Draw text in an ArcLineSegment counter-clockwise

[wizche]

I'm trying to replicate this: (drawing circle and text in the lower arc)

With this code:

        public void Draw(string text)
        {
            var imageSize = 300;
            var fontSize = 18;
            var border = 10;
            var color = Color.ParseHex("#808080");
            FontFamily family = SystemFonts.Get("Arial");
            Font font = new(family, fontSize);
            var radius = imageSize / 2;
            var center = new PointF(radius, radius);

            var img = Image<Rgba32>.Load(@"C:\Users\sergi\Downloads\comet.png");
            img.Mutate(o => o.Resize(new Size(imageSize, imageSize)));
            var topSegment = new ArcLineSegment(center, new SizeF(radius-border, radius-border), 0, -220, 260);
            PathBuilder pathBuilder = new PathBuilder();
            pathBuilder.AddSegment(topSegment);

            var textSegment = new ArcLineSegment(center, new SizeF(radius- fontSize/2 - border, radius- fontSize/2-border), 0, -220, -100);
            IPath textShape = new Polygon(textSegment);
            TextOptions textOptions = new(font)
            {
                WrappingLength = textShape.ComputeLength(),
                VerticalAlignment = VerticalAlignment.Top,
                HorizontalAlignment = HorizontalAlignment.Left,
                TextAlignment = TextAlignment.Center,
                TextDirection = TextDirection.LeftToRight
                
            };
            DrawingOptions options = new()
            {
                GraphicsOptions = new()
                {
                    ColorBlendingMode = PixelColorBlendingMode.Multiply
                }
            };
            IPen pen = Pens.Solid(color, 1);
            img.Mutate(x => x.Draw(color, 2, pathBuilder.Build()));
            img.Mutate(x => x.Draw(color, 2, new PathBuilder().AddSegment(textSegment).Build()));

            IPathCollection glyphs = TextBuilder.GenerateGlyphs(text, textShape, textOptions);
            img.Mutate(i => i.Fill(color, glyphs));
            string fullPath = IOPath.GetFullPath(IOPath.Combine("Output", IOPath.Combine(
                "test.png")));
            IODirectory.CreateDirectory(IOPath.GetDirectoryName(fullPath));
            img.Save(fullPath);
            Console.WriteLine($"Saved to {fullPath}");
        }

I get to this result:

My questions are:

• How do I make the text use the whole space so that the center aligment will actually put the text in center?
• Bonus point, if the text is too long the GenerateGlyphs will fail with 'Should always reach a point along the path.'. Is there a way to scale the text size to fit the arc automatically if the text is too long?

EDIT: I experimented with different segments, I have the same issue with a simpler line segment:

var textSegment = new LinearLineSegment(new PointF(50, 150), new PointF(250, 150));
IPath textShape = new Polygon(textSegment);
TextOptions textOptions = new(font)
{
    //WrappingLength = textShape.ComputeLength(),
    VerticalAlignment = VerticalAlignment.Top,
    HorizontalAlignment = HorizontalAlignment.Center,
    TextAlignment = TextAlignment.Center,
    
};

Similarly TextAligment has no effect and having HorizontalAligment.Center will cause the text to be aligned in the middle of the segment start point:

What is also strange is that the textShape.ComputeLength() returns 400 in the above example, whereas the segment is only 200 long.

Thanks a lot

[wizche]

I received a helpful answer on Stack Overflow after placing a bounty on my question. For reference purposes, I would like to share it here.
https://stackoverflow.com/questions/75768993/draw-text-in-an-arclinesegment-counter-clockwise-with-imagesharp

public void Draw(string text)
{
    var imageSize = 300;
    var fontSize = 18;
    var border = 10;
    var color = Color.ParseHex("#808080");
    FontFamily family = SystemFonts.Get("Arial");
    Font font = new(family, fontSize);
    var radius = imageSize / 2;
    var center = new PointF(radius, radius);

    var img = Image<Rgba32>.Load(@"C:\tmp\comet.png");
    img.Mutate(o => o.Resize(new Size(imageSize, imageSize)));
    var topSegment = new ArcLineSegment(center, new SizeF(radius - border, radius - border), 0, -220, 260);
    PathBuilder pathBuilder = new PathBuilder();
    pathBuilder.AddSegment(topSegment);

    var bottomSegment = new ArcLineSegment(center, new SizeF(radius - fontSize / 2 - border, radius - fontSize / 2 - border), 0, -220, -100);

    // Find the smallest curve encompassing the whole text
    var textSweepAngle = FindLowestSweepAngle(text, font, new SizeF(radius - fontSize / 2 - border, radius - fontSize / 2 - border));

    var fullSweepAngle = 100;
    var gapSweepAngle = (fullSweepAngle - textSweepAngle) / 2;
    var textStartAngle = -220 - gapSweepAngle;

    var textSegment = new ArcLineSegment(center, new SizeF(radius - fontSize / 2 - border, radius - fontSize / 2 - border), 0, textStartAngle, -textSweepAngle);

    IPath textShape = new Polygon(textSegment);
    TextOptions textOptions = new(font)
    {
        WrappingLength = textShape.ComputeLength(),
        VerticalAlignment = VerticalAlignment.Top,
        HorizontalAlignment = HorizontalAlignment.Left,
        TextAlignment = TextAlignment.Start,
        TextDirection = TextDirection.LeftToRight
    };
    DrawingOptions options = new()
    {
        GraphicsOptions = new()
        {
            ColorBlendingMode = PixelColorBlendingMode.Multiply
        }
    };
    IPen pen = Pens.Solid(color, 1);
    img.Mutate(x => x.Draw(color, 2, pathBuilder.Build()));
    img.Mutate(x => x.Draw(color, 2, new PathBuilder().AddSegment(bottomSegment).Build()));

    IPathCollection glyphs = TextBuilder.GenerateGlyphs(text, textShape, textOptions);
    img.Mutate(i => i.Fill(color, glyphs));
    string fullPath = IOPath.GetFullPath(IOPath.Combine("Output", IOPath.Combine("test.png")));
    IODirectory.CreateDirectory(IOPath.GetDirectoryName(fullPath));
    img.Save(fullPath);
    Console.WriteLine($"Saved to {fullPath}");
}

private float FindLowestSweepAngle(string text, Font font, SizeF radius)
{
    var low = 0.1f;
    var high = 359.9f;
    var step = 0.1f;

    while (low < high)
    {
        var mid = (low + high) / 2;
        var arcLineSegment = new ArcLineSegment(PointF.Empty, radius, 0, 0, mid);
        var polygon = new Polygon(arcLineSegment);

        TextOptions textOptions = new(font)
        {
            WrappingLength = polygon.ComputeLength(),
            VerticalAlignment = VerticalAlignment.Top,
            HorizontalAlignment = HorizontalAlignment.Left,
            TextAlignment = TextAlignment.Start,
            TextDirection = TextDirection.LeftToRight
        };

        try
        {
            TextBuilder.GenerateGlyphs(text, polygon, textOptions);
            high = mid - step; // Curve is too big. Keep finding a smaller one
        }
        catch // InvalidOperationException: Should always reach a point along the path
        {
            low = mid + step; // Curve is too small to hold the whole text
        }
    }

    return low;
}