Introduction:
Starting in version 4.5, Imatest is capable of performing the cross-correlation based texture blur measurement which is under consideration for ISO 19567-2: Texture Analysis on Stochastic Pattern. This is a texture-blur-analysis method originally proposed in Description of texture loss using the dead leaves target: Current issues and a new intrinsic approach by Kirk et al at Image Engineering.
This method starts from the same principles as the so-called “Direct” method of Cao et al which is the Power Spectral Density-based method described on the Random Module page. The main difference is the Direct method makes use of assumed statistical properties of the target, the cross-correlation method makes use of the actual true pixel values themselves, promoting it from a semi-reference to a full-reference metric.
The Imatest Spilled Coins (Dead Leaves) Cross chart
Key features of the Spilled coins Random-Cross chart:
- Contrast of the pattern reflectance range is 3:1, with 18% gray background as per the CPIQ new standards.
- 8 Registration marks are placed around the active area to attain a more accurate transformation between the reference image and the test image.
- 16 grayscale OECF patches are included for linearization prior to texture response calculation.
- The chart contains slanted edges (2:1 and 4:1 contrast) for convenient comparisons between Texture MTF and slanted-edge MTF.
- Backwards Compatibility: The chart maintains flat areas of the same mean reflectance as the pattern on either side of the center area, used for Noise Power Spectral Density estimation in the classic “Direct” Texture MTF method supported by our other charts.
- Pattern design is more uniform, i.e., is more shift-invariant than other vendors’ Dead Leaves charts.
This chart can be purchased from the Imatest Store.
Chart configuration selection
The Imatest Dead Leaves Cross chart layout is the first option in the “Chart Configuration” setting list. This can be set in any of three places: Reschart’s “Chart Configuration” menu bar option, the Random Parameters window that pops up during a fixed run of that module, or, as shown below, in the ROI Options window.
Chart configuration option for the Imatest Dead Leaves Cross chart.
Settings
To perform the cross-correlation calculation instead of the “Direct” method (which is based on auto-correlation and noise PSD subtraction), you must have an appropriate chart configuration and select the “Random-Cross Method” Calculation option.
Note: The only chart configurations which the Dead Leaves Cross calculation is available for are the Imatest Dead Leaves Cross chart and the TE276v3. This is reflected in the Calculation option automatically changing to “Direct” and becoming disabled when another chart configuration is chosen. If you select a chart configuration which supports cross-correlation, it will become re-enabled.
Other settings are the same as described on the Random/Spilled Coins Module page.
Reference Image Selection
A unique requirement of the dead leaves cross-correlation measurement is that it requires a reference image to directly compare with the observed image. Thus, it is necessary to specify a reference image file when setting the options for Random results calculation. This selection comes in the form of a file path to the location of the reference file on your computer (as seen above).
Starting in Imatest 5.0, reference images are distributed with the software. They can be found in the resources > reference_images subfolder which is created in the default ini folder location created at installation time.
Note: You can find the location of this folder from the main Imatest window via the top menu bar: File > Open INI file folder
The reference image files distributed by Imatest are high-resolution (at least 8000 x 8000 px), one-channel raster files. Each represents the linearly encoded luminance channel of the patterned area of the chart under test, since this is the only information that is used in the calculation. It is important to select the correct reference file for the chart you are using, otherwise the results will be obviously meaningless.
Shrunken version of the Imatest Dead Leaves Cross chart reference image: the luminance channel of the original pattern used to print the chart.
Imatest distributes the reference files for the Imatest Dead Leaves Cross chart with grayscale pattern area, the same chart with color pattern area, and the Image Engineering TE276v3 (which is also the pattern used as an openly-distributed reference for ISO 19567-2).
Automatic region registration
Since the cross-correlation calculation is based on a reference image which is exactly warped to align with the observed image, registration of the chart in the image is extremely important. (In fact we published a paper at Electronic Imaging 2016 on this topic, PDF and video of the presentation found here.)
This registration accuracy requirement means that we do not leave it to you, a human, to attempt to register the chart with sub-pixel accuracy. Instead, if you have selected the cross-correlation calculation method, automatic registration will be run again to refine the initial location estimate. Note: This will override any manual adjustment you may have done in the ROI selection window during the setup of the module.
Analysis Results
Texture MTF results are reported when using this analysis method, as they would be by the Random/Spilled Coins module for Direct Method results.
However, target and noise PSD results are not returned (as they are with the other method) because they are not part of this calculation. The dead leaves cross method does not explicitly observe and compensate for noise spectra.