Game & Watch Mario Drawing Song animation converter

DrawingSongConv.cs

using System;
using System.IO;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Formats.Gif;
using SixLabors.ImageSharp.IO;
using SixLabors.ImageSharp.Memory;

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

            int dataOffset = 0x12d44;
            using BufferedReadStream ifs = new BufferedReadStream(new Configuration(), File.OpenRead(args[0]));
            using FileStream ofs = File.Create(args[1]);
            BinaryReader br = new BinaryReader(ifs);
            BinaryWriter bw = new BinaryWriter(ofs);
            ifs.Seek(dataOffset, SeekOrigin.Begin);

            // Header
            string signature = new string(br.ReadChars(3));
            if (signature != "GIF") throw new InvalidDataException("Not a GIF");
            bw.Write(signature.ToCharArray());
            string version = new string(br.ReadChars(3));
            if (version != "89a") throw new InvalidDataException("Not GIF 89a");
            bw.Write(version.ToCharArray());

            // Logical screen descriptor
            ushort width = br.ReadUInt16();
            ushort height = br.ReadUInt16();
            byte packed = br.ReadByte();
            byte bgIndex = br.ReadByte();
            byte pixAR = br.ReadByte();

            bw.Write(width);
            bw.Write(height);
            bw.Write(packed);
            bw.Write(bgIndex);
            bw.Write(pixAR);

            // Global color table
            if ((packed & 0x80) == 0) throw new InvalidDataException("No GCT present");
            int colorDepth = (packed & 3) + 1;
            int gctSize = 1 << ((packed & 3) + 1);
            byte[] gct = br.ReadBytes(gctSize * 3);
            bw.Write(gct);

            // Frames
            byte minCodeSize = br.ReadByte();
            int frame = 0;

            var memoryAllocator = new ArrayPoolMemoryAllocator();
            using Buffer2D<byte> buffer = memoryAllocator.Allocate2D<byte>(new Size(width, height));

            int peek;
            while ((peek = br.PeekChar()) >= 0 && peek != 0x3b)
            {
                // Decode frame
                // We want to also find a color to use for transparency
                bool[] isColorUsed = new bool[gctSize];
                using Buffer2D<byte> currFrame = memoryAllocator.Allocate2D<byte>(new Size(width, height));
                using (LzwDecoder decoder = new LzwDecoder(memoryAllocator, ifs))
                    decoder.DecodePixels(minCodeSize, currFrame);

                bool isIncremental = frame > 0;

                // Update current drawing buffer and flip color
                for (int y = 0; y < buffer.Height; ++y)
                    for (int x = 0; x < buffer.Width; ++x)
                    {
                        var pix = currFrame[x, y];
                        if (pix != 0)
                        {
                            pix ^= buffer[x, y];
                            currFrame[x, y] = pix;
                            buffer[x, y] = pix;
                            isColorUsed[pix] = true;
                        }
                        else
                        {
                            if (isIncremental) currFrame[x, y] = 0xff;
                        }
                    }

                // Find first transparent color
                byte transparentColor = 0xff;
                if (isIncremental)
                {
                    for (int i = 0; i < isColorUsed.Length; ++i)
                    {
                        if (!isColorUsed[i])
                        {
                            transparentColor = (byte)i;
                            break;
                        }
                    }
                    if (transparentColor == 0xff)
                        throw new Exception($"Cannot find unused color for transparency on frame {frame}");

                    // Set transparency color
                    for (int y = 0; y < buffer.Height; ++y)
                        for (int x = 0; x < buffer.Width; ++x)
                        {
                            if (currFrame[x, y] == 0xff)
                                currFrame[x, y] = transparentColor;
                        }
                }

                // Graphic control extension
                bw.Write((byte)0x21);
                bw.Write((byte)0xf9);
                bw.Write((byte)4);
                bw.Write((byte)((1 << 2) | (isIncremental ? 1 : 0))); // do not dispose, transparent index specified (if not first frame)
                bw.Write((ushort)(1f / 60 * 5 * 100)); // 60fps, flip every 5 frames
                bw.Write(transparentColor);
                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

                // Image data
                using LzwEncoder encoder = new LzwEncoder(memoryAllocator, colorDepth);
                encoder.Encode(currFrame, ofs);

                byte terminator = br.ReadByte();
                if (terminator != 0) throw new InvalidDataException($"Frame terminator has unexpected value, frame {frame}");
                ++frame;
            }

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

ImageSharp.patch

diff --git a/src/ImageSharp/Formats/Gif/LzwDecoder.cs b/src/ImageSharp/Formats/Gif/LzwDecoder.cs
index 9eaa55566..a0e397c92 100644
--- a/src/ImageSharp/Formats/Gif/LzwDecoder.cs
+++ b/src/ImageSharp/Formats/Gif/LzwDecoder.cs
@@ -13,7 +13,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
     /// <summary>
     /// Decompresses and decodes data using the dynamic LZW algorithms.
     /// </summary>
-    internal sealed class LzwDecoder : IDisposable
+    public sealed class LzwDecoder : IDisposable
     {
         /// <summary>
         /// The max decoder pixel stack size.
diff --git a/src/ImageSharp/Formats/Gif/LzwEncoder.cs b/src/ImageSharp/Formats/Gif/LzwEncoder.cs
index 195a84a1d..c20d0a7c7 100644
--- a/src/ImageSharp/Formats/Gif/LzwEncoder.cs
+++ b/src/ImageSharp/Formats/Gif/LzwEncoder.cs
@@ -34,7 +34,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
     ///              Joe Orost              (decvax!vax135!petsd!joe)
     /// </para>
     /// </remarks>
-    internal sealed class LzwEncoder : IDisposable
+    public sealed class LzwEncoder : IDisposable
     {
         /// <summary>
         /// 80% occupancy
diff --git a/src/ImageSharp/IO/BufferedReadStream.cs b/src/ImageSharp/IO/BufferedReadStream.cs
index acba3eff0..0c0b2ad91 100644
--- a/src/ImageSharp/IO/BufferedReadStream.cs
+++ b/src/ImageSharp/IO/BufferedReadStream.cs
@@ -12,7 +12,7 @@ namespace SixLabors.ImageSharp.IO
     /// A readonly stream that add a secondary level buffer in addition to native stream
     /// buffered reading to reduce the overhead of small incremental reads.
     /// </summary>
-    internal sealed class BufferedReadStream : Stream
+    public sealed class BufferedReadStream : Stream
     {
         private readonly int maxBufferIndex;