servuo-plugins/overlay/Scripts/Custom/Bridge/BridgePng.cs

using System;
using System.IO;
using System.IO.Compression;

namespace Server.Custom.Bridge
{
    /// <summary>
    ///     **A PNG encoder that does not go through GDI+** (docs/link/v8.md §4.4, §4.9 — phase 4).
    ///
    ///     <see cref="BridgeUop"/> decodes into a <c>ushort[]</c> of ARGB1555 rather than into a
    ///     <c>Bitmap</c>, which is the whole point of §4.4's note that the UOP reader is written
    ///     without <c>System.Drawing</c>: libgdiplus was archived in March 2025, and every line of
    ///     extraction that does not depend on it is a line that survives its absence. That leaves
    ///     the encode, and <c>Bitmap.Save(…, ImageFormat.Png)</c> is GDI+ too — so this is the
    ///     other half.
    ///
    ///     It is deliberately the smallest thing that produces a correct file: 8-bit RGBA, one
    ///     IDAT, filter type 0 on every row. No interlacing, no palette, no colour-type choice, no
    ///     filter heuristics. A sprite is a few hundred pixels across and the bytes go straight
    ///     into a base64 field; the compression difference between this and a tuned encoder is a
    ///     rounding error against the wire, and every knob not turned is a way this cannot be
    ///     subtly wrong.
    ///
    ///     Phase 3's <c>BridgeCatalog.ToPng</c> is left exactly as it is. It is measured, shipped,
    ///     and its input really is a <c>Bitmap</c> from the vendored decoder — a path that needs
    ///     GDI+ to produce the pixels in the first place, so encoding them without it buys nothing.
    /// </summary>
    public static class BridgePng
    {
        private static readonly byte[] Signature =
        {
            0x89, (byte)'P', (byte)'N', (byte)'G', 0x0D, 0x0A, 0x1A, 0x0A
        };

        private static readonly uint[] CrcTable = BuildCrcTable();

        private static readonly byte[] Empty = new byte[0];

        /// <summary>
        ///     ARGB1555 to an RGBA8 PNG with a transparent background.
        ///
        ///     The expansion is the same one <c>BridgeCatalog.ToPng</c> documents and for the same
        ///     reason: alpha bit clear is fully transparent, and each 5-bit channel is widened by
        ///     repeating its high bits — <c>(c &lt;&lt; 3) | (c &gt;&gt; 2)</c>, not a plain shift,
        ///     which would cap white at 248 and tint every sprite.
        /// </summary>
        public static byte[] FromArgb1555(ushort[] pixels, int width, int height)
        {
            if (pixels == null || width <= 0 || height <= 0)
                return null;

            if ((long)width * height > pixels.Length)
                return null;

            // One filter byte per row, then RGBA per pixel. This is the PNG "raw" stream, the
            // thing that gets deflated. Bounded by the caller's dimension ceiling
            // (BridgeAssetValidator.MaxArtDimension), so the arithmetic cannot overflow an int —
            // the check is here anyway, because that ceiling lives in another file.
            long size = (((long)width * 4) + 1) * height;

            if (size > Int32.MaxValue / 2)
                return null;

            var raw = new byte[size];

            int at = 0;

            for (int y = 0; y < height; y++)
            {
                raw[at++] = 0; // filter: None

                int row = y * width;

                for (int x = 0; x < width; x++)
                {
                    int p = pixels[row + x];

                    if ((p & 0x8000) == 0)
                    {
                        at += 4; // already zero: transparent black
                        continue;
                    }

                    int r = (p >> 10) & 0x1F;
                    int g = (p >> 5) & 0x1F;
                    int b = p & 0x1F;

                    raw[at++] = (byte)((r << 3) | (r >> 2));
                    raw[at++] = (byte)((g << 3) | (g >> 2));
                    raw[at++] = (byte)((b << 3) | (b >> 2));
                    raw[at++] = 0xFF;
                }
            }

            using (var ms = new MemoryStream(raw.Length / 2))
            {
                ms.Write(Signature, 0, Signature.Length);

                var header = new byte[13];

