Welcome : GDI+ FAQ : Transparent GIF images

Creating Transparent GIF Images

Now includes a VB version of the listing.

This process is easy enough when you know how but is nevertheless a little more complex than it should be.

To save a GIF with a transparency key you need to modify the colour palette of the image. There are a few problems associated with this. If you're creating an image, you'll be using a true colour format such as 24 or 32 bit per pixel non indexed. This is because you cannot obtain a Graphics object using Graphics.FromImage for any images with an indexed pixel format. Saving such an image as a GIF file will create a standard spread palette for you with the range of colours seen in figure 1.

Figure 1: The standard spread palette provided by GDI+

Unfortunately, at no point during the save process are you given the opportunity to choose a transparent colour so you need to take the saved image and re-save it with a modified palette.

This in itself presents a problem because once a GIF image has been created, even though you can get hold of and manipulate the palette using the Bitmap.Palette property, GDI+ refuses to save the image with anything other than its original palette.

To work around these limitations it's necessary to create a new, blank 8 bit per pixel, indexed palette image, modify it's bitmap to be the same as the original images, copy all the pixel data from the original to the new and then save the new image.

As a demonstration of this process, and to provide a useful tool, the code in listing 1 is an application that enables you to load a GIF image, choose a transparent colour and save the GIF. panel1_Click is the method with the actual GIF manipulation.

Listing 1: TransparentGifCreator.cs

using System;

using System.IO;

using System.Drawing;

using System.Drawing.Imaging;

using System.Collections;

using System.ComponentModel;

using System.Windows.Forms;

using System.Data;

namespace TransparentGifCreator

{

/// <summary>

/// Summary description for Form1.

/// </summary>

public class Form1 : System.Windows.Forms.Form

{

private System.Windows.Forms.PictureBox pictureBox1;

private System.Windows.Forms.Panel panel1;

private System.Windows.Forms.Button button1;

private System.Windows.Forms.Button button2;

private System.Windows.Forms.Button button3;

private System.ComponentModel.IContainer components;

Image _gifImage;

private System.Windows.Forms.Timer timer1;

ColorPalette cp;

int CurrentEntry;

public Form1()

{

// Required for Windows Form Designer support

InitializeComponent();

// TODO: Add any constructor code after InitializeComponent call

}

/// <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.components = new System.ComponentModel.Container();

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

this.panel1 = new System.Windows.Forms.Panel();

this.button1 = new System.Windows.Forms.Button();

this.button2 = new System.Windows.Forms.Button();

this.button3 = new System.Windows.Forms.Button();

this.timer1 = new System.Windows.Forms.Timer(this.components);

this.SuspendLayout();

// pictureBox1

this.pictureBox1.Location = new System.Drawing.Point(96, 8);

this.pictureBox1.Name = "pictureBox1";

this.pictureBox1.Size = new System.Drawing.Size(144, 144);

this.pictureBox1.SizeMode = System.Windows.Forms.PictureBoxSizeMode.StretchImage;

this.pictureBox1.TabIndex = 0;

this.pictureBox1.TabStop = false;

// panel1

this.panel1.Location = new System.Drawing.Point(8, 168);

this.panel1.Name = "panel1";

this.panel1.Size = new System.Drawing.Size(144, 144);

this.panel1.TabIndex = 1;

this.panel1.Click += new System.EventHandler(this.panel1_Click);

this.panel1.Paint += new System.Windows.Forms.PaintEventHandler(this.panel1_Paint);

this.panel1.MouseMove += new System.Windows.Forms.MouseEventHandler(this.panel1_MouseMove);

// button1

this.button1.Location = new System.Drawing.Point(200, 176);

this.button1.Name = "button1";

this.button1.Size = new System.Drawing.Size(88, 24);

this.button1.TabIndex = 2;

this.button1.Text = "Open";

this.button1.Click += new System.EventHandler(this.button1_Click);

// button2

this.button2.Location = new System.Drawing.Point(200, 216);

this.button2.Name = "button2";

this.button2.Size = new System.Drawing.Size(88, 24);

this.button2.TabIndex = 2;

this.button2.Text = "Save";

this.button2.Click += new System.EventHandler(this.button2_Click);

