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‎readme.md‎

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@@ -110,6 +110,52 @@ public Task VerifyImage()
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<!-- endSnippet -->
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## SSIM Image Comparison
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By default, image comparison is byte-exact. To tolerate minor rendering differences (anti-aliasing, font hinting, subpixel rendering), enable SSIM (Structural Similarity Index) comparison by passing a threshold to `Initialize`:
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```cs
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[ModuleInitializer]
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public static void Init() =>
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VerifyImageSharp.Initialize(ssimThreshold: 0.999);
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```
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SSIM returns a value between 0.0 (completely different) and 1.0 (identical). Images with an SSIM at or above the threshold are considered equal. Recommended thresholds:
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* `0.999` — tolerates anti-aliasing and subpixel rendering differences
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* `0.995` — tolerates minor font/layout shifts across OS versions
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* `0.99` — tolerates moderate rendering variation
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### Per-test threshold
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Override the global threshold for a specific test:
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<!-- snippet: SsimThreshold -->
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<a id='snippet-SsimThreshold'></a>
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```cs
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[Test]
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public Task VerifyImageWithSsimThreshold()
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{
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var image = new Image<Rgba32>(11, 11)
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{
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[5, 5] = Rgba32.ParseHex("#0000FF")
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};
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return Verify(image)
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.SsimThreshold(0.95);
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}
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```
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<sup><a href='/src/Tests/SsimTests.cs#L61-L74' title='Snippet source file'>snippet source</a> | <a href='#snippet-SsimThreshold' title='Start of snippet'>anchor</a></sup>
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<!-- endSnippet -->
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### Direct SSIM calculation
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The `SsimComparer` class can also be used directly to get the raw SSIM value:
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```cs
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double ssim = SsimComparer.Calculate(receivedStream, verifiedStream);
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```
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## File Samples
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http://file-examples.com/

‎src/Tests/SsimTests.VerifyImageWithSsimThreshold#00.verified.txt‎

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{
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Width: 11,
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Height: 11,
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HorizontalResolution: 96.0,
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VerticalResolution: 96.0,
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ResolutionUnits: PixelsPerInch
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}

‎src/Tests/SsimTests.VerifyImageWithSsimThreshold#01.verified.txt‎

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{
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Width: 11,
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Height: 11,
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HorizontalResolution: 3780.0,
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VerticalResolution: 3780.0,
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ResolutionUnits: PixelsPerMeter
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}

‎src/Tests/SsimTests.VerifyImageWithSsimThreshold.verified.png‎

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‎src/Tests/SsimTests.cs‎

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using SixLabors.ImageSharp;
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using SixLabors.ImageSharp.Formats.Png;
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using SixLabors.ImageSharp.PixelFormats;
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[TestFixture]
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public class SsimTests
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{
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[Test]
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public void IdenticalImages()
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{
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using var image = new Image<Rgba32>(100, 100);
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var stream1 = Encode(image);
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var stream2 = Encode(image);
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var ssim = SsimComparer.Calculate(stream1, stream2);
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Assert.That(ssim, Is.EqualTo(1.0));
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}
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[Test]
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public void CompletelyDifferentImages()
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{
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using var black = new Image<Rgba32>(100, 100);
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using var white = new Image<Rgba32>(100, 100);
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for (var y = 0; y < 100; y++)
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{
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for (var x = 0; x < 100; x++)
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{
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white[x, y] = new Rgba32(255, 255, 255, 255);
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}
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}
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var ssim = SsimComparer.Calculate(Encode(black), Encode(white));
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Assert.That(ssim, Is.LessThan(0.1));
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}
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[Test]
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public void SlightlyDifferentImages()
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{
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using var image1 = new Image<Rgba32>(100, 100);
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using var image2 = image1.Clone();
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// change a few pixels slightly
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for (var x = 0; x < 10; x++)
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{
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image2[x, 0] = new Rgba32(10, 10, 10, 255);
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}
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var ssim = SsimComparer.Calculate(Encode(image1), Encode(image2));
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Assert.That(ssim, Is.GreaterThan(0.99));
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Assert.That(ssim, Is.LessThan(1.0));
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}
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[Test]
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public void DifferentSizeReturnsZero()
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{
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using var small = new Image<Rgba32>(50, 50);
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using var large = new Image<Rgba32>(100, 100);
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var ssim = SsimComparer.Calculate(Encode(small), Encode(large));
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Assert.That(ssim, Is.EqualTo(0));
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}
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#region SsimThreshold
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[Test]
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public Task VerifyImageWithSsimThreshold()
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{
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var image = new Image<Rgba32>(11, 11)
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{
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[5, 5] = Rgba32.ParseHex("#0000FF")
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};
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return Verify(image)
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.SsimThreshold(0.95);
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}
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#endregion
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static MemoryStream Encode(Image image)
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{
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var stream = new MemoryStream();
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image.Save(stream, new PngEncoder());
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stream.Position = 0;
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return stream;
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}
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}

