Convert Label Studio annotations from rectangle to polygon

rectangle-to-polygon.linq

void Main()
{
    // label studio exported json
    var json = File.ReadAllText(@"C:\project-2-at-2022-11-29-16-57-b274c21b.json");
    var jArray = JsonConvert.DeserializeObject<JArray>(json);

    foreach (var obj in jArray)
    {
        foreach (var ann in obj["annotations"])
        {
            foreach (var res in ann["result"])
            {
                // no need to convert
                if (res["type"].ToString() == "polygonlabels")
                {
                    continue;
                }

                var value = res["value"];

                var (x, y, w, h, rotation) = GetRectCoords(value);

                res["type"] = "polygonlabels";
                res["from_name"] = "label_polys";

                var newValue = new
                {
                    polygonlabels = new[] { value["rectanglelabels"][0] },
                    points = new[]
                    {
                        new[] { x, y },
                        new[] { x+w, y },
                        new[] {(x+w), (y+h) },
                        new[] { x, y+h },
                    }
                };

                res["value"] = JToken.FromObject(newValue);
                //res.Dump();
            }
        }
    }

    File.WriteAllText(@"c:\Ml\poly-convert.json", JsonConvert.SerializeObject(jArray, Newtonsoft.Json.Formatting.Indented));
}

// You can define other methods, fields, classes and namespaces here
// Ported from label-studio-converter/converter.py 'def rotated_rectangle(label)'
private (double label_x, double label_y, double label_w, double label_h, double rotation) GetRectCoords(JToken value)
{
    var label_x = (value["x"].Value<double>());
    var label_y = (value["y"].Value<double>());
    var label_w = (value["width"].Value<double>());
    var label_h = (value["height"].Value<double>());
    var rotation = value["rotation"].Value<float>();

    if (Math.Abs(rotation) > 0)
    {
        var alpha = Math.Atan(label_h / label_w);
        var beta = Math.PI * (rotation / 180);  // Label studio defines the angle towards the vertical axis

        var radius = Math.Sqrt(Math.Pow((label_w / 2), 2) + Math.Pow((label_h / 2), 2));

        //Label studio saves the position of top left corner after rotation
        var x_0 = label_x - radius * (Math.Cos(Math.PI - alpha - beta) - Math.Cos(Math.PI - alpha)) + label_w / 2;

        var y_0 = label_y + radius * (Math.Sin(Math.PI - alpha - beta) - Math.Sin(Math.PI - alpha)) + label_h / 2;


        var theta_1 = alpha + beta;
        var theta_2 = Math.PI - alpha + beta;
        var theta_3 = Math.PI + alpha + beta;
        var theta_4 = 2 * Math.PI - alpha + beta;

        var x_coord = new[]
        {
            x_0 + radius * Math.Cos(theta_1),
            x_0 + radius * Math.Cos(theta_2),
            x_0 + radius * Math.Cos(theta_3),
            x_0 + radius * Math.Cos(theta_4)
        };

        var y_coord = new[]
        {
            y_0 + radius * Math.Sin(theta_1),
            y_0 + radius * Math.Sin(theta_2),
            y_0 + radius * Math.Sin(theta_3),
            y_0 + radius * Math.Sin(theta_4)
        };


        label_x = x_coord.Min();
        label_y = y_coord.Min();
        label_w = x_coord.Max() - label_x;
        label_h = y_coord.Max() - label_y;
    }

    return (label_x, label_y, label_w, label_h, rotation);
}