// button3

this.button3.Location = new System.Drawing.Point(200, 256);

this.button3.Name = "button3";

this.button3.Size = new System.Drawing.Size(88, 24);

this.button3.TabIndex = 2;

this.button3.Text = "Exit";

this.button3.Click += new System.EventHandler(this.button3_Click);

// timer1

this.timer1.Enabled = true;

this.timer1.Interval = 250;

this.timer1.Tick += new System.EventHandler(this.timer1_Tick);

// Form1

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

this.ClientSize = new System.Drawing.Size(328, 325);

this.Controls.Add(this.button1);

this.Controls.Add(this.panel1);

this.Controls.Add(this.pictureBox1);

this.Controls.Add(this.button2);

this.Controls.Add(this.button3);

this.Name = "Form1";

this.Text = "Form1";

this.ResumeLayout(false);

}

#endregion

/// <summary>

/// The main entry point for the application.

/// </summary>

[STAThread]

static void Main()

{

Application.Run(new Form1());

}

private void panel1_Paint(object sender, System.Windows.Forms.PaintEventArgs e)

{

if(cp==null)

return;

for(float y=0;y<16;y++)

for(float x=0;x<16;x++)

{

Color c=Color.Black;

if( ((16*y) + x)<cp.Entries.Length)

c=cp.Entries[(int)((16*y)+x)];

SolidBrush sb=new SolidBrush(Color.FromArgb(255,c));

float w=((float)this.panel1.Width)/16;

float h=((float)this.panel1.Height)/16;

e.Graphics.FillRectangle(sb,w*x,h*y,w,h);

if(c.A!=255)

{

if(showTrans)

e.Graphics.DrawRectangle(Pens.Black,w*x,h*y,w-1,h-1);

else

e.Graphics.DrawRectangle(Pens.White,w*x,h*y,w-1,h-1);

}

sb.Dispose();

}

private void panel1_MouseMove(object sender, System.Windows.Forms.MouseEventArgs e)

{

int y=(int)(((float)e.Y)/(((float)this.panel1.Width)/16f));

int x=(int)(((float)e.X)/(((float)this.panel1.Height)/16f));

CurrentEntry=(int)((16*y)+x);

if(cp!=null)

{

if(CurrentEntry>=cp.Entries.Length)

CurrentEntry=cp.Entries.Length-1;

//Little bit of diagnostic for the palette chooser below

//System.Diagnostics.Trace.WriteLine(string.Format("{0},{1}, adjusted={4},{5} entry={2} Colour={3}",e.X,e.Y,CurrentEntry,cp.Entries[CurrentEntry].ToString(),x,y));

}

private void panel1_Click(object sender, System.EventArgs e)

{

//Creates a new GIF image with a modified colour palette

if(cp!=null)

{

//Create a new 8 bit per pixel image

Bitmap bm=new Bitmap(_gifImage.Width,_gifImage.Height,PixelFormat.Format8bppIndexed);

//get it's palette

ColorPalette ncp=bm.Palette;

//copy all the entries from the old palette removing any transparency

int n=0;

foreach(Color c in cp.Entries)

ncp.Entries[n++]=Color.FromArgb(255,c);

//Set the newly selected transparency

ncp.Entries[CurrentEntry]=Color.FromArgb(0,cp.Entries[CurrentEntry]);

//re-insert the palette

bm.Palette=ncp;

//now to copy the actual bitmap data

//lock the source and destination bits

BitmapData src=((Bitmap)_gifImage).LockBits(new Rectangle(0,0,_gifImage.Width,_gifImage.Height),ImageLockMode.ReadOnly,_gifImage.PixelFormat);

BitmapData dst=bm.LockBits(new Rectangle(0,0,bm.Width,bm.Height),ImageLockMode.WriteOnly,bm.PixelFormat);

//uses pointers so we need unsafe code.

