readme.md

Modifying Bitmap Data in Memory

The fastest way to set pixel values of an image is to directly modify the values of its memory. Finding a pixel position in memory is easy because for 32-bit color images each pixel is 4 bytes (A, R, G, B), and from there it can be easily modified This method can be faster than using drawing methods like GetPixel and PutPixel.

Core Concepts

Converting Images To/From Byte Arrays

The System.Runtime.InteropServices.Marshal class is used to provide byte access to Bitmap memory blocks. These functions do not require using any unsafe code blocks.

public byte[] BitmapToBytes(Bitmap bmp)
{
    // return a bitmap (of any image format) as a byte array
    int bytesPerPixel = Image.GetPixelFormatSize(bmp.PixelFormat) / 8;
    Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
    BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.ReadOnly, bmp.PixelFormat);
    byte[] bytes = new byte[bmpData.Stride * bmp.Height * bytesPerPixel];
    Marshal.Copy(bmpData.Scan0, bytes, 0, bytes.Length);
    bmp.UnlockBits(bmpData);
    return bytes;
}

public Bitmap BitmapFromBytes(byte[] bytes, Size size, 
    PixelFormat format = PixelFormat.Format32bppArgb)
{
    // create a bitmap given a byte array of raw data
    Bitmap bmp = new Bitmap(size.Width, size.Height, format);
    Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
    BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.ReadWrite, bmp.PixelFormat);
    Marshal.Copy(bytes, 0, bmpData.Scan0, bytes.Length);
    bmp.UnlockBits(bmpData);
    return bmp;
}

Assign Random Values to Image Bytes

In this example we assign random values to all bytes in an image and render the output on a picturebox.

private Bitmap bmp;
private Graphics gfx;
private Random rand = new Random();

private void InitializeBitmapAndGraphics()
{
    bmp = new Bitmap(pictureBox1.Width, pictureBox1.Height);
    gfx = Graphics.FromImage(bmp);

    byte[] bmpBytes = BitmapToBytes(bmp);
    for (int i = 0; i < bmpBytes.Length; i++)
        bmpBytes[i] = (byte)rand.Next(256);

    Console.WriteLine($"Created new bitmap {bmp.Size} ({bmpBytes.Length} bytes)");
    pictureBox1.Image = BitmapFromBytes(bmpBytes, bmp.Size);

}

Output