measureChromaticAberration

Measure chromatic aberration at slanted edges using Imatest eSFR chart

Syntax

aberrationTable = measureChromaticAberration(chart)

aberrationTable = measureChromaticAberration(chart,Name,Value)

Description

aberrationTable = measureChromaticAberration(chart) measures the chromatic aberration at all slanted edge regions of interest (ROIs) of an Imatest^® Extended eSFR chart [1].

aberrationTable = measureChromaticAberration(chart,Name,Value) measures the chromatic aberration with additional parameters to specify a subset of ROIs to measure.

Examples

Measure Chromatic Aberration of Slanted Edges on 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 60 slanted edge ROIs are labeled with green numbers.

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

Measure the chromatic aberration in all slanted edge ROIs. Examine the contents of the returned table, chTable, for a single ROI.

chTable = measureChromaticAberration(chart);
ROIIndex = 3;
chTable(3,:)

ans=1×5 table
    ROI    aberration    percentAberration     edgeProfile     normalizedEdgeProfile
    ___    __________    _________________    _____________    _____________________

     3       1.9164           0.14883         {348x4 table}        {348x4 table}    

Store the normalized edge profile in a separate variable, edgeProfile, for clarity. Examine the normalized color intensity of the first and last pixel of edgeProfile.

edgeProfile = chTable.normalizedEdgeProfile{ROIIndex};
edgeProfile([1 end],:)

ans=2×4 table
    normalizedEdgeProfile_R    normalizedEdgeProfile_G    normalizedEdgeProfile_B    normalizedEdgeProfile_Y
    _______________________    _______________________    _______________________    _______________________

          -0.0022049                  0.0094112                  0.0088172                  0.0050968       
             0.98511                    0.98953                       1.01                      0.986       

Plot the normalized intensity for the ROI.

npix = length(edgeProfile.normalizedEdgeProfile_R);
plot(1:npix,edgeProfile.normalizedEdgeProfile_R,'r', ...
    1:npix,edgeProfile.normalizedEdgeProfile_G,'g', ...
    1:npix,edgeProfile.normalizedEdgeProfile_B,'b')
xlabel('Pixel')
ylabel('Normalized Intensity')
title(['ROI ' num2str(ROIIndex) ' with Aberration ' num2str(chTable.aberration(ROIIndex))])

The blue channel has a higher intensity than the red and green channels immediately before the edge, and a lower intensity than the red and green channels immediately after the edge. This difference in intensity contributes to the measured value of chromatic aberration.

The measured values of aberration and percentAberration for this edge are relatively small. Visual inspection of the image confirms that the sides of the edge do not have a strong color tint.

Input Arguments

chart — eSFR chart

esfrChart object

eSFR chart, specified as an esfrChart object.

Name-Value Pair Arguments

Specify optional comma-separated pairs of Name,Value arguments. Name is the argument name and Value is the corresponding value. Name must appear inside quotes. You can specify several name and value pair arguments in any order as Name1,Value1,...,NameN,ValueN.

Example: measureChromaticAberration(myChart,'ROIIndex',2) measures the chromatic aberration only of ROI 2.

'ROIIndex' — ROI indices

1:60 (default) | scalar | vector

ROI indices to include in measurements, specified as the comma-separated pair consisting of 'ROIIndex' and a scalar or vector of integers in the range [1, 60]. The indices match the ROI numbers displayed by displayChart.

Note

measureChromaticAberration uses the intersection of ROIs specified by 'ROIIndex' and 'ROIOrientation'.

Example: 29:32

Data Types: single | double | int8 | int16 | int32 | uint8 | uint16 | uint32

'ROIOrientation' — ROI orientation

'both' (default) | 'vertical' | 'horizontal'

ROI orientation, specified as the comma-separated pair consisting of 'ROIOrientation' and 'both', 'vertical', or 'horizontal'. The measureChromaticAberration function performs measurements only on ROIs with the specified orientation.

Note

measureChromaticAberration uses the intersection of ROIs specified by 'ROIIndex' and 'ROIOrientation'.

Example: 'vertical'

Data Types: char | string

Output Arguments

aberrationTable — Chromatic aberration measurements

m-by-5 table

Chromatic aberration measurements, returned as an m-by-5 table. m is the number of sampled ROIs.

The five columns represent these variables:

Variable	Description
ROI	Index of the sampled ROI. The value of ROI is an integer in the range [1, 60].
aberration	Chromatic aberration, measured as the area between the maximum and the minimum red, green, and blue edge intensity profiles. The measured chromatic aberration indicates perceptual chromatic aberration. aberration is a scalar of type double.
percentAberration	Aberration, expressed as a percentage of the distance in pixels between the center of the image and the center of the ROI.
edgeProfile	Intensity profile of each color channel across the edge in the ROI. edgeProfile is an s-by-4 table, where s is the number of samples across the edge. The four columns represent the red, green, blue, and luminance values, averaged along the edge. Luminance (Y) is a linear combination of the red (R), green (G), and blue (B) channels according to: Y = 0.213R + 0.715G + 0.072B Note The sampling rate for the chromatic aberration measurement is about four times the sampling rate of the image.
normalizedEdgeProfile	Intensity profile, normalized between [0, 1] using 5% of the front end and tail end of data. normalizedEdgeProfile is an s-by-4 table with a similar structure to edgeProfile.

Tips

• Chromatic aberration is best measured at slanted edges that are:

  • Roughly orthogonal to the line connecting the center of the image and the center of the ROI

  • Farthest from the center of the image

  Because chromatic aberration increases radially from the center of the image, measurements at slanted edges near the center of the image can be ignored.

• The absolute chromatic aberration reported in the aberration field is measured in the horizontal or vertical direction. However, chromatic aberration is a radial phenomenon, and radial measurements are more accurate.

References

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