//the project is also compiled with /unsafe

unsafe

{

//steps through each pixel

for(int y=0;y<_gifImage.Height;y++)

for(int x=0;x<_gifImage.Width;x++)

{

//transferring the bytes

((byte *)dst.Scan0.ToPointer())[(dst.Stride*y)+x]=((byte *)src.Scan0.ToPointer())[(src.Stride*y)+x];

}

//all done, unlock the bitmaps

((Bitmap)_gifImage).UnlockBits(src);

bm.UnlockBits(dst);

//clear out the picturebox

this.pictureBox1.Image=null;

_gifImage.Dispose();

//set the new image in place

_gifImage=bm;

cp=_gifImage.Palette;

this.pictureBox1.Image=_gifImage;

}

private void button1_Click(object sender, System.EventArgs e)

{

OpenFileDialog dlg=new OpenFileDialog();

dlg.Filter="GIF files|*.GIF";

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

{

_gifImage=Image.FromFile(dlg.FileName);

this.pictureBox1.Image=_gifImage;

cp=_gifImage.Palette;

this.panel1.Invalidate();

}

private void button2_Click(object sender, System.EventArgs e)

{

SaveFileDialog dlg=new SaveFileDialog();

dlg.Filter="GIF files|*.gif";

dlg.DefaultExt=".gif";

dlg.AddExtension=true;

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

{

_gifImage.Save(dlg.FileName,ImageFormat.Gif);

}

private void button3_Click(object sender, System.EventArgs e)

{

Application.Exit();

}

bool showTrans;

private void timer1_Tick(object sender, System.EventArgs e)

{

showTrans^=true;

Graphics g=this.panel1.CreateGraphics();

//I do this rather than invalidate the panel because

//the panel draws its background ans so flickers horribly.

PaintEventArgs pe=new PaintEventArgs(g,new Rectangle(0,0,this.panel1.Width,this.panel1.Height));

this.panel1_Paint(this,pe);

g.Dispose();

}

To perform the same task in VB the Marshal class can be used to access the image byte array as shown in the following listing.

Imports System

Imports System.IO

Imports System.Drawing

Imports System.Drawing.Imaging

Imports System.Collections

Imports System.ComponentModel

Imports System.Windows.Forms

Imports System.Data

Imports System.Runtime.InteropServices

Listing 2. TransparentGifCreator.VB

Namespace TransparentGifCreator

'/ <summary>

'/ Summary description for Form1.

'/ </summary>

Public Class Form1

Inherits System.Windows.Forms.Form

Private pictureBox1 As System.Windows.Forms.PictureBox

Private WithEvents panel1 As System.Windows.Forms.Panel

Private WithEvents button1 As System.Windows.Forms.Button

Private WithEvents button2 As System.Windows.Forms.Button

Private WithEvents button3 As System.Windows.Forms.Button

Private components As System.ComponentModel.IContainer

Private _gifImage As Image

Private WithEvents timer1 As System.Windows.Forms.Timer

Private cp As ColorPalette

Private CurrentEntry As Integer

Public Sub New()

' Required for Windows Form Designer support

InitializeComponent()

End Sub 'New

' TODO: Add any constructor code after InitializeComponent call

'/ <summary>

'/ Clean up any resources being used.

'/ </summary>

Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)

If disposing Then

If Not (components Is Nothing) Then

components.Dispose()

End If

MyBase.Dispose(disposing)

End Sub 'Dispose

#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 Sub InitializeComponent()

Me.components = New System.ComponentModel.Container

Me.pictureBox1 = New System.Windows.Forms.PictureBox

Me.panel1 = New System.Windows.Forms.Panel

Me.button1 = New System.Windows.Forms.Button

Me.button2 = New System.Windows.Forms.Button

Me.button3 = New System.Windows.Forms.Button

Me.timer1 = New System.Windows.Forms.Timer(Me.components)

Me.SuspendLayout()

' pictureBox1

Me.pictureBox1.Location = New System.Drawing.Point(96, 8)

Me.pictureBox1.Name = "pictureBox1"

Me.pictureBox1.Size = New System.Drawing.Size(144, 144)

Me.pictureBox1.SizeMode = System.Windows.Forms.PictureBoxSizeMode.StretchImage

Me.pictureBox1.TabIndex = 0

Me.pictureBox1.TabStop = False

' panel1

Me.panel1.Location = New System.Drawing.Point(8, 168)

