Selecting SFR edge regions based on field distance

November 19, 2012
March 30, 2017

Some customers need to select regions based on a specified center-to-corner distance. This can be done using an Edge ID file (enhanced to be an Edge ID/Zone selection file in Imatest 5.0+), which is available in SFRplus, eSFR ISO, and Checkerboard. The box below contains concise instructions for using these files.

Edge ID/Zone selection file usage

An Edge ID/Zone selection file contains a list of identifiers for slanted edge ROI locations and selection zones, one per line, similar to lists in CSV and JSON output files. There are two types of data line: for individual ROIs and for ROI selection zones. It allows you to freely select edges or zones.

This window opens when you click
 Read EdgeID/Zone selection file. Click to view full size.

Click Read EdgeID file to read the file. Then click OK.

Individual edge ROI selection lines:  The format is  Column_Row_Side,  where

  • Column is the column number (x) relative to the center, e.g., -3, 0, 3, etc.
  • Row is the row number (y) relative to the center, e.g., -2, 0, 2, etc. NOTE: Row number increases in a downward direction, e.g., -1 is ABOVE center, etc.
  • Side is L, R, T, or B  (Left, Right, Top, or Bottom) (SFRplus and eSFR ISO modules). Use H or V (instead of L, R, T, B) for the Checkerboard module-only.
  • The final field from the results files ( _C, _E, etc. for Corner, Edge, etc.) is omitted.

Example: -2_1_L selects the left edge of the square two columns left of center and one row below center.

Zone selection lines (Imatest 5.0+):  The format is Z ii nn or S ii nn, where

  • Z denotes zone for edge selection based on % center-corner distance.
  • S denotes zone for edge selection based on % center-side distance.
  • ii is the minimum % distance. nn is the maximum % distance.

Example:  Z 60 80 selects all edges with centers between 60 and 80% of the way between the image center and the corner. Several zones can be selected.

A recent enhancement to Imatest Master 4.5 makes it easier to view the exact center-to-corner distance of each selected region in the Multi-ROI summary, shown below, using the Display ctr-corner % only setting in Imatest 4.5 (lower right).

Multi-region display with Display ctr-corner % only (and Plot Vertical-only)

In Imatest 5.0+ another selection— Display ctr-corner & EdgeID (shown on the right)— makes it even easier to relate center-to-corner location to Edge ID.

For SFRplus, it’s very important to note that

  • the center-to-corner % distance of an edge is proportional to the bar-bar height % (the top of the top bar – the bottom of the bottom bar in % of the image height) for an image with a fixed aspect ratio (3:2 for the above image, shown just to the right of (WxH).
  • The center-to-corner % distance of the edge is inversely proportional to the camera-to-chart distance.

For example, the Edge 3_2_R (Column 3, Row 2 (below the center), Right edge) is located at 74.8% of the center-to-corner distance. Moving the chart away from the camera reduces this number.

Another example (from a different image), let’s say that we want the two edges on the lower-right be located at an average distance of 70% of the center-to-corner distance. The current average is (81.9+83.8)/2 = 82.85%. The image would have to be framed (by backing away from it or using a smaller chart) so the bar-to-bar height was 89.6*70/82.85 = 75.7% (of the image height).

Another option is available if you can’t get the exact distances. The squares in rows 2 and 4 can be shifted slightly to the left or right of their nominal positions. This does not require a change in the software. (This cannot be done with rows 1, 3, and 5 without affecting the vertical convergence angle measurement.).

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