RichTextRun.End behaves as an inclusive index only when it is the same as the end of the string

[EbiseLutica]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of ImageSharp.Drawing
• 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

According to the API documentation, RichTextRun.End is recognized as an exclusive end index, meaning the character at this position should not be decorated.

However, when RichTextRun.End is set to length - 1 (the same as the last index of the string), the RichTextRun decorates the entire string. Adding one more character at the end causes it to behave as expected.

Steps to Reproduce

Below is the sample code to reproduce:

using SixLabors.Fonts;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Drawing.Processing;
using SixLabors.ImageSharp.PixelFormats;
using SixLabors.ImageSharp.Processing;

var img = new Image<Rgba32>(150, 150);
RichTextOptions GenerateOption(PointF origin) => new(SystemFonts.Get("Arial").CreateFont(24))
{
    Origin = origin,
    TextRuns = new[]
    {
        new RichTextRun { Start = 0, End = 4, Brush = new SolidBrush(Color.Red) }
    }
};

img.Mutate(ctx =>
{
    ctx.Clear(Color.White);
    ctx.DrawText(GenerateOption(new PointF(10, 10)), "abcd", new SolidBrush(Color.Black));
    ctx.DrawText(GenerateOption(new PointF(10, 30)), "abcde", new SolidBrush(Color.Black));
    ctx.DrawText(GenerateOption(new PointF(10, 50)), "abcdef", new SolidBrush(Color.Black));
});

img.SaveAsPng("output.png");

and below is the output file.

System Configuration

• ImageSharp.Drawing version: 2.1.4
• Other ImageSharp packages and versions: SixLabors.ImageSharp 3.1.5, SixLabors.Fonts 2.0.4
• Environment (Operating system, version and so on): Windows 11 Home Build 22621
• .NET Framework version: .NET 8.0.107
• Additional information:

[JimBobSquarePants]

Thanks for this. The underlying issue will be in the Fonts library, so I'll transfer this issue to that repository.