Imatest - Using Test Charts

Introduction Bitmap patterns SVG patterns Options

Bitmap patterns

Bitmap patterns consist of arrays of pixels stored as standard image files, in distinction to Vector graphic patterns (below), which consist of scalable primitives such as lines, polygons, etc. Bitmaps are best for continuous-toned images, especially the sine-modulated images described below.

The first three patterns, SFR:quadrants, SFR rectangles, and Distortion grid or checkerboard , are available in all versions. Most of the remaining patterns are available in Imatest Master only.

Print height can be selected in cm (for prints) or pixels (for standard video/screen sizes). Available print sizes are 25x18 cm (for A4 paper), 25x20 cm (for US letter paper), 40x28 cm (for A3 paper), and 40x30 cm (for Super A3/Super B paper). In addition, files can be created with the following sizes, which correspond to standard display resolutions: 640x480, 600x800, 768x1024, 960x1280, 1024x1280, and 1200x1600 pixels.

SFR: quadrants (All versions)

This is the standard chart for SFR measurements. It should be tilted when it is photographed. Instructions are found in Using SFR, Part 1.

Options include adjustment of pixels per inch (100 is recommended), contrast ratio, gamma, and highlight color.

SFR Quadrants

SFR: two rectangles (All versions)

This chart is also used for SFR measurements. It should be cut into two segments (left and right sides) and tilted in a manner similar to the SFR quadrants chart, above, when photographed.

Options include adjustment of pixels per inch (100 is recommended), contrast ratio, gamma, and highlight color.

SFR Two rectangles

Distortion grid or checkerboard (All versions)

Distortion grid... generates grid or checkerboard test charts for the Distortion module. (a grid is shown on the right.)

Options include PPI, Highlight color, and Type (Grid or Checkerboard). Maximum contrast is always used. High resolution is not necessary. 100-300 pixels per inch is recommended.

Distortion (grid)

A portion of the input dialog box for the grid pattern is shown on the right. Several of the popup menu boxes are changed for Grid.

Horizontal divisions and Vertical divisions are specified in the bottom two boxes. 10-20 Horizontal divisions works well in most cases. Vertical divisions has the option, "Square" that makes the grids square. This is the default, recommended in most cases.

Line width (pxls) is typically set somewhere in the range of .01*PPI to .025*PPI. The printed line should be clear (not pale), but not very thick. 3 pixels is appropriate for a 150 PPI file. Lines in the preview image are thicker relative to the grid size than they are in the final image.

The line width should be thick enough to cover at least two pixels in the captured image. Narrower lines may result in errors (Inconsistent number...). For example, suppose your captured image were 640x480 pixels (VGA). The line width (as shown on the right) would be 3 pixels out of a chart width of 1476. In the captured chart, the line width would be 640*(3/1476) = 1.3, inadequate for this small image size, though this chart would be perfectly adequate for 1+ megapixel cameras.

A portion of the Test Charts input dilog box for
Distortion (showing Checkerboard)

Star chart (Imatest Master only)

Star charts can be used for observing performance (SFR, Moire patterns, and other image processing artifacts) at a variety of angles. In addition to the star (which has a selectable number of bands and either bar or sine form), the chart contains tonal calibration patches and slanted edges for checking results with SFR.

Standard print sizes are 24x18 cm (for A4 paper), 26.67x20 cm (for US letter paper), 37.33x28 cm (for A3 paper), and 40x30 cm (for Super A3/Super B paper). Options include the number of star bands, sine or bar band pattern (sine shown on the right), pixels per inch (600 or 720 is recommended, depending on the printer), contrast ratio, gamma, and highlight color. Star charts will be analyzed in a future version of Imatest.

Star chart

You can also create the star chart included in the proposed update to the ISO 12233 standard using the following settings: PPI: 720 (Epson inkjets) or 600 (HP or Canon inkjets), Height (cm) (as required), Highlight color: White, Contrast ratio: 40, Type: Sine, Gamma: 2.2, Star pattern bands: 144 or 72, Chart lightness: Lightest, ISO standard chart: Yes or Yes (small inner circle): (inner circle will have 1/10 or 1/20 the diameter of the outer, respectively).

A high quality printed version of this chart is available from Image Engineering. They describe its use in Measuring the resolution of digital cameras.

