Welcome : GDI+ FAQ : Converting RGB to 1 bit-per-pixel monochrome

Converting an RGB image to 1 bit-per-pixel monochrome.

This article has been revised to use high-speed LockBits processing on both sides of the conversion and updated to include code in VB.NET.

One bit per pixel images are essentially an indexed format image having two entries in the colour palette. Unlike a GIF or other 256 colour indexed format which uses a single byte to access one of up-to 256 colours in the palette, the one bit per pixel format uses only a single bit to select colour A or colour B from the palette.

Often used in Fax formats, one bit per pixel images are very compact with one byte of information representing eight pixels.

Very often, a more complex RGB image can be used for the basic raw-image that is used to define the 1 bpp array but this operation in GDI+ and .NET systems is a little more complex than one would hope because you cannot simply get the Graphics object for an indexed image and draw on it. In order to do this conversion, you must manipulate the individual bits of the image by locking it's byte array and selecting the correct bit for the desired pixel.

The LockBits method can be used to obtain a BitmapData object which contains the address of the bitmap in memory, the dimensions and particularly, the stride of the image. All images are stored in an array that fills memory to a four-byte boundary so the stride is used to calculate where each successive line begins in the array. A specific line may be addressed by the formula LineAddress=stride*linenumber and NOT LineAddress = pixeldepth*imagewidth*linenumber as many people assume.

For our single bit per pixel image, the pixel indexing is further complicated by the need to select a specific bit in a byte that contains up to 8 pixels (it may have less than 8 pixels because it may be on the end of a scan-line). To perform this selection, a bit mask is used and rotated into position so that the correct bit may be set or reset as desired.

The simple application shown in listing 1 enables you to load any image into a PictureBox control situated on the left of the form. Once loaded, the image is scanned pixel by pixel to check the brightness of the colours contained in the image. If a pixel is darker than 50% brightness, a black pixel is set in the corresponding pixel of a black and white, single bit per pixel image stored in the rightmost PictureBox on the form. Between the two PictureBox controls, a Splitter enables you to see more or less of the original image for comparison with the converted one.

Note in particular how the single bit per pixel image is created in the pictureBox1_Click handler and how the SetIndexedPixel method is used to access the individual pixel bits in the monochrome image.

using System;

using System.Drawing;

using System.Drawing.Drawing2D;

using System.Drawing.Imaging;

using System.Collections;

using System.ComponentModel;

using System.Windows.Forms;

using System.Data;

using System.Runtime.InteropServices;

namespace _1bitmap

{

/// <summary>

/// Summary description for Form1.

/// </summary>

public class Form1 : System.Windows.Forms.Form

{

/// <summary>

/// Required designer variable.

/// </summary>

private System.ComponentModel.Container components = null;

public Form1()

{

// Required for Windows Form Designer support

InitializeComponent();

// TODO: Add any constructor code after InitializeComponent call

SetStyle(ControlStyles.ResizeRedraw,true);

}

/// <summary>

/// Clean up any resources being used.

/// </summary>

protected override void Dispose( bool disposing )

{

if( disposing )

{

if (components != null)

{

components.Dispose();

}

base.Dispose( disposing );

}

#region Windows Form Designer generated code

/// <summary>

/// Required method for Designer support - do not modify

/// the contents of this method with the code editor.

/// </summary>

private void InitializeComponent()

{

this.pictureBox1 = new System.Windows.Forms.PictureBox();

this.splitter1 = new System.Windows.Forms.Splitter();

this.pictureBox2 = new System.Windows.Forms.PictureBox();

this.SuspendLayout();

// pictureBox1

this.pictureBox1.BackColor = System.Drawing.Color.White;

this.pictureBox1.Dock = System.Windows.Forms.DockStyle.Left;

this.pictureBox1.Name = "pictureBox1";

this.pictureBox1.Size = new System.Drawing.Size(100, 397);

this.pictureBox1.TabIndex = 0;

this.pictureBox1.TabStop = false;

this.pictureBox1.Click += new System.EventHandler(this.pictureBox1_Click);

// splitter1

this.splitter1.Location = new System.Drawing.Point(100, 0);

this.splitter1.Name = "splitter1";

this.splitter1.Size = new System.Drawing.Size(3, 397);

this.splitter1.TabIndex = 1;

this.splitter1.TabStop = false;

