Fix #999 and add tests

src/ImageSharp/Processing/Extensions/ResizeExtensions.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 using SixLabors.ImageSharp.Processing.Processors.Transforms;
@@ -12,16 +12,6 @@ namespace SixLabors.ImageSharp.Processing
     /// </summary>
     public static class ResizeExtensions
     {
-        /// <summary>
-        /// Resizes an image in accordance with the given <see cref="ResizeOptions"/>.
-        /// </summary>
-        /// <param name="source">The image to resize.</param>
-        /// <param name="options">The resize options.</param>
-        /// <returns>The <see cref="IImageProcessingContext"/> to allow chaining of operations.</returns>
-        /// <remarks>Passing zero for one of height or width within the resize options will automatically preserve the aspect ratio of the original image or the nearest possible ratio.</remarks>
-        public static IImageProcessingContext Resize(this IImageProcessingContext source, ResizeOptions options)
-            => source.ApplyProcessor(new ResizeProcessor(options, source.GetCurrentSize()));
-
         /// <summary>
         /// Resizes an image to the given <see cref="Size"/>.
         /// </summary>
@@ -128,7 +118,18 @@ public static IImageProcessingContext Resize(
             Rectangle sourceRectangle,
             Rectangle targetRectangle,
             bool compand)
-            => source.ApplyProcessor(new ResizeProcessor(sampler, width, height, source.GetCurrentSize(), targetRectangle, compand), sourceRectangle);
+        {
+            var options = new ResizeOptions
+            {
+                Size = new Size(width, height),
+                Mode = ResizeMode.Manual,
+                Sampler = sampler,
+                TargetRectangle = targetRectangle,
+                Compand = compand
+            };
+
+            return source.ApplyProcessor(new ResizeProcessor(options, source.GetCurrentSize()), sourceRectangle);
+        }
 
         /// <summary>
         /// Resizes an image to the given width and height with the given sampler and source rectangle.
@@ -150,6 +151,27 @@ public static IImageProcessingContext Resize(
             IResampler sampler,
             Rectangle targetRectangle,
             bool compand)
-            => source.ApplyProcessor(new ResizeProcessor(sampler, width, height, source.GetCurrentSize(), targetRectangle, compand));
+        {
+            var options = new ResizeOptions
+            {
+                Size = new Size(width, height),
+                Mode = ResizeMode.Manual,
+                Sampler = sampler,
+                TargetRectangle = targetRectangle,
+                Compand = compand
+            };
+
+            return Resize(source, options);
+        }
+
+        /// <summary>
+        /// Resizes an image in accordance with the given <see cref="ResizeOptions"/>.
+        /// </summary>
+        /// <param name="source">The image to resize.</param>
+        /// <param name="options">The resize options.</param>
+        /// <returns>The <see cref="IImageProcessingContext"/> to allow chaining of operations.</returns>
+        /// <remarks>Passing zero for one of height or width within the resize options will automatically preserve the aspect ratio of the original image or the nearest possible ratio.</remarks>
+        public static IImageProcessingContext Resize(this IImageProcessingContext source, ResizeOptions options)
+            => source.ApplyProcessor(new ResizeProcessor(options, source.GetCurrentSize()));
     }
-}
+}

src/ImageSharp/Processing/Processors/Transforms/Resize/ResizeHelper.cs

@@ -36,6 +36,11 @@ public static (Size, Rectangle) CalculateTargetLocationAndBounds(Size sourceSize
             int width = options.Size.Width;
             int height = options.Size.Height;
 
+            if (width <= 0 && height <= 0)
+            {
+                ThrowInvalid($"Target width {width} and height {height} must be greater than zero.");
+            }
+
             // Ensure target size is populated across both dimensions.
             // These dimensions are used to calculate the final dimensions determined by the mode algorithm.
             // If only one of the incoming dimensions is 0, it will be modified here to maintain aspect ratio.
@@ -51,9 +56,6 @@ public static (Size, Rectangle) CalculateTargetLocationAndBounds(Size sourceSize
                 height = (int)MathF.Max(Min, MathF.Round(sourceSize.Height * width / (float)sourceSize.Width));
             }
 
-            Guard.MustBeGreaterThan(width, 0, nameof(width));
-            Guard.MustBeGreaterThan(height, 0, nameof(height));
-
             switch (options.Mode)
             {
                 case ResizeMode.Crop:
@@ -66,8 +68,10 @@ public static (Size, Rectangle) CalculateTargetLocationAndBounds(Size sourceSize
                     return CalculateMaxRectangle(sourceSize, width, height);
                 case ResizeMode.Min:
                     return CalculateMinRectangle(sourceSize, width, height);
+                case ResizeMode.Manual:
+                    return CalculateManualRectangle(options, width, height);
 
