Using Test Charts
Creates test charts for high quality inkjet printers
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Using Test ChartsCreates test charts for high quality inkjet printers | |
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Test Charts creates test chart files for printing on high quality inkjet printers. Two types of chart can be created: bitmaps and Scalable Vector Graphics (SVG), which can also be sent out for precision printing. A variety of options is available. You can select chart pattern, type (bar or sine), contrast, highlight color, printer gamma, and more.
To run Test Charts, run Imatest and click on in the Imatest main window. The following dialog box, which contains the initial (default) settings, appears. The available settings change for different patterns.
The file pixel size and the dimensions when printed at the specified PPI (bitmap files only) is shown on the lower left. The file may be printed at any desired size. Select the Pattern (the type of chart), the options, and the folder (directory) and file name for the test chart file. When you are ready to create the file, click on . You may cancel by clicking on . When the calculations are complete a figure showing the chart (not for printing) appears. You can zoom in to see the fine detail. The figure with the image of the chart is for preview only: it shows you what the chart looks like and lets you zoom in if you want. Do not print the chart directly from this figure. Load bitmap Print Test output files (chart_temp.tif in the above example) into an image editor (such as Photoshop, Picture Window Pro, or Irfanview (a great free image viewer that can read and write nearly every known format)) and print it from the editor. SVG (Scalable Vector Graphics) files can be printed from Inkscape (or imported into Adobe Illustrator or Corel Draw). This will allow you to select the size of the print, add annotations, use color management, etc. 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. Bitmap patternsBitmap 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 Pro 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)
SFR: two rectangles (All versions)
Distortion grid or checkerboard (All versions)
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.
Star chart (Imatest Pro only)
Log frequency-contrast gradient charts (or Stepcharts) (1 and 2-band charts in Imatest Pro 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).
Each chart consists of the following patterns in 1, 2, or 4 bands. From top to bottom,
Options include
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 Pro only)
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
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 Pro can create two types of SVG chart, discussed in detail in SVG (Structured Vector Graphics) Test Charts. SVG Squares & Wedges (letter, A4, ...)
Slanted squares on m x n gridsThe 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.
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 Pro only. Additional details, as well as instructions on how to print these charts, are found on SVG (Structured Vector Graphics) Test Charts. OptionsPixels 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
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 lightnessChoices: 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 bandsThe 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 ratioChoices: 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. GammaChoices: 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 compensationcorrects 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 figureThe 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.
You can zoom in on the chart by selecting an area or clicking on a point of interest. Double-clicking restores the complete image.
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(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.. 