// pictureBox2

this.pictureBox2.BackColor = System.Drawing.Color.FromArgb(((System.Byte)(128)), ((System.Byte)(255)), ((System.Byte)(255)));

this.pictureBox2.Dock = System.Windows.Forms.DockStyle.Fill;

this.pictureBox2.Location = new System.Drawing.Point(103, 0);

this.pictureBox2.Name = "pictureBox2";

this.pictureBox2.Size = new System.Drawing.Size(473, 397);

this.pictureBox2.TabIndex = 2;

this.pictureBox2.TabStop = false;

// Form1

this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);

this.ClientSize = new System.Drawing.Size(576, 397);

this.Controls.AddRange(new System.Windows.Forms.Control[] {

this.pictureBox2,

this.splitter1,

this.pictureBox1});

this.Name = "Form1";

this.Text = "Form1";

this.Load += new System.EventHandler(this.Form1_Load);

this.ResumeLayout(false);

}

#endregion

/// <summary>

/// The main entry point for the application.

/// </summary>

[STAThread]

static void Main()

{

Application.Run(new Form1());

}

protected void SetIndexedPixel(int x,int y,BitmapData bmd, bool pixel)

{

int index=y*bmd.Stride+(x>>3);

byte p=Marshal.ReadByte(bmd.Scan0,index);

byte mask=(byte)(0x80>>(x&0x7));

if(pixel)

p |=mask;

else

p &=(byte)(mask^0xff);

Marshal.WriteByte(bmd.Scan0,index,p);

}

private System.Windows.Forms.PictureBox pictureBox1;

private System.Windows.Forms.Splitter splitter1;

private System.Windows.Forms.PictureBox pictureBox2;

Bitmap bm;

private void Form1_Load(object sender, System.EventArgs e)

{

}

private void pictureBox1_Click(object sender, System.EventArgs e)

{

//Load a file

OpenFileDialog dlg = new OpenFileDialog();

dlg.Filter="Image files|*.bmp;*.gif;*.jpg";

if(dlg.ShowDialog()==DialogResult.OK)

{

Bitmap img = (Bitmap)Image.FromFile(dlg.FileName);

//Ensure that it's a 32 bit per pixel file

if(img.PixelFormat!=PixelFormat.Format32bppPArgb)

{

Bitmap temp=new Bitmap(img.Width,img.Height,PixelFormat.Format32bppPArgb);

Graphics g=Graphics.FromImage(temp);

g.DrawImage(img, new Rectangle(0,0,img.Width,img.Height),0,0,img.Width,img.Height,GraphicsUnit.Pixel);

img.Dispose();

g.Dispose();

img=temp;

}

this.pictureBox1.Image=img;

//lock the bits of the original bitmap

BitmapData bmdo=img.LockBits(new Rectangle(0,0,img.Width,img.Height),ImageLockMode.ReadOnly,img.PixelFormat);

//and the new 1bpp bitmap

bm=new Bitmap(this.pictureBox1.Image.Width,this.pictureBox1.Image.Height,PixelFormat.Format1bppIndexed);

BitmapData bmdn=bm.LockBits(new Rectangle(0,0,bm.Width,bm.Height),ImageLockMode.ReadWrite,PixelFormat.Format1bppIndexed);

//for diagnostics

DateTime dt=DateTime.Now;

//scan through the pixels Y by X

int x,y;

for(y=0;y<img.Height;y++)

{

for(x=0;x<img.Width;x++)

{

//generate the address of the colour pixel

int index=y*bmdo.Stride+(x*4);

//check its brightness

if(Color.FromArgb(Marshal.ReadByte(bmdo.Scan0,index+2),

Marshal.ReadByte(bmdo.Scan0,index+1),

Marshal.ReadByte(bmdo.Scan0,index)).GetBrightness()>0.5f)

this.SetIndexedPixel(x,y,bmdn,true); //set it if its bright.

}

//tidy up

bm.UnlockBits(bmdn);

img.UnlockBits(bmdo);

//show the time taken to do the conversion

TimeSpan ts=dt-DateTime.Now;

System.Diagnostics.Trace.WriteLine("Conversion time was:"+ts.ToString());

//display the 1bpp image.

this.pictureBox2.Image=bm;

}

Figure 1 shows the application in action.

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