Star chart (proposed ISO std)

Log frequency-contrast gradient charts (or Stepcharts) (1 and 2-band charts in Imatest Master only)

These charts display bar or sine patterns of increasing spatial frequency, for use with two Rescharts modules: Log Frequency and Log F-Contrast. Two options available for creating Step charts. Several features resemble the Koren 2003 test chart described in Lens testing (developed prior to Imatest).

This chart is valuable for viewing Moiré fringing (colored bands caused by aliasing) and analyzing low contrast detail lost as a result of software noise reduction, which takes place inside the camera and/or RAW converter. Software noise reduction is a type of lowpass filtering (reduction of contrast at high spatial frequencies) that is turned on in regions where edges or strong image features are absent. Some may take place even if you try to turn it off. It cannot distinguish noise from high-spatial frequency, low-contrast detail, located in the lower right of the 1 and 2-band charts. The two and four band charts are designed to be cut into segments to be placed around the target, possibly in different orientations (landscape, portrait).

	Log F-Contrast chart

Each chart consists of the following patterns in 1, 2, or 4 bands. From top to bottom,

A bar pattern of increasing spatial frequency (which could be used to determine spatial frequency (not used in Imatest).
A sine or bar pattern of increasing spatial frequency (horizontally) for all charts and decreasing contrast (vertically) for 1 and 2-band charts. The contrast is proportional to the square of the distance from the bottom of this band. That emphasizes lower contrast areas, where noise reduction causes a loss of detail. It also corresponds to the way the eye sees, i.e., when the pattern contrast is one quarter of its maximum value, it appears to be about half the maximum. The contrast is constant for the 4-band chart, which is available in Imatest Studio.
(1-band chart only) 20 rectangular patches. 1 and 20 are black (0,0,0), 2 and 19 are middle gray, and 3-18 (16 patches) are a stepchart with density steps of 0.1. If the chart is printed correctly, i.e., if printer gamma matches the Test Charts setting, these patches will closely match the first 16 patches of reflective step charts with 0.1 density steps such as the Kodak Q-13 and Q-14..

Options include

Standard options: Pixels per inch (PPI; 600 or 720 is recommended, depending on the printer), Highlight color, Gamma, and Height. The standard print size is 25 x (approximately) 18 cm (9.83 x (approximately) 7.08 inches; A4 paper), 25 x (approximately) 20 cm (9.83 x (approximately) 7.87 inches; US letter paper), 40 x (approximately) 28 cm (15.7 x (approximately) 11.02 inches; A3 paper), 40 x (approximately) 30 cm (15.7 x (approximately) 11.8 inches; Super A3/Super B paper).
Contrast ratio and Chart lightness: Do not affect patterns with vertically-varying contrast.
Type: Bar or sine (single pattern), bar or sine (double pattern), or bar and sine without contrast variation (quad pattern; available in Imatest Studio).
Frequency ratio (maximum to minimum) See Log F-Contrast for recommendations.
Maximum spatial frequency in cycles/width. (The actual spatial frequency will be detected by the analysis program.) See Log F-Contrast for recommendations.

The two Stepchart options include a chart with 20 steps (similar to the Kodak Q-13) and with16 steps with black and middle gray bands at the ends. Both have a density increment of 0.1. They can be printed any size (small or large) and sliced into narrow strips for specialized testing. Stepcharts should be printed with care in a color-managed environment. If an ICC profile is used for printing, Black Point Compensation should be turned off. For most applications, standard charts such as the Kodak Q-13 and Q-14 are recommended for best accuracy.

Zone plate (Imatest Master only)

Zone plates (based on Fresnel diffraction) are useful for observing aliasing, particularly Moire patterns and other image processing artifacts. They have a much larger area of high spatial frequencies than the star chart. The spatial frequency is inversely proportional to the square of the radius (the distance from the center of the pattern). The chart also contains tonal calibration patches and slanted edges for checking results with SFR. Thanks to Bart van der Wolf, who has done some interesting work with zone plates.

Zone plate

Print sizes are the same as the star chart. Options include a divisor for the maximum spatial frequency (0.5 cycles per pixel at the outer radius when set to 1), sine or bar band pattern (sine shown on the right), pixels per inch, contrast ratio, gamma, and highlight color. Low ppi charts are useful for testing resizing performance.

