Negative height rectangles cause ArgumentOutOfRangeException for property MaxDegreeOfParallelism in IterateRows

[atruskie]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of ImageSharp
• I have verified if the problem exist in both DEBUG and RELEASE mode
• I have searched open and closed issues to ensure it has not already been reported

Description

I made a mistake when cloning and cropping an image.

The problem is that the exception thrown was quite obscure. The error was related to MaxDegreeOfParallelism being invalid. Here is the stack trace:

SixLabors.ImageSharp.ImageProcessingException: An error occurred when processing the image using DrawImageProcessor`2. See the inner exception for more detail. ---> SixLabors.ImageSharp.ImageProcessingException: An error occurred when processing the image using DrawImageProcessor`2. See the inner exception for more detail. ---> System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values.
Parameter name: MaxDegreeOfParallelism
   at System.Threading.Tasks.ParallelOptions.set_MaxDegreeOfParallelism(Int32 value)
   at SixLabors.ImageSharp.ParallelUtils.ParallelHelper.IterateRows(Rectangle rectangle, ParallelExecutionSettings& parallelSettings, Action`1 body)
   at SixLabors.ImageSharp.Processing.Processors.Drawing.DrawImageProcessor`2.OnFrameApply(ImageFrame`1 source, Rectangle sourceRectangle, Configuration configuration)
   at SixLabors.ImageSharp.Processing.Processors.ImageProcessor`1.Apply(ImageFrame`1 source, Rectangle sourceRectangle, Configuration configuration)
   --- End of inner exception stack trace ---
   at SixLabors.ImageSharp.Processing.Processors.ImageProcessor`1.Apply(ImageFrame`1 source, Rectangle sourceRectangle, Configuration configuration)
   at SixLabors.ImageSharp.Processing.Processors.ImageProcessor`1.Apply(Image`1 source, Rectangle sourceRectangle)
   --- End of inner exception stack trace ---
   at SixLabors.ImageSharp.Processing.Processors.ImageProcessor`1.Apply(Image`1 source, Rectangle sourceRectangle)
   at SixLabors.ImageSharp.Processing.DefaultInternalImageProcessorContext`1.ApplyProcessor(IImageProcessor`1 processor, Rectangle rectangle)
   at SixLabors.ImageSharp.Processing.DefaultInternalImageProcessorContext`1.ApplyProcessor(IImageProcessor`1 processor)
   at SixLabors.ImageSharp.Processing.DrawImageExtensions.DrawImage[TPixelDst,TPixelSrc](IImageProcessingContext`1 source, Image`1 image, Point location, GraphicsOptions options)

I've tracked down the root cause to:

ImageSharp/src/ImageSharp/Common/ParallelUtils/ParallelHelper.cs

Lines 50 to 64 in 2849fd9

	int maxSteps = DivideCeil(rectangle.Width * rectangle.Height, parallelSettings.MinimumPixelsProcessedPerTask);
	int numOfSteps = Math.Min(parallelSettings.MaxDegreeOfParallelism, maxSteps);
	// Avoid TPL overhead in this trivial case:
	if (numOfSteps == 1)
	{
	var rows = new RowInterval(rectangle.Top, rectangle.Bottom);
	body(rows);
	return;
	}
	int verticalStep = DivideCeil(rectangle.Height, numOfSteps);
	var parallelOptions = new ParallelOptions() { MaxDegreeOfParallelism = numOfSteps };

If the rectangle height is negative, then maxSteps will be negative, which will then set the MaxDegreeOfParallelism to a negative value.

Steps to Reproduce

It took me ages to work out what happened! I got my top and left variables back to front.

My crop was:

image.Mutate(x => x.DrawImage(source, new Point(top, left), GraphicsOptions.Default));

And should have been:

image.Mutate(x => x.DrawImage(source, new Point(left, top), GraphicsOptions.Default));

System Configuration

• ImageSharp version: (local build) 2849fd9
• Other ImageSharp packages and versions: (local build) 2849fd9
• Environment (Operating system, version and so on): Win 10, 1809
• .NET Framework version: ".NETFramework,Version=v4.6.2"
• Additional information:

I think the ideal solution here is a couple more guards to provide a better error message when someone (like me) does something silly and messes up their coordinates.

[JimBobSquarePants]

Well discovered!

This is the kind of simple fix that would be perfect for a first time PR if you're up for that.

[atruskie]

Yeah, I'll give it a go.