Automatic processing of Imatest charts

It turns out that people sometimes want to process an image of an Imatest SFRplus type chart in MTF Mapper. Or at the very least, I have received requests about this.

When I first released MTF Mapper, I took it as a given that people would just print out the MTF Mapper charts if they wanted to use MTF Mapper. In the meantime I have gotten wiser, and I now know how hard (or expensive?) it is to print high-quality test charts. So it actually makes perfect sense to use a good quality chart that you already own (e.g., an SFRplus chart) with MTF Mapper.

Unfortunately the design of the SFRplus-style charts includes a black bar that runs through the top of the top row of target squares, like in this example:

An example of an SFRplus chart. I blatantly copied this example from Imatest's website (please don't sue me).

This black bar causes MTF Mapper to see the entire top row of squares plus the black bar as a single object, and since this compound object does not resemble a square, it ignores it. The same thing happens with the black bar at the bottom of the chart. As a result, MTF Mapper only processes the interior squares, like so:

Ignore the actual MTF50 values, but do note that only the interior square targets were detected automatically

Other than the obvious spurious detections on the non-target squares (which you can cover up with post-its or such if necessary) the output is usable, but you lose the top and bottom rows of squares, which is not ideal.

A simple solution is to just crop your image to exclude the bars, and then to process the cropped image with MTF Mapper's "-b" option. This works, but it is rather clunky. So I added a convenience feature that will do this automatically.

You can choose the new File/Open Imatest image(s) option in the GUI, or you can add the --imatest-chart option if you use the command-line interface. Because the gray target squares of the SFRplus chart cover more of the white background than a typical MTF Mapper chart, you probably have to adjust the "Threshold" value (-t on the CLI, or under Settings/Preferences/Advanced in the GUI) a little to detect all the target squares. The default Threshold is 0.55, and bumping it down to 0.4 should work a little better. For our test image above, we then get this:

Much better; all target squares detected

Note that the top edges of the top row of squares are tagged with "N/A" rather than MTF50 values; this is just MTF Mapper's way to indicate that these edges do not represent valid measurements. If you are parsing the "edge_mtf_values.txt" file produced by the "-q" output option, these edges will have an MTF50 value of 1.0 (which is an impossible / invalid MTF50 value). Or you could identify them by their pixel coordinates, which is probably the better way.

This feature is available from MTF Mapper 0.6.21 onward.