Fix off-by-one error when centering a transform.

Author: JimBobSquarePants

src/ImageSharp/Processing/AffineTransformBuilder.cs

@@ -11,7 +11,8 @@ namespace SixLabors.ImageSharp.Processing;
 /// </summary>
 public class AffineTransformBuilder
 {
-    private readonly List<Func<Size, Matrix3x2>> matrixFactories = new List<Func<Size, Matrix3x2>>();
+    private readonly List<Func<Size, Matrix3x2>> transformMatrixFactories = new();
+    private readonly List<Func<Size, Matrix3x2>> boundsMatrixFactories = new();
 
     /// <summary>
     /// Prepends a rotation matrix using the given rotation angle in degrees
@@ -29,7 +30,9 @@ public AffineTransformBuilder PrependRotationDegrees(float degrees)
     /// <param name="radians">The amount of rotation, in radians.</param>
     /// <returns>The <see cref="AffineTransformBuilder"/>.</returns>
     public AffineTransformBuilder PrependRotationRadians(float radians)
-        => this.Prepend(size => TransformUtils.CreateRotationMatrixRadians(radians, size));
+        => this.Prepend(
+            size => TransformUtils.CreateRotationTransformMatrixRadians(radians, size),
+            size => TransformUtils.CreateRotationBoundsMatrixRadians(radians, size));
 
     /// <summary>
     /// Prepends a rotation matrix using the given rotation in degrees at the given origin.
@@ -65,7 +68,9 @@ public AffineTransformBuilder AppendRotationDegrees(float degrees)
     /// <param name="radians">The amount of rotation, in radians.</param>
     /// <returns>The <see cref="AffineTransformBuilder"/>.</returns>
     public AffineTransformBuilder AppendRotationRadians(float radians)
-        => this.Append(size => TransformUtils.CreateRotationMatrixRadians(radians, size));
+        => this.Append(
+            size => TransformUtils.CreateRotationTransformMatrixRadians(radians, size),
+            size => TransformUtils.CreateRotationBoundsMatrixRadians(radians, size));
 
     /// <summary>
     /// Appends a rotation matrix using the given rotation in degrees at the given origin.
@@ -140,7 +145,9 @@ public AffineTransformBuilder AppendScale(Vector2 scales)
     /// <param name="degreesY">The Y angle, in degrees.</param>
     /// <returns>The <see cref="AffineTransformBuilder"/>.</returns>
     public AffineTransformBuilder PrependSkewDegrees(float degreesX, float degreesY)
-        => this.Prepend(size => TransformUtils.CreateSkewMatrixDegrees(degreesX, degreesY, size));
+        => this.Prepend(
+            size => TransformUtils.CreateSkewTransformMatrixDegrees(degreesX, degreesY, size),
+            size => TransformUtils.CreateSkewBoundsMatrixDegrees(degreesX, degreesY, size));
 
     /// <summary>
     /// Prepends a centered skew matrix from the give angles in radians.
@@ -149,7 +156,9 @@ public AffineTransformBuilder PrependSkewDegrees(float degreesX, float degreesY)
     /// <param name="radiansY">The Y angle, in radians.</param>
     /// <returns>The <see cref="AffineTransformBuilder"/>.</returns>
     public AffineTransformBuilder PrependSkewRadians(float radiansX, float radiansY)
-        => this.Prepend(size => TransformUtils.CreateSkewMatrixRadians(radiansX, radiansY, size));
+        => this.Prepend(
+            size => TransformUtils.CreateSkewTransformMatrixRadians(radiansX, radiansY, size),
+            size => TransformUtils.CreateSkewBoundsMatrixRadians(radiansX, radiansY, size));
 
     /// <summary>
     /// Prepends a skew matrix using the given angles in degrees at the given origin.
@@ -178,7 +187,9 @@ public AffineTransformBuilder PrependSkewRadians(float radiansX, float radiansY,
     /// <param name="degreesY">The Y angle, in degrees.</param>
     /// <returns>The <see cref="AffineTransformBuilder"/>.</returns>
     public AffineTransformBuilder AppendSkewDegrees(float degreesX, float degreesY)
-        => this.Append(size => TransformUtils.CreateSkewMatrixDegrees(degreesX, degreesY, size));
+        => this.Append(
+            size => TransformUtils.CreateSkewTransformMatrixDegrees(degreesX, degreesY, size),
+            size => TransformUtils.CreateSkewBoundsMatrixDegrees(degreesX, degreesY, size));
 
     /// <summary>
     /// Appends a centered skew matrix from the give angles in radians.
@@ -187,7 +198,9 @@ public AffineTransformBuilder AppendSkewDegrees(float degreesX, float degreesY)
     /// <param name="radiansY">The Y angle, in radians.</param>
     /// <returns>The <see cref="AffineTransformBuilder"/>.</returns>
     public AffineTransformBuilder AppendSkewRadians(float radiansX, float radiansY)
-        => this.Append(size => TransformUtils.CreateSkewMatrixRadians(radiansX, radiansY, size));
+        => this.Append(
+            size => TransformUtils.CreateSkewTransformMatrixRadians(radiansX, radiansY, size),
+            size => TransformUtils.CreateSkewBoundsMatrixRadians(radiansX, radiansY, size));
 
