Add Clear method to matrix builders.

Author: JimBobSquarePants

src/ImageSharp/Processing/AffineTransformBuilder.cs

@@ -349,6 +349,16 @@ public SizeF GetTransformedSize(Rectangle sourceRectangle)
     internal static SizeF GetTransformedSize(Rectangle sourceRectangle, Matrix3x2 matrix)
         => TransformUtilities.GetRawTransformedSize(matrix, sourceRectangle.Size);
 
+    /// <summary>
+    /// Clears all accumulated transform matrices, resetting the builder to its initial state.
+    /// </summary>
+    /// <returns>The <see cref="AffineTransformBuilder"/>.</returns>
+    public AffineTransformBuilder Clear()
+    {
+        this.transformMatrixFactories.Clear();
+        return this;
+    }
+
     private static void CheckDegenerate(Matrix3x2 matrix)
     {
         if (TransformUtilities.IsDegenerate(matrix))

src/ImageSharp/Processing/ProjectiveTransformBuilder.cs

@@ -397,6 +397,16 @@ public SizeF GetTransformedSize(Rectangle sourceRectangle)
     internal static SizeF GetTransformedSize(Rectangle sourceRectangle, Matrix4x4 matrix)
         => TransformUtilities.GetRawTransformedSize(matrix, sourceRectangle.Size);
 
+    /// <summary>
+    /// Clears all accumulated transform matrices, resetting the builder to its initial state.
+    /// </summary>
+    /// <returns>The <see cref="ProjectiveTransformBuilder"/>.</returns>
+    public ProjectiveTransformBuilder Clear()
+    {
+        this.transformMatrixFactories.Clear();
+        return this;
+    }
+
     private static void CheckDegenerate(Matrix4x4 matrix)
     {
         if (TransformUtilities.IsDegenerate(matrix))

tests/ImageSharp.Tests/Processing/Transforms/AffineTransformBuilderTests.cs

@@ -58,6 +58,9 @@ protected override void PrependSkewRadians(AffineTransformBuilder builder, float
     protected override void PrependTranslation(AffineTransformBuilder builder, PointF translate)
         => builder.PrependTranslation(translate);
 
+    protected override void ClearBuilder(AffineTransformBuilder builder)
+        => builder.Clear();
+
     protected override Vector2 Execute(
         AffineTransformBuilder builder,
         Rectangle rectangle,

tests/ImageSharp.Tests/Processing/Transforms/ProjectiveTransformBuilderTests.cs

@@ -55,6 +55,9 @@ protected override void PrependTranslation(ProjectiveTransformBuilder builder, P
     protected override void PrependRotationRadians(ProjectiveTransformBuilder builder, float radians, Vector2 origin) =>
         builder.PrependRotationRadians(radians, origin);
 
+    protected override void ClearBuilder(ProjectiveTransformBuilder builder)
+        => builder.Clear();
+
     protected override Vector2 Execute(
         ProjectiveTransformBuilder builder,
         Rectangle rectangle,

tests/ImageSharp.Tests/Processing/Transforms/TransformBuilderTestBase.cs

@@ -248,6 +248,48 @@ public void ThrowsForInvalidMatrix()
             });
     }
 
+    [Fact]
+    public void Clear_ResetsBuilderToIdentity()
+    {
+        Size size = new(100, 100);
+        Rectangle rectangle = new(Point.Empty, size);
+        Vector2 source = new(10, 20);
+
+        TBuilder builder = this.CreateBuilder();
+
+        // Apply a transform that changes the point.
+        this.AppendScale(builder, new SizeF(2, 3));
+        this.AppendTranslation(builder, new PointF(50, 50));
+        Vector2 transformed = this.Execute(builder, rectangle, source);
+        Assert.NotEqual(source, transformed, Comparer);
+
+        // Clear and verify the builder produces identity behavior.
+        this.ClearBuilder(builder);
+        Vector2 afterClear = this.Execute(builder, rectangle, source);
+        Assert.Equal(source, afterClear, Comparer);
+    }
+
+    [Fact]
+    public void Clear_AllowsReuse()
+    {
+        Size size = new(100, 100);
+        Rectangle rectangle = new(Point.Empty, size);
+        Vector2 source = new(10, 20);
+
+        TBuilder builder = this.CreateBuilder();
+
+        // First transform: scale by 2.
+        this.AppendScale(builder, new SizeF(2, 2));
+        Vector2 scaled = this.Execute(builder, rectangle, source);
+        Assert.Equal(new Vector2(20, 40), scaled, Comparer);
+
+        // Clear and apply a different transform: translate.
+        this.ClearBuilder(builder);
+        this.AppendTranslation(builder, new PointF(5, 10));
+        Vector2 translated = this.Execute(builder, rectangle, source);
+        Assert.Equal(new Vector2(15, 30), translated, Comparer);
+    }
+
     protected abstract TBuilder CreateBuilder();
 
     protected abstract void AppendRotationDegrees(TBuilder builder, float degrees);
@@ -282,5 +324,7 @@ public void ThrowsForInvalidMatrix()
 
     protected abstract void PrependTranslation(TBuilder builder, PointF translate);
 
+    protected abstract void ClearBuilder(TBuilder builder);
+
     protected abstract Vector2 Execute(TBuilder builder, Rectangle rectangle, Vector2 sourcePoint);
 }