Scalable Vector Graphics (SVG) patterns

Imatest Master SVG charts are discussed in detail in SVG (Structured Vector Graphics) Test Charts for improved MTF measurements.

Vector graphic images consist of primitives such as lines, polygons, etc., that can be scaled to any size and do not suffer from the pixellation problems of bitmap images. They print at the printer's maximum quality, regardless of size. Vector graphics are excellent for generating test charts consisting of lines, curves, and areas that don't require complex tone variations such as sine modulation.

Scalable Vector Graphics (SVG) is an XML file format recognized by the World Wide Web Consortium (W3C). SVG files can be read, edited, and printed by Inkscape— an excellent free open-source program (described in Wikipedia). They can also be imported into Adobe Illustrator, Corel Draw, and others. SVG implementations are listed here.

Imatest Master can create two types of SVG chart, discussed in detail in SVG (Structured Vector Graphics) Test Charts.

SVG Squares & Wedges (letter, A4, ...)

This was the first SVG pattern to be included in Imatest. It consists of tilted squares, "hyperbolic" wedges, bars of varying spatial frequencies, and a replica of the Kodak Q-13 step chart with density steps of 0.1. It is optimized to be printed A4 or letter size on a high quality inkjet printer and mounted on a 3x3 grid, as shown on the right— though it can be printed any size without loss of quality. In this arrangement it will have a field width of around 32 inches (80 cm), sufficient for measuring the performance of the best modern DSLRs.

The chart below was created using Contrast ratio = 20, Gamma = 2.2, Highlight color = White, and Chart lightness = Light. Additional details are found on SVG (Structured Vector Graphics) Test Charts.

3x3 chart arrangement (for letter/A4 prints)

Slanted squares on m x n grids

The rest of the SVG charts consist of slanted squares arranged on an m x n grid. A step chart (density increment = 0.1) and a pair of hyperbolic wedges may be optionally added. The chart's contrast, brightness, and several other details are adjustable. A typical SVG SFR chart, consisting of a 5x9 grid, is shown below.

SVG SFR chart: 5x9 grid, 20:1 contrast, with optional hyperbolic wedge
and step patterns (0.1 density increment) .

This chart is optimized to print at 20x36 inches (50x90 cm) or larger. It should not be printed small (letter-size or A4) on inkjet printers because SFR test results would be limited by the printer's resolution. Other grid patterns are available that are optimized for printing at different sizes. Grid choices include 1x2, 2x3, 3x5, 4x7, 5x9, and 7x11. The larger grids (3x5 and up) are optimized for printing on wide format printers, and are available in Imatest Master only. Additional details, as well as instructions on how to print these charts, are found on SVG (Structured Vector Graphics) Test Charts.

Options

Pixels per inch (PPI)

(bitmap charts only) Choices: 720, 600, 400, 360, 320, 300, 180, 150, 120, 100, 72

Along with chart height, PPI determines the size of bitmap test chart image files. For the two SFR charts (1 and 2 above) it can be a low number— 100 PPI, because there are only vertical and horizontal edges. For the Star, Log frequency, and Zone plate charts, the native DPI (dots per inch) of the printer is recommended: 720 for Epson printers (1440 or 2880 dpi); 600 for Canon or HP (1200, 2400, etc. DPI). High resolution files can be somewhat slow to create.

Chart height

(bitmap charts only) Choices: 30 (Super A3/B), 28 (A3), 20 (US Letter), 18 (A4)

The intended height of printed bitmap charts, assuming landscape format (where width > height). The chart can be printed any size, but when it is printed at the intended height, the the specified PPI (above) is sent to the printer. This gives optimum results. The width is larger than the height by a roughly 4:3 ratio (depending on the pattern). The specified heights are 18 cm (7.08 inches; A4 paper), 20 cm (7.87 inches; 11x8.5 inch US letter paper), 28 cm (11.02 inches; A3 paper), and 30 cm (11.8 inches; 19x13 inch Super A3/Super B paper).

Highlight color

In addition to the standard White/Black (or White/Gray) charts, you can replace white with any additive or subtractive primary color: R, G, B, C, M, or Y.