     /// <summary>
     /// Appends a skew matrix using the given angles in degrees at the given origin.
@@ -254,7 +267,7 @@ public AffineTransformBuilder AppendTranslation(Vector2 position)
     public AffineTransformBuilder PrependMatrix(Matrix3x2 matrix)
     {
         CheckDegenerate(matrix);
-        return this.Prepend(_ => matrix);
+        return this.Prepend(_ => matrix, _ => matrix);
     }
 
     /// <summary>
@@ -270,7 +283,7 @@ public AffineTransformBuilder PrependMatrix(Matrix3x2 matrix)
     public AffineTransformBuilder AppendMatrix(Matrix3x2 matrix)
     {
         CheckDegenerate(matrix);
-        return this.Append(_ => matrix);
+        return this.Append(_ => matrix, _ => matrix);
     }
 
     /// <summary>
@@ -281,7 +294,7 @@ public AffineTransformBuilder AppendMatrix(Matrix3x2 matrix)
     public Matrix3x2 BuildMatrix(Size sourceSize) => this.BuildMatrix(new Rectangle(Point.Empty, sourceSize));
 
     /// <summary>
-    /// Returns the combined matrix for a given source rectangle.
+    /// Returns the combined transform matrix for a given source rectangle.
     /// </summary>
     /// <param name="sourceRectangle">The rectangle in the source image.</param>
     /// <exception cref="DegenerateTransformException">
@@ -296,11 +309,11 @@ public Matrix3x2 BuildMatrix(Rectangle sourceRectangle)
         Guard.MustBeGreaterThan(sourceRectangle.Height, 0, nameof(sourceRectangle));
 
         // Translate the origin matrix to cater for source rectangle offsets.
-        var matrix = Matrix3x2.CreateTranslation(-sourceRectangle.Location);
+        Matrix3x2 matrix = Matrix3x2.CreateTranslation(-sourceRectangle.Location);
 
         Size size = sourceRectangle.Size;
 
-        foreach (Func<Size, Matrix3x2> factory in this.matrixFactories)
+        foreach (Func<Size, Matrix3x2> factory in this.transformMatrixFactories)
         {
             matrix *= factory(size);
         }
@@ -310,6 +323,32 @@ public Matrix3x2 BuildMatrix(Rectangle sourceRectangle)
         return matrix;
     }
 
+    /// <summary>
+    /// Returns the size of a rectangle large enough to contain the transformed source rectangle.
+    /// </summary>
+    /// <param name="sourceRectangle">The rectangle in the source image.</param>
+    /// <exception cref="DegenerateTransformException">
+    /// The resultant matrix is degenerate containing one or more values equivalent
+    /// to <see cref="float.NaN"/> or a zero determinant and therefore cannot be used
+    /// for linear transforms.
+    /// </exception>
+    /// <returns>The <see cref="Size"/>.</returns>
+    public Size GetTransformedSize(Rectangle sourceRectangle)
+    {
+        Size size = sourceRectangle.Size;
+
+        // Translate the origin matrix to cater for source rectangle offsets.
+        Matrix3x2 matrix = Matrix3x2.CreateTranslation(-sourceRectangle.Location);
+
+        foreach (Func<Size, Matrix3x2> factory in this.boundsMatrixFactories)
+        {
+            matrix *= factory(size);
+            CheckDegenerate(matrix);
+        }
+
+        return TransformUtils.GetTransformedSize(size, matrix);
+    }
+
     private static void CheckDegenerate(Matrix3x2 matrix)
     {
         if (TransformUtils.IsDegenerate(matrix))
@@ -318,15 +357,17 @@ private static void CheckDegenerate(Matrix3x2 matrix)
         }
     }
 
-    private AffineTransformBuilder Prepend(Func<Size, Matrix3x2> factory)
+    private AffineTransformBuilder Prepend(Func<Size, Matrix3x2> transformFactory, Func<Size, Matrix3x2> boundsFactory)
     {
-        this.matrixFactories.Insert(0, factory);
+        this.transformMatrixFactories.Insert(0, transformFactory);
+        this.boundsMatrixFactories.Insert(0, boundsFactory);
         return this;
     }
 
-    private AffineTransformBuilder Append(Func<Size, Matrix3x2> factory)
+    private AffineTransformBuilder Append(Func<Size, Matrix3x2> transformFactory, Func<Size, Matrix3x2> boundsFactory)
     {
-        this.matrixFactories.Add(factory);
+        this.transformMatrixFactories.Add(transformFactory);
+        this.boundsMatrixFactories.Add(boundsFactory);
         return this;
     }
 }

src/ImageSharp/Processing/Extensions/Transforms/TransformExtensions.cs

@@ -51,7 +51,7 @@ public static IImageProcessingContext Transform(
         IResampler sampler)
     {
         Matrix3x2 transform = builder.BuildMatrix(sourceRectangle);
-        Size targetDimensions = TransformUtils.GetTransformedSize(sourceRectangle.Size, transform);
+        Size targetDimensions = builder.GetTransformedSize(sourceRectangle);
         return source.Transform(sourceRectangle, transform, targetDimensions, sampler);
     }
 
