Align Measure standard deviation with ImageJ (#59)

Author: yangfei

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

@@ -10,7 +10,7 @@ ImageJCsharp Measure behavior.
 | 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` |
+| ImageJCsharp commit compared initially | `762eae3` |
 | Operating system | Windows |
 
 ImageJ was run through its Java API with `ij.plugin.filter.Analyzer` using these
@@ -53,7 +53,7 @@ The ROI covers these four pixels:
 
 ### Full Image
 
-| Measurement | ImageJ 1.54s99 | ImageJCsharp | Status |
+| Measurement | ImageJ 1.54s99 | ImageJCsharp at `762eae3` | Initial Status |
 | --- | ---: | ---: | --- |
 | Area | 12 | 12 | Match |
 | Mean | 55 | 55 | Match |
@@ -63,7 +63,7 @@ The ROI covers these four pixels:
 
 ### Rectangle ROI
 
-| Measurement | ImageJ 1.54s99 | ImageJCsharp | Status |
+| Measurement | ImageJ 1.54s99 | ImageJCsharp at `762eae3` | Initial Status |
 | --- | ---: | ---: | --- |
 | Area | 4 | 4 | Match |
 | Mean | 75 | 75 | Match |
@@ -82,13 +82,13 @@ deviation.
 
 ## Follow-Up
 
-Follow-up issue:
+Follow-up issue created from this comparison:
 
 ```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.
-
+Issue #57 aligned ImageJCsharp Measure with ImageJ for these StdDev cases by
+using sample standard deviation in `Measurements.Measure`. Automated Core tests
+now assert the ImageJ 1.54s99 full-image and rectangle-ROI StdDev values listed
+above.

src/ImageJCsharp.Core/Measurements.cs

@@ -138,7 +138,9 @@ private static MeasurementResult Measure(
         }
 
         var mean = sum / count;
-        var variance = Math.Max(0, (sumSquares / count) - (mean * mean));
+        var variance = count > 1
+            ? Math.Max(0, (sumSquares - ((sum * sum) / count)) / (count - 1))
+            : 0;
         var area = count * calibration.PixelWidth * calibration.PixelHeight;
 
         return new MeasurementResult(count, area, mean, min, max, Math.Sqrt(variance));

tests/ImageJCsharp.Core.Tests/CoreImageTests.cs

@@ -104,6 +104,36 @@ public void MeasureComputesStatisticsInsideRectRoi()
         Assert.Equal(60, result.Max);
     }
 
+    [Fact]
+    public void MeasureUsesImageJCompatibleSampleStandardDeviation()
+    {
+        var image = GrayImage.FromPixels(4, 3, new ushort[]
+        {
+            0, 10, 20, 30,
+            40, 50, 60, 70,
+            80, 90, 100, 110
+        });
+
+        var result = Measurements.Measure(image, new RectRoi(0, 0, 4, 3), PixelCalibration.Identity);
+
+        Assert.Equal(36.05551275463989, result.StandardDeviation, 12);
+    }
+
+    [Fact]
+    public void MeasureRoiUsesImageJCompatibleSampleStandardDeviation()
+    {
+        var image = GrayImage.FromPixels(4, 3, new ushort[]
+        {
+            0, 10, 20, 30,
+            40, 50, 60, 70,
+            80, 90, 100, 110
+        });
+
+        var result = Measurements.Measure(image, new RectRoi(1, 1, 2, 2), PixelCalibration.Identity);
+
+        Assert.Equal(23.804761428476166, result.StandardDeviation, 12);
+    }
+
     [Fact]
     public void MeasureUsesPixelCalibrationForArea()
     {