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Using SFRplus Part 3

Imatest SFRplus results

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Imatest SFRplus performs highly automated measurements of several key image quality factors using a specially-designed test chart. Unlike other modules, the user never has to manually select Regions of Interest (ROIs). Image quality factors include

• Sharpness, expressed as Spatial Frequency Response (SFR), also known as the Modulation Transfer Function (MTF),
• Noise,
• Lateral Chromatic Aberration,
• Distortion (with less detailed output than the Distortion module), and
• Tonal response (again, with less detail than Stepchart; no noise statistics)

This document illustrates SFRplus results. Part 1 introduced SFRplus and explained how to obtain and photograph the chart. Part 2 showed how to run SFRplus inside Rescharts and how to save settings for automated runs.

SFRplus results

When calculations are complete, results are displayed in the Rescharts window, which allows a number of displays to be selected. A general description can be found on Using Rescharts.

Multi-ROI summary display

SFRplus results in Rescharts window: Multiple region (ROI) summary

The multi-ROI summary results shown in the Rescharts window (above) is the best summary of SFRplus results, but it is just one of several available displays. It is described in detail in Multiple ROI (Region of Interest) plot. The upper left contains the image in muted gray tones. The selected regions are surrounded by red rectangles and displayed with full contrast. Four results boxes are displayed next to each region. There is a legend below the image. The table below explains the contents in more detail.

N: Ctr-corner distance %	Region number (N) and distance from the center as a percentage of center-to-corner distance
MTF50  LW/PH or cycles/pxl	MTF50 in units specified in the MTF plots entry in Display options in the SFRplus settings and options window.
MTF20  LW/PH or cycles/pxl	MTF20 in the same units as MTF50. May be replaced by the secondary readout in an upcoming release.
Chrom Aberration (pxl area)	Chromatic Aberration in pixel area. % of center-to-corner distance may be an option in a future release. See Chromatic Aberration and Chromatic Aberration ... plots.

Distortion statistics are shown in the lower left. SMIA TV distortion is the simplest overall measure of distortion. k₁ (the third order distortion coefficient) and the Picture Window Pro arctangent/tangent coefficients are discussed on the page on Distortion.

The upper right contains a display of Weighted MTF50 results that covers the small image visible in the other plots. The weighted sum has weights of 1 for ROIs in the central region (inside the inner dotted circle), 0.75 for the middle region (between the two dotted circles), and 0.25 for the outer region (outside the outer dotted circle). The results are independent of the number of ROIs in each region.

Plot settings area

A small number of options are available in the Plot settings area, on the lower right, below the Display box (fewer than for other Rescharts modules).

All displays contain the dropdown menu for selecting the primary channel to analyze. If R, G, B,  or Y (Luminance) is selected, all channels are analyzed, but the selected channel is emphasized. There is also an option to analyze any of the channels alone— useful where one of the secondary channels is dark and may cause a run to crash. Luminance (Y) is shown above. If it is changed, results are recalculated.

All displays except Multi-ROI summary and Tonal response & gamma allow you to select the ROI for viewing results.

The Edge and MTF display has a dropdown window for selecting the maximum MTF display frequency: 2x Nyquist (the default), Nyquist, 0.5x Nyquist, and 0.2x Nyquist.

Edge and MTF display

Edge and MTF display in Rescharts window

This display is identical to the SFR Edge and MTF display. MTF is explained in Sharpness: What is it and how is it measured? The edge (or line spread function) is plotted on the top and the MTF is plotted on the bottom. There are a number of readouts, including 10-90% rise distance, MTF50, MTF50P (the spatial frequency where MTF is 50% of the peak value; differing from MTF50 only for oversharpened pulses), the secondary readout (MTF20 in this case), and the MTF at the Nyquist frequency (0.5 cycles/pixel).

An important (and optional) readout in the upper plot is

• The Chart contrast (entered in the Display options section of the SFRplus settings & options window),
• The Contrast factor: the ratio between the chart contrast derived from the pixel ratio and the input value of gamma (0.5 in the above display),
• Gamma (from chart): a measurement of gamma derived from the chart pixel levels and the Chart contrast (an input value, as described above). Gamma has a relatively small effect on the MTF measurement, especially for moderate to low contrast targets (10:1 or under).