@@ -113,7 +113,7 @@ public static IImageProcessingContext Transform(
         IResampler sampler)
     {
         Matrix4x4 transform = builder.BuildMatrix(sourceRectangle);
-        Size targetDimensions = TransformUtils.GetTransformedSize(sourceRectangle.Size, transform);
+        Size targetDimensions = builder.GetTransformedSize(sourceRectangle);
         return source.Transform(sourceRectangle, transform, targetDimensions, sampler);
     }
 

src/ImageSharp/Processing/Processors/Transforms/Linear/LinearTransformUtility.cs

@@ -38,8 +38,8 @@ public static float GetSamplingRadius<TResampler>(in TResampler sampler, int sou
     /// </summary>
     /// <param name="radius">The radius.</param>
     /// <param name="center">The center position.</param>
-    /// <param name="min">The min allowed amouunt.</param>
-    /// <param name="max">The max allowed amouunt.</param>
+    /// <param name="min">The min allowed amount.</param>
+    /// <param name="max">The max allowed amount.</param>
     /// <returns>The <see cref="int"/>.</returns>
     [MethodImpl(InliningOptions.ShortMethod)]
     public static int GetRangeStart(float radius, float center, int min, int max)
@@ -51,8 +51,8 @@ public static int GetRangeStart(float radius, float center, int min, int max)
     /// </summary>
     /// <param name="radius">The radius.</param>
     /// <param name="center">The center position.</param>
-    /// <param name="min">The min allowed amouunt.</param>
-    /// <param name="max">The max allowed amouunt.</param>
+    /// <param name="min">The min allowed amount.</param>
+    /// <param name="max">The max allowed amount.</param>
     /// <returns>The <see cref="int"/>.</returns>
     [MethodImpl(InliningOptions.ShortMethod)]
     public static int GetRangeEnd(float radius, float center, int min, int max)

src/ImageSharp/Processing/Processors/Transforms/Linear/RotateProcessor.cs

@@ -28,14 +28,15 @@ public RotateProcessor(float degrees, Size sourceSize)
     /// <param name="sourceSize">The source image size</param>
     public RotateProcessor(float degrees, IResampler sampler, Size sourceSize)
         : this(
-            TransformUtils.CreateRotationMatrixDegrees(degrees, sourceSize),
+            TransformUtils.CreateRotationTransformMatrixDegrees(degrees, sourceSize),
+            TransformUtils.CreateRotationBoundsMatrixDegrees(degrees, sourceSize),
             sampler,
             sourceSize)
         => this.Degrees = degrees;
 
     // Helper constructor
-    private RotateProcessor(Matrix3x2 rotationMatrix, IResampler sampler, Size sourceSize)
-        : base(rotationMatrix, sampler, TransformUtils.GetTransformedSize(sourceSize, rotationMatrix))
+    private RotateProcessor(Matrix3x2 rotationMatrix, Matrix3x2 boundsMatrix, IResampler sampler, Size sourceSize)
+        : base(rotationMatrix, sampler, TransformUtils.GetTransformedSize(sourceSize, boundsMatrix))
     {
     }
 

src/ImageSharp/Processing/Processors/Transforms/Linear/SkewProcessor.cs

@@ -30,7 +30,8 @@ public SkewProcessor(float degreesX, float degreesY, Size sourceSize)
     /// <param name="sourceSize">The source image size</param>
     public SkewProcessor(float degreesX, float degreesY, IResampler sampler, Size sourceSize)
         : this(
-             TransformUtils.CreateSkewMatrixDegrees(degreesX, degreesY, sourceSize),
+             TransformUtils.CreateSkewTransformMatrixDegrees(degreesX, degreesY, sourceSize),
+             TransformUtils.CreateSkewBoundsMatrixDegrees(degreesX, degreesY, sourceSize),
              sampler,
              sourceSize)
     {
@@ -39,8 +40,8 @@ public SkewProcessor(float degreesX, float degreesY, IResampler sampler, Size so
     }
 
     // Helper constructor:
-    private SkewProcessor(Matrix3x2 skewMatrix, IResampler sampler, Size sourceSize)
-        : base(skewMatrix, sampler, TransformUtils.GetTransformedSize(sourceSize, skewMatrix))
+    private SkewProcessor(Matrix3x2 skewMatrix, Matrix3x2 boundsMatrix, IResampler sampler, Size sourceSize)
+        : base(skewMatrix, sampler, TransformUtils.GetTransformedSize(sourceSize, boundsMatrix))
     {
     }