‎src/Verify.ImageSharp/SsimComparer.cs‎

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using SixLabors.ImageSharp.PixelFormats;
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namespace VerifyTests;
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public static class SsimComparer
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{
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const double k1 = 0.01;
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const double k2 = 0.03;
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const double l = 255.0;
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const double c1 = k1 * l * k1 * l;
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const double c2 = k2 * l * k2 * l;
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public static double Calculate(Stream received, Stream verified)
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{
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using var imgReceived = Image.Load<Rgba32>(received);
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using var imgVerified = Image.Load<Rgba32>(verified);
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if (imgReceived.Width != imgVerified.Width ||
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imgReceived.Height != imgVerified.Height)
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{
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return 0;
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}
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var width = imgReceived.Width;
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var height = imgReceived.Height;
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var pixelCount = (double) (width * height);
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double sumR1 = 0, sumG1 = 0, sumB1 = 0;
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double sumR2 = 0, sumG2 = 0, sumB2 = 0;
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double sumR1Sq = 0, sumG1Sq = 0, sumB1Sq = 0;
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double sumR2Sq = 0, sumG2Sq = 0, sumB2Sq = 0;
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double sumR12 = 0, sumG12 = 0, sumB12 = 0;
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imgReceived.ProcessPixelRows(imgVerified, (accessorReceived, accessorVerified) =>
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{
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for (var y = 0; y < height; y++)
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{
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var row1 = accessorReceived.GetRowSpan(y);
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var row2 = accessorVerified.GetRowSpan(y);
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for (var x = 0; x < width; x++)
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{
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var p1 = row1[x];
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var p2 = row2[x];
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double r1 = p1.R, g1 = p1.G, b1 = p1.B;
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double r2 = p2.R, g2 = p2.G, b2 = p2.B;
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sumR1 += r1;
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sumG1 += g1;
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sumB1 += b1;
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sumR2 += r2;
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sumG2 += g2;
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sumB2 += b2;
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sumR1Sq += r1 * r1;
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sumG1Sq += g1 * g1;
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sumB1Sq += b1 * b1;
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sumR2Sq += r2 * r2;
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sumG2Sq += g2 * g2;
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sumB2Sq += b2 * b2;
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sumR12 += r1 * r2;
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sumG12 += g1 * g2;
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sumB12 += b1 * b2;
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}
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}
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});
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var ssimR = CalculateChannel(pixelCount, sumR1, sumR2, sumR1Sq, sumR2Sq, sumR12);
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var ssimG = CalculateChannel(pixelCount, sumG1, sumG2, sumG1Sq, sumG2Sq, sumG12);
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var ssimB = CalculateChannel(pixelCount, sumB1, sumB2, sumB1Sq, sumB2Sq, sumB12);
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return (ssimR + ssimG + ssimB) / 3.0;
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}
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static double CalculateChannel(double n, double sum1, double sum2, double sum1Sq, double sum2Sq, double sum12)
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{
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var mu1 = sum1 / n;
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var mu2 = sum2 / n;
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var sigma1Sq = sum1Sq / n - mu1 * mu1;
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var sigma2Sq = sum2Sq / n - mu2 * mu2;
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var sigma12 = sum12 / n - mu1 * mu2;
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return (2 * mu1 * mu2 + c1) * (2 * sigma12 + c2) /
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((mu1 * mu1 + mu2 * mu2 + c1) * (sigma1Sq + sigma2Sq + c2));
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}
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}

‎src/Verify.ImageSharp/VerifyImageSharp.cs‎

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{
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public static bool Initialized { get; private set; }
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public static void Initialize()
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public static void Initialize(double ssimThreshold = 1.0)
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{
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if (Initialized)
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{
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VerifierSettings.RegisterStreamConverter("png", ConvertPng);
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VerifierSettings.RegisterStreamConverter("tif", ConvertTiff);
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if (ssimThreshold < 1.0)
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{
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Task<CompareResult> Compare(Stream received, Stream verified, IReadOnlyDictionary<string, object> context)
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{
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var threshold = context.TryGetValue("ImageSharpSsimThreshold", out var value)
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? (double) value
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: ssimThreshold;
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var ssim = SsimComparer.Calculate(received, verified);
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var result = ssim >= threshold
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? CompareResult.Equal
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: CompareResult.NotEqual($"SSIM: {ssim:F6}, threshold: {threshold:F6}");
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return Task.FromResult(result);
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}
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VerifierSettings.RegisterStreamComparer("bmp", Compare);
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VerifierSettings.RegisterStreamComparer("gif", Compare);
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VerifierSettings.RegisterStreamComparer("jpg", Compare);
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VerifierSettings.RegisterStreamComparer("png", Compare);
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VerifierSettings.RegisterStreamComparer("tif", Compare);
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}
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var encoder = new PngEncoder();
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VerifierSettings.RegisterFileConverter<Image>((image, context) => ConvertImage(null, image, context, "png", encoder));
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}
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public static void SsimThreshold(this VerifySettings settings, double threshold) =>
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settings.Context["ImageSharpSsimThreshold"] = threshold;
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public static SettingsTask SsimThreshold(this SettingsTask settings, double threshold)
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{
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settings.CurrentSettings.SsimThreshold(threshold);
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return settings;
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}
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static void EncodeAs<TEncoder>(this VerifySettings settings, string extension, IImageEncoder? encoder)
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where TEncoder : IImageEncoder, new()
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{

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