BitmapMaker.cs

static class BitmapMaker
{
    // 8-color palette — one per button slot (cycles when > 8 buttons)
    public static readonly (byte R, byte G, byte B)[] Palette =
    [
        (0x00, 0x78, 0xD7), // blue
        (0x10, 0x7C, 0x10), // green
        (0xD8, 0x3B, 0x01), // orange
        (0x87, 0x64, 0xB8), // purple
        (0x00, 0x82, 0x72), // teal
        (0xE8, 0x11, 0x23), // red
        (0xFF, 0xB9, 0x00), // amber
        (0x00, 0xB7, 0xC3), // cyan
    ];

    public static byte[] Make16x16((byte R, byte G, byte B) color) =>
        Make16x16(color.R, color.G, color.B);

    // 16x16 24-bit BMP: colored background, white cross, black border
    public static byte[] Make16x16(byte r, byte g, byte b)
    {
        const int width = 16, height = 16, bpp = 3;
        int rowSize    = width * bpp;       // 48 — already DWORD-aligned
        int pixelBytes = rowSize * height;  // 768
        int fileSize   = 14 + 40 + pixelBytes; // 822

        var buf = new byte[fileSize];
        int p = 0;

        // BITMAPFILEHEADER
        buf[p++] = 0x42; buf[p++] = 0x4D;
        WriteI32(buf, ref p, fileSize);
        WriteI16(buf, ref p, 0);
        WriteI16(buf, ref p, 0);
        WriteI32(buf, ref p, 54);

        // BITMAPINFOHEADER
        WriteI32(buf, ref p, 40);
        WriteI32(buf, ref p, width);
        WriteI32(buf, ref p, height);
        WriteI16(buf, ref p, 1);
        WriteI16(buf, ref p, 24);
        WriteI32(buf, ref p, 0);
        WriteI32(buf, ref p, 0);
        WriteI32(buf, ref p, 0);
        WriteI32(buf, ref p, 0);
        WriteI32(buf, ref p, 0);
        WriteI32(buf, ref p, 0);

        // Pixel data — bottom-up, BGR order
        for (int y = 0; y < height; y++)
        for (int x = 0; x < width; x++)
        {
            bool border = x == 0 || x == 15 || y == 0 || y == 15;
            bool cross  = x == 7 || x == 8  || y == 7 || y == 8;
            if      (border) { buf[p++] = 0x00; buf[p++] = 0x00; buf[p++] = 0x00; }
            else if (cross)  { buf[p++] = 0xFF; buf[p++] = 0xFF; buf[p++] = 0xFF; }
            else             { buf[p++] = b;    buf[p++] = g;    buf[p++] = r;    }
        }

        return buf;
    }

    static void WriteI32(byte[] buf, ref int p, int v)
    {
        buf[p++] = (byte)( v        & 0xFF);
        buf[p++] = (byte)((v >>  8) & 0xFF);
        buf[p++] = (byte)((v >> 16) & 0xFF);
        buf[p++] = (byte)((v >> 24) & 0xFF);
    }

    static void WriteI16(byte[] buf, ref int p, short v)
    {
        buf[p++] = (byte)( v       & 0xFF);
        buf[p++] = (byte)((v >> 8) & 0xFF);
    }
}