Chromatic Aberration Lateral Chromatic Aberration (LCA), also known as "color fringing," is most visible on tangential boundaries near the edges of the image. Much of the plot is grayed out if the selected region (ROI) is too close to the center (less than 30% of the distance to the corner) to accurately measure CA. The area between the highest and lowest of the edge curves (shown for the R, G, B, and Y (luminance) channels) is a perceptual measurement of LCA. It has units of pixels because the curves are normalized to an amplitude of 1 and the x-direction (normal to the edge) is in units of pixels. It is displayed as a magenta curve.	Lateral Chromatic Aberration
Perceptual LCA is also expressed as percentage of the distance from center to corner, which tends to be more reflective of system performance: less sensitive to location and pixel count than the pixel measurement. Values under 0.04% of the distance from the center are insignificant; LCA over 0.15% can be quite visible and serious. Information for correction LCA (R-G and B-G crossing distances) is also given in units of % (center-to-corner) and pixels. LCA can be corrected most effectively before demosaicing. Results are explained in Chromatic Aberration ... plot.
SQF (Subjective Quality Factor) SQF is a perceptual measurement of the sharpness of a display (monitor image or print). MTF, by comparison is device sharpness (not perceptual sharpness). SQF includes the effects of the human visual system's Contrast Sensitivity Function (CSF), print (or display) size, and viewing distance (which is assumed to be proportional to print height, by default). See Introduction to SQF for more detail.	SQF (Subjective Quality Factor)
Tonal response & gamma This display is derived from the 4x5 stepchart pattern, located just below the central square of the SFRplus test chart. It resembles the third figure in Stepchart. The upper plot shows the tonal response for all colors. The lower plot shows instantaneous gamma— the slope (derivative) of tonal response. The value of gamma may differ slightly from the values in the Edge response and MTF display because it's calculated differently-- based on the average slope of the light to middle tone squares of the stepchart.	Tonal response & gamma
Histograms and noise analysis This display contains histograms of pixel levels for individual ROIs (original on top and linearized using input gamma on bottom). The black (background) histogram contains pixel levels for the entire ROI. The red histogram is for the right (dark) region, away from the transition, used in the noise statistics calculation. The cyan histogram is for the left (light) region. Sharpening may cause extra bumps to appear in the black histogram. A detailed level and noise analysis is displayed below the image. Dark, light levels Original pixel levels normalized to 1 and linearized levels, also normalized to 1 Estimated chart contrast (for gamma = ...) The correct chart contrast must be entered in SFRplus parameters & setup window. Noise (dark, light, mean; norml pixels %) RMS noise expressed in pixels, normalized to 100% (for 255 in 8-bit files) S/N (...; norml pixels) Signal/Noise, where signal is mean pixel level of ROI at a distance from the transition. Noise (...; linearized %) Noise, linearized using gamma (input); normalized to 100% S/N (...; linearized) Signal/Noise, linearized	Histograms and noise stats
Noise calculations are made in portions of the ROIs (Regions of Interest) away from the transitions. They are facilitated by selecting wide ROIs. The noise spectrum on the lower right contains qualitative information about noise visibility and software noise reduction, which generally reduces high frequency noise below 0.5, typical of demosaicing alone. The region on the right is shown in red; left is shown in cyan. See also Noise.
Color & lightness uniformity profiles This display is similar in some respects to the profile plots in Light Falloff. It consists of profiles of the light areas (average values of rectangles between squares). Left-to-right profiles of the top, middle, and bottom of the image are shown on the left side of the display, and top-to-bottom profiles at the left, center, and right of the image are shown on the right. The following display options are available. R, G, B, and Y unnormalized (max = 1) R, G, B, and Y normalized (max = 1) R, G, B, and Y unnormalized (max = 255) R/G (Red) and B/G (Blue) unnormalized R/G (Red) and B/G (Blue) normalized (max = 1) Delta-L* (gray), a* (R), b* (B), chroma c* (G) For best results with this display, you should make every effort to illuminate the target uniformly using techniques in The Imatest Test Lab. Brightness-related results (R, G, B, Y, and Delta-L*) cannot be measured as accurately as with Light Falloff, where the recommended technique calls for a clear uniform image field photographed through opal diffusing glass. But they can still be useful. The color ratios (R/G, etc.) can be especially useful for diagnosing uneven color response.	Uniformity profiles
Image display This display allows several aspects of the image to be viewed in detail. It shows the selected regions for MTF/noise analysis (violet rectangles) as well as the yellow rectangles used for generating the uniformity profiles. Display options include (original) Color image Red, Green, or Blue channel Color image with HSV satuartion boosted 4X or 10X.	Image (shown with exaggerated saturation)
Summary & EXIF data This plot contains summary results for individual ROIs and EXIF data (metadata that describes camera and lens settings). When first installed, Imatest reads EXIF data from JPEG files only. Enhanced EXIF support requires a special download of ExifTool, described here.	Summary & EXIF data