Different colors are of particular interest in analyzing Bayer sensors, where alternate rows of pixels are sensitive to {R, G, R, B, R, ...} and {G, B, G, B, G, ...}. Since there are only half as many R and B sensors as green sensors (and one quarter as many as total pixels), one would expect MTF to be poorer for red-black and blue-black charts than for white-black). Demosaicing algorithms typically make use of different colored pixels for calculating detail (i.e., an edge in the red channel would be sharper if information is present in the other channels). Also, different algorithms perform differently. A lot can be learned from targets of different colors.

Chart lightness

Choices: Lightest, Light, Mid-light, Middle, Mid-dark, Darkest

Determines the grayscale level of both the light and dark areas of the chart. Useful with charts that contain grayscale information (star, zone plate, and Log frequency patterns with sine modulation), where best pattern quality can be obtained if this setting is not "lightest" (highlights pure white). Also valuable for SFR edge charts, where middle tones are likely to be in the camera's linear response region.

Type (bar or sine)

(bitmap charts only) The features of the star, zone plate, and Log frequency patterns can be either bars (shown in the Magenta star pattern, above), or sine (shown in the White-Black Star, Log frequency-contrast, and Zone plate patterns, above). Sine patterns are not available in SVG charts.

Star pattern bands

The number of bands in a star pattern is selectable. It is always a multiple of four. The nearby Magenta pattern has 36 bands (the default); the White-Black pattern shown above has 20 bands. Commercially-available star patterns come in a wide variety.

Contrast ratio

Choices: None, Maximum, 80, 50, 40, 25, 20, 10, 5, 4, 2, 1.414, 1.2, 1.1

The ratio of the reflectivity of the light to dark areas (the upper zone only in the Log frequency chart). The maximum contrast ratio depends on the printer and paper. It is typically around 100 for glossy (or luster) surfaces, but only around 40 for matte surfaces. Maximum print density = -log₁₀(minimum reflectivity) can be measured in Print Test or estimated by Gamutvision. Available selections are Maximum and 80 through 1.1.

Clipping is often a problem with high contrast targets. The ISO-12233 standard calls for a contrast ratio between 40 and 80, but recent experiments have shown that lower contrast ratios produce more accurate and consistent results, which are less affected by gamma and camera nonlinearities. In 2007 the ISO standard will change to recommend lower contrast chart. The default value for Test Chart is 40 (the minimum recommended in the ISO standard), but 20 is preferred in many cases.

Contrast ratio affects MTF (SFR) measurements in cameras that perform nonlinear signal processing, i.e., noise reduction and sharpening with thresholds— nearly universal in consumer cameras. Charts with at least two contrast ratios: 20 and 2 (or lower) are recommended for characterizing cameras.

Gamma

Choices: 2.5, 2.2, 2.0, 1.8, 1.5, 1.0

You should use the value of gamma for your typical color space and workflow (2.2 is standard for Windows; 1.8 for older Mac systems). The default value is 2.2. Gamma affects the accuracy of sine patterns and the actual printed contrast ratio: it must be the same value used for printing (the value of the color space, i.e., 2.2 for Adobe RGB (1998), if color management is used) to get the correct contrast ratio.

Ink spread compensation

corrects for ink spread (or "dot gain") in the Log frequency chart. (The Star chart may be added later.) The best value has to be determined experimentally.

Output figure

The output figure contains an image of the test chart. It is not anti-aliased, hence lines may be missing (in Grid) or curious aliasing patterns may appear, visible below. Only the circle in the middle is real-- all the others are the result of aliasing.

This figure is not intended for printing. To print a chart open the image file (C:\Imatest\Data_distortion\chart_temp.tif in the example below) in an Image editor and print it from the editor. A high quality (photographic) inkjet printer is recommended. The best way to print the charts is to convert the TIFF files from black & white into 24-bit color and print them using a color-managed workflow with ICC profiles. Manufacturer-supplied profiles are usually sufficient; custom profiles may be better when tonal values are critical.

Preview image of Zone plate showing extreme Moiré

You can zoom in on the chart by selecting an area or clicking on a point of interest. Double-clicking restores the complete image.

Preview image of Zone plate, zoomed in

Using Test Charts

Creates test charts for high quality inkjet printers