save .ANI (doesn't work, not sure why)

aniexport.cs

        public static void SaveAsANI(Image inputImage, string outputFilePath)
        {
            ArgumentNullException.ThrowIfNull(inputImage);
            ArgumentNullException.ThrowIfNull(outputFilePath);
            using var stream = new FileStream(outputFilePath, FileMode.Create);
            SaveAsANI(inputImage, stream);
        }

        public static void SaveAsANI(Image inputImage, Stream outputStream)
        {
            ArgumentNullException.ThrowIfNull(inputImage);
            ArgumentNullException.ThrowIfNull(outputStream);

            var dimension = new FrameDimension(inputImage.FrameDimensionsList[0]);
            var frameCount = inputImage.GetFrameCount(dimension);

            // see https://en.wikipedia.org/wiki/Resource_Interchange_File_Format on RIFF format
            // see https://www.gdgsoft.com/anituner/help/aniformat.htm for example on .ANI format
            using var writer = new BinaryWriter(outputStream, Encoding.ASCII);

            // riff format
            writeString("RIFF");

            // get pos & write dummy value,  we'll rewrite that later
            var riffSizePos = outputStream.Position;
            writeInt32(0); // 'RIFF' chunk size

            writeString("ACON"); // .ani format

            // header
            writeString("anih");
            writeInt32(36); // 'anih' chunk size

            writeInt32(36); // cbSizeof
            writeInt32(frameCount); // cFrames
            writeInt32(frameCount); // cSteps
            // unused in .ani case
            writeInt32(0); // cx
            writeInt32(0); // cy
            writeInt32(32); // cBitCount
            writeInt32(1); // cPlanes
            writeInt32(3); // jifRate
            writeInt32(1); // flags (AF_ICON = 1)

            // some other RIFF chunks
            writeString("LIST");

            // get pos & write dummy value,  we'll rewrite that later
            var listSizePos = outputStream.Position;
            writeInt32(0); // 'LIST' chunk size

            writeString("fram");
            // note: we use the same buffer for all frames
            using var ms = new MemoryStream();
            for (var i = 0; i < frameCount; i++)
            {
                // get image frame #i
                inputImage.SelectActiveFrame(dimension, i);

                writeString("icon");
                // get pos & write dummy value,  we'll rewrite that later
                var iconSizePos = outputStream.Position;
                writeInt32(0); // 'icon' chunk size

                // get png bytes
                ms.Position = 0;
                inputImage.Save(ms, ImageFormat.Png);

                var beforeIconPos = outputStream.Position;
                WritePNGCursor(outputStream, (uint)inputImage.Width, (uint)inputImage.Height, ms.GetBuffer(), 0, (int)ms.Length);
                var afterIconPos = outputStream.Position;

                // now we can get back to write final icon size
                outputStream.Position = iconSizePos;
                writeInt32((int)(afterIconPos - beforeIconPos));

                // restore pos
                outputStream.Position = afterIconPos;
            }

            // now we can get back to write final sizes
            outputStream.Position = riffSizePos;
            writeInt32((int)(outputStream.Length - riffSizePos - 4));

            outputStream.Position = listSizePos;
            writeInt32((int)(outputStream.Length - listSizePos - 4));

            void writeString(string str) => outputStream.Write(Encoding.ASCII.GetBytes(str));
            void writeInt32(int i) => outputStream.Write(BitConverter.GetBytes(i));
        }

        // utility methods to write .cur files with PNG format (the easiest)
        public static void WritePNGCursor(Stream iconFileStream, uint width, uint height, byte[] pngBytes, int pngBytesIndex = 0, int pngBytesCount = int.MaxValue)
        {
            ArgumentNullException.ThrowIfNull(iconFileStream);
            ArgumentNullException.ThrowIfNull(pngBytes);

            var startPos = iconFileStream.Position;
            if (pngBytesCount == int.MaxValue || pngBytesCount < 0)
            {
                pngBytesCount = pngBytes.Length;
            }

            var writer = new BinaryWriter(iconFileStream);
            writer.Write((short)0); // reserved
            writer.Write((short)2); // image type (2 for .cur)
            writer.Write((short)1); // count
            const int maxIconSize = 256;
            writer.Write(width == maxIconSize ? (byte)0 : (byte)width);
            writer.Write(height == maxIconSize ? (byte)0 : (byte)height);
            writer.Write((byte)0); // number of colors in the color palette
            writer.Write((byte)0); // reserved
            writer.Write((short)0); // h hotspot
            writer.Write((short)0); // v hotspot 

            var offsets = iconFileStream.Position;
            writer.Write(0); // size of image data, we'll rewrite that later
            writer.Write(0); // offset of data from beginning;

            var pngOffset = iconFileStream.Position - startPos;
            writer.Write(pngBytes, pngBytesIndex, pngBytesCount);

            // save end position
            var endPos = iconFileStream.Position;
            iconFileStream.Seek(offsets, SeekOrigin.Begin);
            writer.Write(pngBytesCount);
            writer.Write((int)pngOffset);
            iconFileStream.Position = endPos; // restore end position
        }

        public static void WritePNGCursor(string iconFilePath, uint width, uint height, byte[] pngBytes, int pngBytesIndex = 0, int pngBytesCount = int.MaxValue)
        {
            ArgumentNullException.ThrowIfNull(iconFilePath);
            ArgumentNullException.ThrowIfNull(pngBytes);
            using var stream = File.OpenWrite(iconFilePath);
            WritePNGCursor(stream, width, height, pngBytes, pngBytesIndex, pngBytesCount);
        }