using System;

public class BitmapBytes
{
    /// <summary>
    /// Converts RGBA bytes into an RGBA Bitmap.
    /// Pixels go left to right, bottom to top.
    /// Can write to file with, e.g:
    /// File.WriteAllBytes("example.bmp", BitmapBytes.FromRGBABytes(imageData, width));
    /// </summary>
    /// <param name="rgba">array of r,g,b,a pixels, must be divisible by 4</param>
    /// <param name="width">the image width, in pixels, height is omitted as it gets calculated</param>
    /// <returns>a byte array in bmp format</returns>
    public static byte[] FromRGBABytes(byte[] rgba, int width)
    {
        int height = (rgba.Length / 4) / width;
        var bytes = new byte[rgba.Length + bmpHeader.Length];
        //write header
        Buffer.BlockCopy(bmpHeader, 0, bytes, 0, bmpHeader.Length);
        //write filesize to offset 2
        Buffer.BlockCopy(BitConverter.GetBytes(bytes.Length), 0, bytes, 2, 4);
        //write width to offset 18
        Buffer.BlockCopy(BitConverter.GetBytes(width), 0, bytes, 18, 4);
        //write height to offset 22
        Buffer.BlockCopy(BitConverter.GetBytes(height), 0, bytes, 22, 4);
        //write image size to offset 34
        Buffer.BlockCopy(BitConverter.GetBytes(width * height * 4), 0, bytes, 34, 4);
        //finally, write the image data
        Buffer.BlockCopy(rgba, 0, bytes, bmpHeader.Length, rgba.Length);
        return bytes;
    }

    static byte[] bmpHeader = new byte[] // All values are little-endian
     {
            0x42, 0x4D,             // Signature 'BM'
            0xaa, 0x00, 0x00, 0x00, // Size: e.g. 170 bytes
            0x00, 0x00,             // Unused
            0x00, 0x00,             // Unused
            0x8a, 0x00, 0x00, 0x00, // Offset to image data

            0x7c, 0x00, 0x00, 0x00, // DIB header size (124 bytes)
            0x04, 0x00, 0x00, 0x00, // Width (e.g. 4px)
            0x02, 0x00, 0x00, 0x00, // Height (e.g. 2px)
            0x01, 0x00,             // Planes (1)
            0x20, 0x00,             // Bits per pixel (32)
            0x03, 0x00, 0x00, 0x00, // Format (bitfield = use bitfields | no compression)
            0x20, 0x00, 0x00, 0x00, // Image raw size (32 bytes)
            0x13, 0x0B, 0x00, 0x00, // Horizontal print resolution (2835 = 72dpi * 39.3701)
            0x13, 0x0B, 0x00, 0x00, // Vertical print resolution (2835 = 72dpi * 39.3701)
            0x00, 0x00, 0x00, 0x00, // Colors in palette (none)
            0x00, 0x00, 0x00, 0x00, // Important colors (0 = all)
            0xFF, 0x00, 0x00, 0x00, // R bitmask (00FF0000)
            0x00, 0xFF, 0x00, 0x00, // G bitmask (0000FF00)
            0x00, 0x00, 0xFF, 0x00, // B bitmask (000000FF)
            0x00, 0x00, 0x00, 0xFF, // A bitmask (FF000000)
            0x42, 0x47, 0x52, 0x73, // sRGB color space
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // Unused R, G, B entries for color space
            0x00, 0x00, 0x00, 0x00, // Unused Gamma X entry for color space
            0x00, 0x00, 0x00, 0x00, // Unused Gamma Y entry for color space
            0x00, 0x00, 0x00, 0x00, // Unused Gamma Z entry for color space

            0x00, 0x00, 0x00, 0x00, // Unknown
            0x00, 0x00, 0x00, 0x00, // Unknown
            0x00, 0x00, 0x00, 0x00, // Unknown
            0x00, 0x00, 0x00, 0x00, // Unknown
     };

}