SFRplus summary

• SFRplus analyzes images of the SFRplus test chart, framed so that there is white space above and below the horizontal bars in the chart, i.e., the bars must not run off the top or bottom of the image. They may, however, run off the sides of the image. The white space should be between 0.5% and 25% of the image height. There should be few or no interfering patterns (bars, etc.) outside the image of the chart itself.
• Lighting should be even and glare-free. Lighting and alignment recommendations are given in Building a test lab.
• The first time SFRplus is run, it should be run through Rescharts. This allows
  • parameters to be adjusted and saved for later use in the automatic version of SFRplus, which is opened with the SFRplus button in the Imatest main window.
  • results (listed above) to be examined interactively in the Rescharts window.

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Result file names—   The roots of the file names are the same as the image file name. The channel (Y, R, G, or B) is included in the file name. If a Region of Interest has been selected from a complete digital camera image, information about the location of the ROI is included in the file name following the channel. For example, if the center of the ROI is above-right of the image, 20% of the distance from the center to the corner, the characters AR20 are included in the file name.

Output files for  filename.jpg  (Y-channel)
(default location: subfolder Results)

Figures (.PNG image files)
filename_YA17_cpp.png	Plot with x-axis in cycles/pixel (c/p), Y-channel,17% of the way to the corner above the center of the uncropped image.
filename_YA17_lwph.png	Plot with x-axis in Line Widths per Picture Height (LW/PH).
filename_YA17_ca.png	Plot of Chromatic Aberration, with noise statistics and Shannon information capacity.
Excel .CSV (ASCII text files that can be opened in Excel)
CSV output files are explained in detail below. Click link for detail on individual file.
filename_YA17_MTF.csv	Excel .CSV file of MTF results for this run. All channels (R, G, B, and Y (luminance) ) are displayed.The first row has the headers: cy/pxl, LW/PH, MTF(nchan), MTF(corr), MTF(R), MTF(G), MTF(B), MTF(Y), where nchan is the selected channel. The remaining lines contain the data. Can easily be plotted or combined with data from other files.
filename_Y_multi.csv	Excel .CSV file of summary results for a multiple ROI run.
filename_Y_sfrbatch.csv	Excel .CSF file combining the results of batch runs (several files) with multiple ROIs. Particularly useful for generating easily-readable Excel plots.

Excel .CSV (Comma-Separated Variables) and XML output

Imatest SFR creates or updates output files for use with Microsoft Excel. The files are in CSV (Comma-Separated Variable) format, and are written to the Results subfolder by default. .CSV files are ASCII text files that look pretty ugly when viewed in a text editor:

