No longer throws when DrawImage source does not overlap target

Previously, when DrawImage was used to overlay an image, in cases where the source image did not overlap the target image, a very confusing error was reported: "System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values.
Parameter name: MaxDegreeOfParallelism"

Now, when this case happens, the DrawImage method will simply not affect the target image, which is the same way FillRegionProcessor handles such cases.

ParallelHelper.IterRows also now does more validation of the input rectangle so that any further cases of this kind of problem throw a more relvant exception. Note I switched from DebugGuard to Guard because IterRows is a public API and thus should always validate its inputs.

Fixes SixLabors#875

src/ImageSharp.Drawing/Processing/Processors/Drawing/DrawImageProcessor.cs

@@ -81,6 +81,12 @@ protected override void OnFrameApply(ImageFrame<TPixelDst> source, Rectangle sou
 
             var workingRect = Rectangle.FromLTRB(minX, minY, maxX, maxY);
 
+            if (workingRect.Width < 0 || workingRect.Height < 0)
+            {
+                // no effect because rectangle does not overlap with this image.
+                return;
+            }
+
             ParallelHelper.IterateRows(
                 workingRect,
                 configuration,

src/ImageSharp/Common/ParallelUtils/ParallelHelper.cs

@@ -45,7 +45,8 @@ public static void IterateRows(
             in ParallelExecutionSettings parallelSettings,
             Action<RowInterval> body)
         {
-            DebugGuard.MustBeGreaterThan(rectangle.Width, 0, nameof(rectangle));
+            Guard.MustBeGreaterThan(rectangle.Width, 0, nameof(rectangle));
+            Guard.MustBeGreaterThan(rectangle.Height, 0, nameof(rectangle));
 
             int maxSteps = DivideCeil(rectangle.Width * rectangle.Height, parallelSettings.MinimumPixelsProcessedPerTask);
 

tests/ImageSharp.Tests/Drawing/DrawImageTest.cs

@@ -132,6 +132,26 @@ public void ImageShouldHandlePositiveLocation(TestImageProvider<Rgba32> provider
             }
         }
 
+        [Theory]
+        [WithSolidFilledImages(100, 100, 255, 255, 255, PixelTypes.Rgba32, -30, -30)]
+        [WithSolidFilledImages(100, 100, 255, 255, 255, PixelTypes.Rgba32, 130, -30)]
+        [WithSolidFilledImages(100, 100, 255, 255, 255, PixelTypes.Rgba32, 130, 130)]
+        [WithSolidFilledImages(100, 100, 255, 255, 255, PixelTypes.Rgba32, -30, 130)]
+        public void NonOverlappingImageHasNoEffect(TestImageProvider<Rgba32> provider, int x, int y)
+        {
+            using (Image<Rgba32> original = provider.GetImage())
+            using (Image<Rgba32> background = provider.GetImage())
+            using (var overlay = new Image<Rgba32>(Configuration.Default, 10, 10, Rgba32.Black))
+            {
+                background.Mutate(context => context.DrawImage(overlay, new Point(x, y), GraphicsOptions.Default));
+
+                // background image should be unmodified
+                ImageComparer.Exact.CompareImagesOrFrames(original, background);
+
+                background.DebugSave(provider, testOutputDetails: "NonOverlapping");
+            }
+        }
+
         private static void VerifyImage<TPixel>(
             TestImageProvider<TPixel> provider,
             PixelColorBlendingMode mode,

tests/ImageSharp.Tests/Helpers/ParallelHelperTests.cs

@@ -334,5 +334,20 @@ void FillRow(int y, Buffer2D<Point> buffer)
                 TestImageExtensions.CompareBuffers(expected.Span, actual.Span);
             }
         }
+
+        [Theory]
+        [InlineData(0, 10)]
+        [InlineData(10, 0)]
+        [InlineData(-10, 10)]
+        [InlineData(10, -10)]
+        public void IterateRowsRequiresValidRectangle(int width, int height)
+        {
+            var parallelSettings = new ParallelExecutionSettings();
+
+            var rect = new Rectangle(0, 0, width, height);
+
+            Assert.Throws<ArgumentOutOfRangeException>(
+                () => { ParallelHelper.IterateRows(rect, parallelSettings, (rows) => { }); });
+        }
     }
 }