COM3D2.MeidoPhotoStudio/src/MeidoPhotoStudio.Converter/Utility/PngUtility.cs

using System;
using System.IO;

namespace MeidoPhotoStudio.Converter.Utility
{
    internal static class PngUtility
    {
        private static readonly byte[] PngHeader = { 137, 80, 78, 71, 13, 10, 26, 10 };
        private static readonly byte[] PngEnd = System.Text.Encoding.ASCII.GetBytes("IEND");

        public static byte[]? ExtractPng(Stream stream)
        {
            var memoryStream = new MemoryStream();

            var headerBuffer = new byte[PngHeader.Length];

            stream.Read(headerBuffer, 0, headerBuffer.Length);

            if (!MeidoPhotoStudio.Plugin.Utility.BytesEqual(headerBuffer, PngHeader))
                return null;

            memoryStream.Write(headerBuffer, 0, headerBuffer.Length);

            var fourByteBuffer = new byte[4];
            var chunkBuffer = new byte[1024];

            try
            {
                do
                {
                    // chunk length
                    var read = stream.Read(fourByteBuffer, 0, 4);
                    memoryStream.Write(fourByteBuffer, 0, read);

                    if (BitConverter.IsLittleEndian)
                        Array.Reverse(fourByteBuffer);

                    var length = BitConverter.ToUInt32(fourByteBuffer, 0);

                    // chunk type
                    read = stream.Read(fourByteBuffer, 0, 4);
                    memoryStream.Write(fourByteBuffer, 0, read);

                    if (chunkBuffer.Length < length + 4L)
                        chunkBuffer = new byte[length + 4L];

                    // chunk data + CRC
                    read = stream.Read(chunkBuffer, 0, (int)(length + 4L));
                    memoryStream.Write(chunkBuffer, 0, read);
                } while (!MeidoPhotoStudio.Plugin.Utility.BytesEqual(fourByteBuffer, PngEnd));
            }
            catch
            {
                return null;
            }

            return memoryStream.ToArray();
        }
    }
}