Ico Image Encoder

IcoEncoder.cs

using System;
using System.Linq;
using System.IO;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Processing;
using SixLabors.ImageSharp.PixelFormats;
using SixLabors.ImageSharp.Advanced;

namespace EQL.IconAutosizer.ImageEncoders
{
    // https://en.wikipedia.org/wiki/ICO_%28file_format%29
    // https://fileformats.fandom.com/wiki/Icon

    public static class IcoEncoder
    {
        private static void ResizeImageToPngBinaryWriter(BinaryWriter binaryWriter, Image<Bgra32> image, int size, out int imageSize)
        {
            using (var sizedImage = image.Clone(i => i.Resize(size, size)))
            {
                var position = binaryWriter.BaseStream.Position;
                sizedImage.SaveAsPng(binaryWriter.BaseStream);
                imageSize = (int)(binaryWriter.BaseStream.Position - position);
            }
        }

        private static void ResizeImageToBmpBinaryWriter(BinaryWriter binaryWriter, Image<Bgra32> image, int size, out int imageSize)
        {
            using (var sizedImage = image.Clone(i => i.Resize(size, size).Flip(FlipMode.Vertical)))
            {
                var position = binaryWriter.BaseStream.Position;
                var pixelData = sizedImage.GetPixelSpan().ToArray();
                binaryWriter.Write((uint)40);
                binaryWriter.Write(size);
                binaryWriter.Write(size * 2);
                binaryWriter.Write((ushort)1);
                binaryWriter.Write((ushort)32);
                binaryWriter.Write((uint)0);
                binaryWriter.Write((uint)((size * size) * 5));
                binaryWriter.Write(0);
                binaryWriter.Write(0);
                binaryWriter.Write((uint)0);
                binaryWriter.Write((uint)0);
                foreach (var pixel in pixelData)
                {
                    binaryWriter.Write(pixel.B);
                    binaryWriter.Write(pixel.G);
                    binaryWriter.Write(pixel.R);
                    binaryWriter.Write(pixel.A);
                }
                binaryWriter.Write(new byte[size * size]);
                imageSize = (int)(binaryWriter.BaseStream.Position - position);
            }
        }


        public static void Encode(string filePath, Image<Bgra32> image)
        {
            using (var fileStream = File.Create(filePath))
            {
                Encode(fileStream, image);
            }
        }

        private static int[] GetIconSizes()
        {
            return new int[] { 256, 48, 32, 16 };
        }

        public static void Encode(Stream stream, Image<Bgra32> image)
        {
            if (!stream.CanSeek)
            {
                throw new Exception("Seekable stream is required");
            }

            if (image.Width != image.Height || image.Width < 16)
            {
                throw new Exception("Image should be square and at least 16px in size");
            }

            var iconSizes = GetIconSizes().Where(i => i < image.Width).ToArray();

            using (var binaryWriter = new BinaryWriter(stream))
            {
                // Write IconDir
                binaryWriter.Write((ushort)0);
                binaryWriter.Write((ushort)1);
                binaryWriter.Write((ushort)iconSizes.Length);

                const int iconDirEntrySize = 16;

                // Write IconDirEntries
                foreach (var iconSize in iconSizes)
                {
                    binaryWriter.Write((byte)(iconSize == 256 ? 0 : iconSize));
                    binaryWriter.Write((byte)(iconSize == 256 ? 0 : iconSize));
                    binaryWriter.Write((byte)0);
                    binaryWriter.Write((byte)0);
                    binaryWriter.Write((ushort)0);
                    binaryWriter.Write((ushort)32);
                    binaryWriter.Write((uint)0);
                    binaryWriter.Write((uint)0);
                }

                var imageSize = 0;

                // Write image data and update IconDirEntries
                var imageDataOffset = 6 + (iconSizes.Length * iconDirEntrySize);
                for (var i = 0; i < iconSizes.Length; i++)
                {
                    stream.Seek(0, SeekOrigin.End);

                    var iconSize = iconSizes[i];
                    switch (iconSize)
                    {
                        case 256:
                            ResizeImageToPngBinaryWriter(binaryWriter, image, iconSize, out imageSize);
                            break;
                        default:
                            ResizeImageToBmpBinaryWriter(binaryWriter, image, iconSize, out imageSize);
                            break;
                    }

                    stream.Position = 6 + (i * iconDirEntrySize) + 8;
                    binaryWriter.Write((uint)imageSize);
                    binaryWriter.Write((uint)imageDataOffset);
                    imageDataOffset += imageSize;
                }

                binaryWriter.Flush();
            }
        }
    }
}