drawrectangle; roiPos(cnt,:) = hrect.Position; end
Load the reference L*a*b* values of the color patches, which are saved in a file called RGBColorPatches_refLab.mat. The white point of the reference values is the CIE standard illuminant D50.
load("RGBColorPatches_refLab.mat");Calculate the color reproduction values for these selected edges, specifying the white point.
colorValues = measureColor(I,roiPos,refLab,ReferenceLABWhitePoint="d50");On a color patch diagram, display the measured and reference colors and the color error.
displayColorPatch(colorValues)
Input Arguments
Name-Value Arguments
Output Arguments
More About
References
[1] Imatest. "Esfr". https://www.imatest.com/mathworks/esfr/.
[2] Calibrite. "ColorChecker Classic". https://calibrite.com/us/product/colorchecker-classic/.
