UVtools/UVtools.Core/Extensions/DrawingExtensions.cs

using System;
using System.Drawing;

namespace UVtools.Core.Extensions
{
    public static class DrawingExtensions
    {
        public static Color FactorColor(this Color color, byte pixelColor, byte min = 0, byte max = byte.MaxValue) =>
            FactorColor(color, pixelColor / 255f, min, max);

        public static Color FactorColor(this Color color, float factor, byte min = 0, byte max = byte.MaxValue)
        {
            byte r = (byte)(color.R == 0 ? 0 :
                Math.Min(Math.Max(min, color.R * factor), max));

            byte g = (byte)(color.G == 0 ? 0 :
                Math.Min(Math.Max(min, color.G * factor), max));
            
            byte b = (byte)(color.B == 0 ? 0 :
                Math.Min(Math.Max(min, color.B * factor), max));
            return Color.FromArgb(r, g, b);
        }

        public static double CalculatePolygonSideLengthFromRadius(double radius, int sides)
        {
            return 2 * radius * Math.Sin(Math.PI / sides);
        }

        public static double CalculatePolygonVerticalLengthFromRadius(double radius, int sides)
        {
            return radius * Math.Cos(Math.PI / sides);
        }

        public static double CalculatePolygonRadiusFromSideLength(double length, int sides)
        {
            var theta = 360.0 / sides;
            return length / (2 * Math.Cos((90 - theta / 2) * Math.PI / 180.0));
        }

        public static Point[] GetPolygonVertices(int sides, int radius, Point center, double startingAngle = 0, bool flipHorizontally = false, bool flipVertically = false)
        {
            if (sides < 3)
                throw new ArgumentException("Polygons can't have less than 3 sides...", nameof(sides));

            var vertices = new Point[sides];

            double deg = 360.0 / sides;//calculate the rotation angle
            var rad = Math.PI / 180.0;

            var x0 = center.X + radius * Math.Cos(-(((180 - deg) / 2) + startingAngle) * rad);
            var y0 = center.Y - radius * Math.Sin(-(((180 - deg) / 2) + startingAngle) * rad);

            var x1 = center.X + radius * Math.Cos(-(((180 - deg) / 2) + deg + startingAngle) * rad);
            var y1 = center.Y - radius * Math.Sin(-(((180 - deg) / 2) + deg + startingAngle) * rad);

            vertices[0] = new(
                (int) Math.Round(x0),
                (int) Math.Round(y0)
                );

            vertices[1] = new(
                (int) Math.Round(x1),
                (int) Math.Round(y1)
                );

            for (int i = 0; i < sides - 2; i++)
            {
                double dsinrot = Math.Sin((deg * (i + 1)) * rad);
                double dcosrot = Math.Cos((deg * (i + 1)) * rad);

                vertices[i + 2] = new(
                        (int)Math.Round(center.X + dcosrot * (x1 - center.X) - dsinrot * (y1 - center.Y)),
                        (int)Math.Round(center.Y + dsinrot * (x1 - center.X) + dcosrot * (y1 - center.Y))
                );
            }

            if (flipHorizontally)
            {
                var startX = center.X - radius;
                var endX = center.X + radius;
                for (int i = 0; i < sides; i++)
                {
                    vertices[i].X = endX - (vertices[i].X - startX);
                }
            }

            if (flipVertically)
            {
                var startY = center.Y - radius;
                var endY = center.Y + radius;
                for (int i = 0; i < sides; i++)
                {
                    vertices[i].Y = endY - (vertices[i].Y - startY);
                }
            }

            return vertices;
        }
    }
}