PICO-8 PNG CARTRIDGE LOADING ROUTINE

pico8png.cs

// emulator variables
byte[] cart = new byte[32769];
byte[] ram = new byte[32768];

public string LoadCart(string fn)
{
    string code = "";

    // load raw data from png
    Bitmap bmp = new Bitmap(fn);
    int x, y;

    for (y = 0; y < bmp.Height; y++)
    {
        for (x = 0; x < bmp.Width; x++)
        {
            int pos = x + y * bmp.Width;
            
            // if we reached limit we are not doing anything
            if (pos > 32768) continue;

            Color c = bmp.GetPixel(x, y);
            byte a = (byte)(c.A & 0x03);
            byte r = (byte)(c.R & 0x03);
            byte g = (byte)(c.G & 0x03);
            byte b = (byte)(c.B & 0x03);

            byte z = (byte)((a << 6) + (r << 4) + (g << 2) + b);

            cart[pos] = z;
        }
    }

    // copy cart to ram
    Buffer.BlockCopy(cart, 0, ram, 0, 0x4300);

    // decode source
    bool compressed = (cart[0x8000] == 1);
    int size = (cart[0x4304] << 8) + cart[0x4305];

    if (compressed)
    {
        int i = 0;
        string chars = " 0123456789abcdefghijklmnopqrstuvwxyz!#%(){}[]<>+=/*:;.,~_";
        while (i < size && 0x4308 + i < 0x8000)
        {
            byte b = cart[0x4308 + i];

            if (b == 0x00) // extended
            {
                code += (char)cart[0x4308 + i + 1];
                i += 2;
            }
            else if (b == 0x01) // new line
            {
                code += "\n";
                i++;
            }
            else if (b >= 0x02 && b <= 0x3B) // characters
            {
                code += chars[b - 2];
                i++;
            }
            else // copy block
            {
                byte b2 = cart[0x4308 + i + 1];

                int offset = (b - 0x3C) * 16 + (b2 & 0xF);
                int length = (b2 >> 4) + 2;
                string part = "";

                for (int c = 0; c < length; c++)
                {
                    part += code[code.Length - offset + c];
                }

                code += part;

                i += 2;
            }
        }
    }
    else // uncompressed
    {
        for (int i = 0; i < size; i++)
        {
            code += (char)cart[0x4308 + i];
        }
    }
    
    return code;
}