Chunks encoder decoder

[hannespreishuber]

have any solution on read write chunk (eg itxt) like https://www.nayuki.io/page/png-file-chunk-inspector
would even pay for it

[EliotJones]

Depending on your needs you can hook into the parsing of individual chunks using the IChunkVisitor (here), you can implement the interface with any logic you need and pass it to Png.Open https://github.com/EliotJones/BigGustave/blob/master/src/BigGustave/Png.cs#L59

[stanac]

For those who are trying to set and read data:

To set data:

class Program
{
    private const string Key = "testKey";
    private const string Value = "testValue123";

    private const string SourceFile = @"c:\temp\PNG_transparency_demonstration_1.png";
    private const string DestinationFile = @"c:\temp\PNG_transparency_demonstration_2.png";

    static void Main(string[] args)
    {
        byte[] pngBytes = File.ReadAllBytes(SourceFile);

        Png png = Png.Open(pngBytes);
        PngBuilder builder = PngBuilder.FromPng(png);

        builder.StoreText(Key, Value);
        byte[] newPng = builder.Save(new PngBuilder.SaveOptions {AttemptCompression = false, MaxDegreeOfParallelism = 1});

        if (File.Exists(DestinationFile))
        {
            File.Delete(DestinationFile);
        }

        File.WriteAllBytes(DestinationFile, newPng);
    }
}

To read data:

Png.Open(DestinationFile, new TextVisitor());

And TextVisitor:

public class TextVisitor : IChunkVisitor
{
    public void Visit(Stream stream, ImageHeader header, ChunkHeader chunkHeader, byte[] data, byte[] crc)
    {
        if (chunkHeader.Name == "iTXt")
        {
            string key = GetTextKey(data, out int keyLength);

            if (key == "testKey")
            {
                string value = GetTextValue(data, keyLength);

                Console.WriteLine(value);
            }
        }
    }

    private static string GetTextKey(byte[] data, out int keyLength)
    {
        const int MaxKeyLength = 79;
        keyLength = 0;
        
        for (int i = 0; i < MaxKeyLength - 3; i++)
        {
            if (data[i] == 0 && data[i + 1] == 0 && data[i + 2] == 0 && data[i + 3] == 0)
            {
                keyLength = i;
                break;
            }
        }

        return Encoding.UTF8.GetString(data, 0, keyLength);
    }

    private static string GetTextValue(byte[] data, int keyLength)
    {
        int count = data.Length - keyLength - 5;
        return Encoding.UTF8.GetString(data, keyLength + 5, count);
    }
}