Analysis of random scale-invariant patterns,
including the Spilled Coins (Random-Cross Method) Pattern,
for measuring texture sharpness
Random/Dead Leaves Cross, which runs under the interactive Rescharts interface, measures SFR (Spatial Frequency Response) or MTF (Modulation Transfer Function) from random scale-invariant (or approximately scale-invariant) test charts, including “Dead Leaves” and “Spilled Coins” charts.
The Imatest Spilled Coins (Random/Dead Leaves- Cross) chart
Key features of the Spilled coins Random-Cross chart:
- The Spilled Coins colored (Random Cross) pattern in the central region is almost perfectly scale-invariant (unlike conventional dead leaves charts), allowing it to be used for reliable MTF measurements.
- Maximum contrast range is 4:1 with 18% gray background as per the CPIQ new standards.
- It is more uniform, i.e., is more shift-invariant than other Dead Leaves charts.
- It contains slanted edges (2:1 and 4:1 contrast) for convenient comparisons with the Spilled Coins pattern.
- Noise removal: The Noise removal calculations are based on subtracting cross Power Spectral Density (PSD) of reference image(Ground truth) to the Observed/Test image PSD of the active spilled coins area.
- 8 Registration marks are placed around the active area and those are used to transform the reference image to match with the Observed/Test image to find the MTF.
- 16 grayscale patches are included. The linear levels used to create the grayscale patches are 0 through 255 in steps of 17 (same as the Siemens Star chart in the draft of the upcoming ISO 12233 standard). The black outline makes it easy to align the chart.
Obtaining and photographing the chart
The Spilled Coins Random-Cross chart is available from the Imatest store.
A TIFF file for printing a Random Scale-Invariant test chart can be created using Imatest Test Charts. Typical settings are shown below.
Test Charts settings for standard random scale-invariant pattern
Resolution varies over the image for most cameras and lenses, the chart should not take up too much of the frame— in most cases its dimensions should be no more than about 1/3 of the corresponding image dimensions (height and width), i.e., the area of the random pattern should not exceed about 1/10 of the total chart area (except in very low resolution systems such as VGA, where you need all the pixels you can get).
Running the program
Running interactively in Rescharts
Open Imatest, then click . The Rescharts window is described in the Rescharts page. Click on the Region selection dropdown and choose the appropriate setting for the chart to be analyzed.
Automatic region detection:
Automatic region detection is available for Imatest Spilled Coins/Random-Cross charts starting with Imatest 4.5. Load the test image and it will detect all regions automatically.
ROI for a Spilled Coins (or Random-Cross) image
If you press Yes, Continue (Express mode is not selected), the settings window shown below appears. This window can be at any time by pressing the More settings button.
Title defaults to the file name. You can enter a description of the system if needed.
Select Random-Cross Method from calculation popup menu to run the Random-Cross method. Once you select the Random-Cross Method then DL Cross Ref image will become active and ask you to load the Random-Cross reference image.
Download Reference image here
MTF plots selects the x-axis scaling. If Cycles/inch, Cycles/mm, or any of the Cycles/angle settings are selected, the pixel spacing (um/pixel, pixels/inch, or pixels/mm) should be entered.
X-axis scaling for linear plots selects the maximum spatial frequency to be displayed in linear plots.
Secondary readout allows up to two secondary redouts (MTFnn, MTFnnP, or MTF at a specified spatial frequency) to be displayed on the MTF plots. Details here.
Don’t worry about getting all settings correct: You can always open this dialog box by clicking on Rescharts window.in the
After you press, calculations are performed and the most recently-selected display appears.
Plot (not for Rescharts)
The five plots in this section are available as output figures. They are referenced as Plot n in fixed module in the table, below. EXIF data appears on the right side of Plots 1 and 4.
The Display box in the Rescharts window, shown below, allows you to select any of several displays. Display options are set in boxes that appear below Display. All displays except Exif data have a channel selection option (Red, Green, Blue, or Luminance (Y) (0.3R + 0.59G + 0.11B).
|1. MTF (linear frequency)
Plot 1 in fixed module
|MTF for up to 8 segments (in frequency domain). Linear frequency display.|
|2. MTF (log frequency)||MTF for up to 8 segments. Logarithmic frequency display.|
|3. MTFnn or MTFnnP
Plot 2 in fixed module (polar)
|Display MTF contours (MTF70 through MTF10, if available) in a polar or rectangular plot. Only meaningful if >1 Calc. segments selected.|
|4. EXIF data and linearization
Plot 3 in fixed module
|Show EXIF data if available as well as linearization curves (used to calculate gamma from the chart).|
|5. Signal & Noise PSD (linear f)||Show Signal + Noise, Noise, and Signal (noise removed) Power Spectral Density with a linear frequency display.|
|6. Signal & Noise PSD (logarithmic f)
Plot 4 in fixed module
|Show Signal + Noise, Noise, and Signal (noise removed) Power Spectral Density with a logarithmic frequency display (generally more useful).|
Plot 5 in fixed module
|Display Subjective Quality Factor or CPIQ Acutance. The CPIQ JND (Just Noticeable Difference) can also be plotted.|
|8. Display image||Display the image with any of several options (cropping, lighten, color boost, etc.).|
|In addition to the displays, two buttons allow you to save results.|
|Saves an image of the Starchart window as a PNG file. If you check Display screen in the Save screen dialog box, the image will be opened in the editor/viewer of your choice. (Irfanview works well, and it’s free.)|
|Saves detailed results in a CSV file that can be opened by Excel and also in an XML file.|
The spatial frequency is automatically calculated from the image, under the assumption that log frequency increases linearly with distance. The number of chart cycles is also determined automatically.
The plots are explained clearly in http://www.imatest.com/docs/random/