                WriteBigEndian(header, 0, (uint)width);
                WriteBigEndian(header, 4, (uint)height);

                header[8] = 8;  // bit depth
                header[9] = 6;  // colour type: truecolour with alpha
                header[10] = 0; // compression: deflate
                header[11] = 0; // filter method 0
                header[12] = 0; // no interlace

                WriteChunk(ms, "IHDR", header, 0, header.Length);

                byte[] deflated = Zlib(raw);

                WriteChunk(ms, "IDAT", deflated, 0, deflated.Length);
                WriteChunk(ms, "IEND", Empty, 0, 0);

                return ms.ToArray();
            }
        }

        /// <summary>
        ///     A zlib stream around .NET Framework's raw-deflate-only <c>DeflateStream</c>: the
        ///     two-byte header PNG requires, the deflate data, and the adler32 trailer computed
        ///     here because nothing in the framework will do it. Written by hand for exactly the
        ///     same reason <see cref="BridgeUop"/> reads one by hand — net48 exposes deflate and
        ///     calls it zlib, and the two are not the same format.
        /// </summary>
        private static byte[] Zlib(byte[] data)
        {
            using (var ms = new MemoryStream(data.Length / 2))
            {
                // CMF 0x78 (deflate, 32K window) and FLG 0x9C (default level, no dictionary):
                // 0x789C is the pair whose value is divisible by 31, which is the check a decoder
                // applies.
                ms.WriteByte(0x78);
                ms.WriteByte(0x9C);

                using (var deflate = new DeflateStream(ms, CompressionMode.Compress, true))
                    deflate.Write(data, 0, data.Length);

                uint adler = Adler32(data);

                ms.WriteByte((byte)(adler >> 24));
                ms.WriteByte((byte)(adler >> 16));
                ms.WriteByte((byte)(adler >> 8));
                ms.WriteByte((byte)adler);

                return ms.ToArray();
            }
        }

        private static void WriteChunk(Stream to, string type, byte[] data, int offset, int length)
        {
            var head = new byte[8];

            WriteBigEndian(head, 0, (uint)length);

            head[4] = (byte)type[0];
            head[5] = (byte)type[1];
            head[6] = (byte)type[2];
            head[7] = (byte)type[3];

            to.Write(head, 0, head.Length);

            if (length > 0)
                to.Write(data, offset, length);

            // The CRC covers the type and the data, and not the length.
            uint crc = Crc32(head, 4, 4, 0xFFFFFFFF);

            if (length > 0)
                crc = Crc32(data, offset, length, crc);

            crc ^= 0xFFFFFFFF;

            var tail = new byte[4];

            WriteBigEndian(tail, 0, crc);

            to.Write(tail, 0, tail.Length);
        }

        private static void WriteBigEndian(byte[] into, int at, uint value)
        {
            into[at] = (byte)(value >> 24);
            into[at + 1] = (byte)(value >> 16);
            into[at + 2] = (byte)(value >> 8);
            into[at + 3] = (byte)value;
        }

        private static uint[] BuildCrcTable()
        {
            var table = new uint[256];

            for (uint n = 0; n < 256; n++)
            {
                uint c = n;

                for (int k = 0; k < 8; k++)
                    c = (c & 1) != 0 ? 0xEDB88320 ^ (c >> 1) : c >> 1;

                table[n] = c;
            }

            return table;
        }

        private static uint Crc32(byte[] data, int offset, int length, uint crc)
        {
            for (int i = 0; i < length; i++)
                crc = CrcTable[(crc ^ data[offset + i]) & 0xFF] ^ (crc >> 8);

            return crc;
        }

        private static uint Adler32(byte[] data)
        {
            const uint Mod = 65521;

            uint a = 1, b = 0;

            for (int i = 0; i < data.Length; i++)
            {
                a = (a + data[i]) % Mod;
                b = (b + a) % Mod;
            }

            return (b << 16) | a;
        }
    }
}