Document ImageJ Measure comparison

docs/imagej-measure-comparison-2026-05-16.md

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+# ImageJ Measure Comparison - 2026-05-16
+
+This document records the first focused comparison between ImageJ Measure and
+ImageJCsharp Measure behavior.
+
+## Environment
+
+| Item | Value |
+| --- | --- |
+| ImageJ build | ImageJ 1.54s99 |
+| ImageJ source | `https://wsr.imagej.net/jars/ij.jar` |
+| Java | `C:\Java21\jdk-21.0.2\bin\java.exe` |
+| ImageJCsharp commit | `762eae3` |
+| Operating system | Windows |
+
+ImageJ was run through its Java API with `ij.plugin.filter.Analyzer` using these
+measurement flags:
+
+```text
+AREA | MEAN | MIN_MAX | STD_DEV
+```
+
+ImageJCsharp was run through `ImageJCsharp.Core.Measurements.Measure` with
+`PixelCalibration.Identity`.
+
+## Test Image
+
+The comparison used a synthetic 4 x 3 8-bit grayscale image.
+
+```text
+0    10   20   30
+40   50   60   70
+80   90   100  110
+```
+
+The rectangle ROI used ImageJ/ImageJCsharp zero-based coordinates:
+
+```text
+x = 1
+y = 1
+width = 2
+height = 2
+```
+
+The ROI covers these four pixels:
+
+```text
+50   60
+90   100
+```
+
+## Results
+
+### Full Image
+
+| Measurement | ImageJ 1.54s99 | ImageJCsharp | Status |
+| --- | ---: | ---: | --- |
+| Area | 12 | 12 | Match |
+| Mean | 55 | 55 | Match |
+| Min | 0 | 0 | Match |
+| Max | 110 | 110 | Match |
+| StdDev | 36.05551275463989 | 34.5205252953466 | Differs |
+
+### Rectangle ROI
+
+| Measurement | ImageJ 1.54s99 | ImageJCsharp | Status |
+| --- | ---: | ---: | --- |
+| Area | 4 | 4 | Match |
+| Mean | 75 | 75 | Match |
+| Min | 50 | 50 | Match |
+| Max | 100 | 100 | Match |
+| StdDev | 23.804761428476166 | 20.6155281280883 | Differs |
+
+## Compatibility Notes
+
+The first comparison shows that area, mean, minimum, and maximum match for the
+tested full-image and rectangle-ROI cases.
+
+Standard deviation differs. ImageJ reports the sample standard deviation for
+these cases, while ImageJCsharp currently reports the population standard
+deviation.
+
+## Follow-Up
+
+Follow-up issue:
+
+```text
+https://github.com/yangfei/ImageJCsharp/issues/57
+```
+
+That issue tracks the decision and implementation work for aligning
+ImageJCsharp standard deviation behavior with ImageJ, if that is the desired
+compatibility target.
+