Julia detect png dpi

detect_png_dpi.jl

function detect_png_dpi(io::IO)
    # PNG header signature
    png_signature = UInt8['\u89', 'P', 'N', 'G', '\r', '\n', '\u1a', '\n']

    # Reset the IO position to the beginning
    seekstart(io)

    # Check if the PNG signature is present
    if [read(io, UInt8) for _ in 1:length(png_signature)] != png_signature
        error("Not a png file")
    end

    # Function to read a 4-byte chunk length
    function read_chunk_length()
        return ntoh(read(io, UInt32))
    end

    # Function to read a 4-byte chunk type
    function read_chunk_type()
        return [read(io, UInt8) for _ in 1:4]
    end

    while true
        # Read the chunk length
        chunk_length = read_chunk_length()
        # Read the chunk type
        chunk_type = read_chunk_type()

        s = String(chunk_type)
        if s == "pHYs"
            # Read the horizontal and vertical DPI values
            x_px_per_unit = Int(ntoh(read(io, UInt32)))
            y_px_per_unit = Int(ntoh(read(io, UInt32)))
            in_meters = read(io, Bool)

            x_dpi = in_meters ? x_px_per_unit / 100 * 2.54 : x_px_per_unit
            y_dpi = in_meters ? y_px_per_unit / 100 * 2.54 : y_px_per_unit

            aspect_ratio_only = !in_meters

            return (x_dpi, y_dpi, aspect_ratio_only)
        elseif s == "IDAT"
            return nothing  # pHYs always comes before IDAT if it exists
        end

        # Skip the chunk data and CRC
        skip(io, chunk_length)
        skip(io, 4)
    end

    return nothing  # No pHYs chunk found
end