Add another test and make some methods public

src/ImageSharp/Processing/Transforms/TransformExtensions.cs

@@ -86,26 +86,25 @@ namespace SixLabors.ImageSharp.Processing.Transforms
         /// <param name="source">The image to transform.</param>
         /// <param name="matrix">The transformation matrix.</param>
         /// <returns>The <see cref="Image{TPixel}"/></returns>
-        internal static IImageProcessingContext<TPixel> Transform<TPixel>(this IImageProcessingContext<TPixel> source, Matrix4x4 matrix)
+        public static IImageProcessingContext<TPixel> Transform<TPixel>(this IImageProcessingContext<TPixel> source, Matrix4x4 matrix)
             where TPixel : struct, IPixel<TPixel>
             => Transform(source, matrix, KnownResamplers.Bicubic);
 
         /// <summary>
         /// Applies a projective transform to the image by the given matrix using the specified sampling algorithm.
-        /// TODO: Doesn't work yet! Implement tests + Finish implementation + Document Matrix4x4 behavior
         /// </summary>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image to transform.</param>
         /// <param name="matrix">The transformation matrix.</param>
         /// <param name="sampler">The <see cref="IResampler"/> to perform the resampling.</param>
         /// <returns>The <see cref="Image{TPixel}"/></returns>
-        internal static IImageProcessingContext<TPixel> Transform<TPixel>(this IImageProcessingContext<TPixel> source, Matrix4x4 matrix, IResampler sampler)
+        public static IImageProcessingContext<TPixel> Transform<TPixel>(this IImageProcessingContext<TPixel> source, Matrix4x4 matrix, IResampler sampler)
             where TPixel : struct, IPixel<TPixel>
             => source.ApplyProcessor(new ProjectiveTransformProcessor<TPixel>(matrix, sampler, source.GetCurrentSize()));
 
         /// <summary>
         /// Applies a projective transform to the image by the given matrix using the specified sampling algorithm.
-        /// TODO: Doesn't work yet! Implement tests + Finish implementation + Document Matrix4x4 behavior
+        /// TODO: Should we be offsetting the matrix here?
         /// </summary>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image to transform.</param>

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

@@ -61,6 +61,27 @@ namespace SixLabors.ImageSharp.Tests.Processing.Transforms
             }
         }
 
+        [Theory]
+        [WithSolidFilledImages(100, 100, 0, 0, 255, PixelTypes.Rgba32)]
+        public void RawTransformMatchesDocumentedExample<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            // This test matches the output described in the example at
+            // https://docs.microsoft.com/en-us/xamarin/xamarin-forms/user-interface/graphics/skiasharp/transforms/non-affine
+            using (Image<TPixel> image = provider.GetImage())
+            {
+                Matrix4x4 m = Matrix4x4.Identity;
+                m.M13 = 0.01F;
+
+                image.Mutate(i => { i.Transform(m); });
+
+                image.DebugSave(provider);
+
+                // TODO: Enable and add more tests.
+                // image.CompareToReferenceOutput(ValidatorComparer, provider, resamplerName);
+            }
+        }
+
         private static IResampler GetResampler(string name)
         {
             PropertyInfo property = typeof(KnownResamplers).GetTypeInfo().GetProperty(name);