File        ,Date/time        ,PH,Ch,H/V,10-90U,10-90C,Over-,Over-,MTF50U,MTF50C,MTF,Camera,Lens,FL,f-stop,Loc,Misc. ,,,,,/PH,/PH,shoot%,sharp%,LW/PH,LW/PH,Nyq,,,(mm),,,settings canon_eos10d_sfr.jpg,2004-03-19 22:21:34, 2048,Y,H, 1422, 1447, 19.5, -0.7, 1334, 1340,0.154,,,,, canon_g5_sfr.jpg,2004-03-19 22:24:30, 1955,Y,H, 1973, 1301, 48.0, 21.3, 1488, 1359,0.268,Canon G5,—,14,5.6,ctr, sigma_sd9_sfr.jpg,2004-03-19 22:27:55, 1504,Y,H, 1432, 1676,  2.4, -7.7, 1479, 1479,0.494,,,,, sigma_sd10_sfr.jpg,2004-03-19 22:28:32, 1504,Y,H, 1563, 1628, 11.9, -2.0, 1586, 1587,0.554,,,,,

But they look fine when opened in Excel.

.CSV files can be edited with standard text editors, but it makes more sense to edit them in Excel, where columns as well as rows can be selected, moved, and/or deleted. Some fields are truncated in the above display, and Date/time is displayed as a sequence of pound signs (#####...).

The format can be changed by dragging the boundaries between cells on the header row (A, B, C, ...) and by selecting the first two rows and setting the text to Bold. This makes the output look better. The modified file can be saved with formatting as an Excel Worksheet (XLS) file. This, of course, is just the beginning.

It's easy to customize the Excel spreadsheet to your liking. For example, suppose you want to make a concise chart. You can delete Date/time (Row B; useful when you're testing but not so interesting later) and Channel (all Y = luminance). You can add a blank line under the title, then you can select the data (rows A4 through J7 in the image below) and sort on any value you choose. Corrected MTF50 (column I) has been sorted in descending order. Modified worksheets should be saved in XLS format, which maintains formatting.

There are no limits. With moderate skill you can plot columns of results. I've said enough. ( I'm not an Excel expert! )

Summary .CSV and XML files for MTF and other data

An optional .CSV (comma-separated variable) output file contains results for MTF and other data. Its name is [root name]_[channel location]_MTF.csv, where channel is (R, G, B, or Y) and the location BL75 means below-left, 75% of the distance to the corner (from the center). An example is Canon_17-40_24_f4_C1_1408_YBL75_MTF.csv. Excerpts are shown below, opened in Excel.

A portion of the summary CSV file, opened in Excel

The format is as follows:

Line 1	Imatest, release (1.n.x), version (Light, Pro, Eval), module (SFR, SFR multi-ROI, Colorcheck, Stepchart, etc.).
File	File name (title).
Run date	mm/dd/yyyy hh:mm of run.
(blank line)
Tables	Separated by blank lines if more than one. Two tables are produced.
	The first table contains MTF. The columns are Spatial frequency in Cy/mm, LW/PH, MTF (selected channel), MTF (Red), MTF (Green), MTF (Blue), MTF (Luminance = Y). (...) represent rows omitted for brevity.
	The second table contains the edge. Columns are x (location in pixels), Red edge, Green edge, Blue edge, Luminance (Y) edge, and Chromatic Aberration (the difference between the maximum and minimum).
(blank line)
Additional data	The first entry is the name of the data; the second (and additional) entries contain the value. Names are generally self-explanatory (similar to the figures).
(blank line)
EXIF data	Displayed if available. EXIF data is image file metadata that contains important camera, lens, and exposure settings. By default, Imatest uses a small program, jhead.exe, which works only with JPEG files, to read EXIF data. To read detailed EXIF data from all image file formats, we recommend downloading, installing, and selecting Phil Harvey's ExifTool, as described here.

This format is similar for all modules. Data is largely self-explanatory. Enhancements to .CSV files will be listed in the Change Log.

The optional XML output file contains results similar to the .CSV file. Its contents are largely self-explanatory. It is stored in [root name].xml. XML output will be used for extensions to Imatest, such as databases, to be written by Imatest and third parties. Contact us if you have questions or suggestions.

An optional .CSV file is also produced for multiple ROI runs. Its name is [root name]_multi.csv.

Digital Image Quality Testing - Copyright © 2009 Imatest LLC