-                // Last case ResizeMode.Stretch:
+                // case ResizeMode.Stretch:
                 default:
                     return (new Size(width, height), new Rectangle(0, 0, width, height));
             }
@@ -397,5 +401,28 @@ private static (Size, Rectangle) CalculatePadRectangle(
             // Target image width and height can be different to the rectangle width and height.
             return (new Size(width, height), new Rectangle(targetX, targetY, targetWidth, targetHeight));
         }
+
+        private static (Size, Rectangle) CalculateManualRectangle(
+            ResizeOptions options,
+            int width,
+            int height)
+        {
+            if (!options.TargetRectangle.HasValue)
+            {
+                ThrowInvalid("Manual resizing requires a target location and size.");
+            }
+
+            Rectangle targetRectangle = options.TargetRectangle.Value;
+
+            int targetX = targetRectangle.X;
+            int targetY = targetRectangle.Y;
+            int targetWidth = targetRectangle.Width > 0 ? targetRectangle.Width : width;
+            int targetHeight = targetRectangle.Height > 0 ? targetRectangle.Height : height;
+
+            // Target image width and height can be different to the rectangle width and height.
+            return (new Size(width, height), new Rectangle(targetX, targetY, targetWidth, targetHeight));
+        }
+
+        private static void ThrowInvalid(string message) => throw new InvalidOperationException(message);
     }
 }

src/ImageSharp/Processing/Processors/Transforms/Resize/ResizeProcessor.cs

@@ -13,45 +13,6 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
     /// </summary>
     public class ResizeProcessor : IImageProcessor
     {
-        /// <summary>
-        /// Initializes a new instance of the <see cref="ResizeProcessor"/> class.
-        /// </summary>
-        /// <param name="sampler">The <see cref="IResampler"/>.</param>
-        /// <param name="width">The width.</param>
-        /// <param name="height">The height.</param>
-        /// <param name="sourceSize">The size of the source image.</param>
-        /// <param name="targetRectangle">The target rectangle to resize into.</param>
-        /// <param name="compand">A value indicating whether to apply RGBA companding.</param>
-        public ResizeProcessor(IResampler sampler, int width, int height, Size sourceSize, Rectangle targetRectangle, bool compand)
-        {
-            Guard.NotNull(sampler, nameof(sampler));
-
-            // Ensure target size is populated across both dimensions.
-            // If only one of the incoming dimensions is 0, it will be modified here to maintain aspect ratio.
-            // If it is not possible to keep aspect ratio, make sure at least the minimum is is kept.
-            const int Min = 1;
-            if (width == 0 && height > 0)
-            {
-                width = (int)MathF.Max(Min, MathF.Round(sourceSize.Width * height / (float)sourceSize.Height));
-                targetRectangle.Width = width;
-            }
-
-            if (height == 0 && width > 0)
-            {
-                height = (int)MathF.Max(Min, MathF.Round(sourceSize.Height * width / (float)sourceSize.Width));
-                targetRectangle.Height = height;
-            }
-
-            Guard.MustBeGreaterThan(width, 0, nameof(width));
-            Guard.MustBeGreaterThan(height, 0, nameof(height));
-
-            this.Sampler = sampler;
-            this.TargetWidth = width;
-            this.TargetHeight = height;
-            this.TargetRectangle = targetRectangle;
-            this.Compand = compand;
-        }
-
         /// <summary>
         /// Initializes a new instance of the <see cref="ResizeProcessor"/> class.
         /// </summary>
@@ -71,18 +32,6 @@ public ResizeProcessor(ResizeOptions options, Size sourceSize)
             this.Compand = options.Compand;
         }
 
-        /// <summary>
-        /// Initializes a new instance of the <see cref="ResizeProcessor"/> class.
-        /// </summary>
-        /// <param name="sampler">The sampler to perform the resize operation.</param>
-        /// <param name="width">The target width.</param>
-        /// <param name="height">The target height.</param>
-        /// <param name="sourceSize">The source image size</param>
-        public ResizeProcessor(IResampler sampler, int width, int height, Size sourceSize)
-            : this(sampler, width, height, sourceSize, new Rectangle(0, 0, width, height), false)
-        {
-        }
-
         /// <summary>
         /// Gets the sampler to perform the resize operation.
         /// </summary>

src/ImageSharp/Processing/ResizeMode.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 namespace SixLabors.ImageSharp.Processing
@@ -42,6 +42,11 @@ public enum ResizeMode
         /// <summary>
         /// Stretches the resized image to fit the bounds of its container.
         /// </summary>
-        Stretch
+        Stretch,
+
+        /// <summary>
+        /// The target location and size of the resized image has been manually set.
+        /// </summary>
+        Manual
     }
 }

src/ImageSharp/Processing/ResizeOptions.cs

@@ -41,5 +41,10 @@ public class ResizeOptions
         /// or expand individual pixel colors the value on processing.
         /// </summary>
         public bool Compand { get; set; } = false;
+
+        /// <summary>
+        /// Gets or sets the target rectangle to resize into.
+        /// </summary>
+        public Rectangle? TargetRectangle { get; set; }
     }
 }