Game & Watch splash converter

GameAndWatchBackdropToGif.cs

using System;
using System.IO;

namespace GameAndWatchBackdropToGif
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length != 2)
            {
                Console.WriteLine("Usage: GameAndWatchBackdropToGif <decrypted_flash_path> <output_path>");
                return;
            }

            int[] offsets = new int[]
            {
                0xcd858,
                0xc58f8,
                0xec318,
                0xe16f8,
                0xd6c78
            };

            using FileStream ifs = File.OpenRead(args[0]);
            BinaryReader br = new BinaryReader(ifs);
            Directory.CreateDirectory(args[1]);
            for (int i = 0; i < offsets.Length; ++i)
            {
                ifs.Seek(offsets[i], SeekOrigin.Begin);
                Convert(br, Path.Combine(args[1], i + ".gif"));
            }
        }

        static void Rgb565ToRgba32(ushort pix, out byte r, out byte g, out byte b)
        {
            r = (byte)(((pix >> 11) * 255 + 15) / 31);
            g = (byte)((((pix >> 5) & 0x3f) * 255 + 31) / 63);
            b = (byte)(((pix & 0x1f) * 255 + 15) / 31);
        }

        static void Convert(BinaryReader br, string path)
        {
            using FileStream ofs = File.Create(path);
            BinaryWriter bw = new BinaryWriter(ofs);

            // Header
            bw.Write("GIF".ToCharArray());
            bw.Write("89a".ToCharArray());

            // Logical screen descriptor
            ushort width = br.ReadUInt16();
            ushort height = br.ReadUInt16();
            byte paletteSize = br.ReadByte();
            br.ReadByte(); // padding
            ushort[] palette = new ushort[paletteSize];
            for (int i = 0; i < palette.Length; ++i)
            {
                palette[i] = br.ReadUInt16();
            }

            int gctSize = 0;
            int calcGctSize = 2;
            while (calcGctSize < paletteSize)
            {
                ++gctSize;
                calcGctSize <<= 1;
            }

            bw.Write(width);
            bw.Write(height);
            bw.Write((byte)((1 << 7) | gctSize));
            bw.Write((byte)0);
            bw.Write((byte)0);

            // Global color table
            for (int i = 0; i < calcGctSize; ++i)
            {
                if (i < palette.Length)
                {
                    Rgb565ToRgba32(palette[i], out byte r, out byte g, out byte b);
                    bw.Write(r);
                    bw.Write(g);
                    bw.Write(b);
                }
                else
                {
                    bw.Write((byte)0);
                    bw.Write((byte)0);
                    bw.Write((byte)0);
                }
            }

            // Image descriptor
            bw.Write((byte)0x2c);
            bw.Write((ushort)0); // x
            bw.Write((ushort)0); // y
            bw.Write(width);
            bw.Write(height);
            bw.Write((byte)0); // no flags

            // Frame
            byte minCodeSize = br.ReadByte();
            bw.Write(minCodeSize);

            while (true)
            {
                byte blockSize = br.ReadByte();
                bw.Write(blockSize);
                if (blockSize == 0) break;
                byte[] block = br.ReadBytes(blockSize);
                bw.Write(block);
            }

            byte trailer = br.ReadByte();
            if (trailer != 0x3b) throw new InvalidDataException("Invalid GIF trailer");
            bw.Write(trailer);
        }
    }
}