Class StructuralSimilarity

Implements the Structural Similarity Index Measure (SSIM) for texture/image comparison. SSIM is widely used in image quality assessment (including digital video). The SSIM index is computed over sliding Gaussian-weighted windows on the reference and test images, and the per-window results are averaged across the full image. The window size and Gaussian parameters can be configured via StructuralSimilaritySettings. A window size of 11 and a Gaussian σ of 1.0–1.5 are common defaults. The resulting value typically lies between 0 and 1, where 1 indicates identical images and values near 0 indicate very low structural similarity. Note: Extreme cases (e.g., constant black vs. constant white) usually yield SSIM near 0; exact 0 depends on implementation details and stabilizing constants.

Inheritance
object
TextureComparisonAlgorithm
StructuralSimilarity
Inherited Members
Namespace: UnityEngine.TestTools.Graphics
Assembly: UnityEngine.TestTools.Graphics.dll
Syntax
public class StructuralSimilarity : TextureComparisonAlgorithm

Constructors

StructuralSimilarity(ITextureComparisonSettings)

Initialized a new instance of the Structural Similarity Index Measure (SSIM) algorithm with the given threshold.

Declaration
public StructuralSimilarity(ITextureComparisonSettings settings)
Parameters
Type Name Description
ITextureComparisonSettings settings

Index measure under which the test constraint fails

Methods

Compare(Texture2D, Texture2D)

Compares two Texture2D and returns a Structural Similarity Index Measure.

Declaration
public override ITextureComparisonResult Compare(Texture2D expected, Texture2D actual)
Parameters
Type Name Description
Texture2D expected

The reference texture
Texture2D actual

The texture being evaluated
Returns
Type Description
ITextureComparisonResult

The Structural Similarity Index Measure. higher being better, 1 being the same texture.
Overrides

Compare(Texture2D[], Texture2D[])

Compares two arrays of textures using the Structural Similarity Index Measure (SSIM) algorithm. Both arrays need to have the same length and are compared by corresponding index.

Declaration
public override ITextureComparisonResult Compare(Texture2D[] expectedTextures, Texture2D[] actualTextures)
Parameters
Type Name Description
Texture2D[] expectedTextures

The reference textures
Texture2D[] actualTextures

The textures being evaluated
Returns
Type Description
ITextureComparisonResult

The Structural Similarity Index Measure; higher being better, 30 being the human perceptible threshold.
Overrides

CompareAsync(Texture2D, Texture2D)

Asynchronously compares two textures using the Structural Similarity Index Measure (SSIM) algorithm.

Declaration
public override Task<ITextureComparisonResult> CompareAsync(Texture2D expected, Texture2D actual)
Parameters
Type Name Description
Texture2D expected

The reference image
Texture2D actual

The texture being evaluated against the reference
Returns
Type Description
Task<ITextureComparisonResult>

A task with a result
Overrides
Exceptions
Type Condition
NotSupportedException

Async comparison is not supported for this algorithm

DescribeActual(ITextureComparisonResult)

Describes the measured values of a comparison result produced by this algorithm, as named values for structured result reporting. The algorithm owns the keys. Returns null when the algorithm does not provide structured values (or the result is not one of its own); reporting then falls back to the result's ToString().

Declaration
public override IReadOnlyDictionary<string, object> DescribeActual(ITextureComparisonResult result)
Parameters
Type Name Description
ITextureComparisonResult result

The result of a comparison run by this algorithm.
Returns
Type Description
IReadOnlyDictionary<string, object>

The measured values as named values, or null.
Overrides

DescribeExpected()

Describes the thresholds this algorithm compares against, as named values for structured result reporting. The algorithm owns the keys. Returns null when the algorithm does not provide structured values; reporting then falls back to Description.

Declaration
public override IReadOnlyDictionary<string, object> DescribeExpected()
Returns
Type Description
IReadOnlyDictionary<string, object>

The configured thresholds as named values, or null.
Overrides

Evaluate(ITextureComparisonResult)

Evaluates the comparison result against the defined threshold.

Declaration
public override (object, bool) Evaluate(ITextureComparisonResult result)
Parameters
Type Name Description
ITextureComparisonResult result

The SSIM comparison result
Returns
Type Description
(object, bool)

A tuple containing the result and whether it passes the evaluation
Overrides