Me.panel1.Name = "panel1"

Me.panel1.Size = New System.Drawing.Size(144, 144)

Me.panel1.TabIndex = 1

' button1

Me.button1.Location = New System.Drawing.Point(200, 176)

Me.button1.Name = "button1"

Me.button1.Size = New System.Drawing.Size(88, 24)

Me.button1.TabIndex = 2

Me.button1.Text = "Open"

' button2

Me.button2.Location = New System.Drawing.Point(200, 216)

Me.button2.Name = "button2"

Me.button2.Size = New System.Drawing.Size(88, 24)

Me.button2.TabIndex = 2

Me.button2.Text = "Save"

' button3

Me.button3.Location = New System.Drawing.Point(200, 256)

Me.button3.Name = "button3"

Me.button3.Size = New System.Drawing.Size(88, 24)

Me.button3.TabIndex = 2

Me.button3.Text = "Exit"

' timer1

Me.timer1.Enabled = True

Me.timer1.Interval = 250

' Form1

Me.AutoScaleBaseSize = New System.Drawing.Size(5, 13)

Me.ClientSize = New System.Drawing.Size(328, 325)

Me.Controls.Add(button1)

Me.Controls.Add(panel1)

Me.Controls.Add(pictureBox1)

Me.Controls.Add(button2)

Me.Controls.Add(button3)

Me.Name = "Form1"

Me.Text = "Form1"

Me.ResumeLayout(False)

End Sub 'InitializeComponent

#End Region

'/ <summary>

'/ The main entry point for the application.

'/ </summary>

<STAThread()> _

Shared Sub Main()

Application.Run(New Form1)

End Sub 'Main

Private Sub panel1_Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles panel1.Paint

If cp Is Nothing Then

Return

End If

Dim y As Single

For y = 0 To 15

Dim x As Single

For x = 0 To 15

Dim c As Color = Color.Black

If 16 * y + x < cp.Entries.Length Then

c = cp.Entries(CInt(16 * y + x))

End If

Dim sb As New SolidBrush(Color.FromArgb(255, c))

Dim w As Single = CSng(Me.panel1.Width) / 16

Dim h As Single = CSng(Me.panel1.Height) / 16

e.Graphics.FillRectangle(sb, w * x, h * y, w, h)

If c.A <> 255 Then

If showTrans Then

e.Graphics.DrawRectangle(Pens.Black, w * x, h * y, w - 1, h - 1)

Else

e.Graphics.DrawRectangle(Pens.White, w * x, h * y, w - 1, h - 1)

End If

sb.Dispose()

Next x

Next y

End Sub 'panel1_Paint

Private Sub panel1_MouseMove(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles panel1.MouseMove

Dim y As Integer = CInt(CSng(e.Y) / (CSng(Me.panel1.Width) / 16.0F))

Dim x As Integer = CInt(CSng(e.X) / (CSng(Me.panel1.Height) / 16.0F))

CurrentEntry = CInt(16 * y + x)

If Not (cp Is Nothing) Then

If CurrentEntry >= cp.Entries.Length Then

CurrentEntry = cp.Entries.Length - 1

End If 'Little bit of diagnostic for the palette chooser below

End If 'System.Diagnostics.Trace.WriteLine(string.Format("{0},{1}, adjusted={4},{5} entry={2} Colour={3}",e.X,e.Y,CurrentEntry,cp.Entries[CurrentEntry].ToString(),x,y));

End Sub 'panel1_MouseMove

Private Sub panel1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles panel1.Click

'Creates a new GIF image with a modified colour palette

If Not (cp Is Nothing) Then

'Create a new 8 bit per pixel image

Dim bm As New Bitmap(_gifImage.Width, _gifImage.Height, PixelFormat.Format8bppIndexed)

'get it's palette

Dim ncp As ColorPalette = bm.Palette

'copy all the entries from the old palette removing any transparency

Dim n As Integer = 0

Dim c As Color

For Each c In cp.Entries

ncp.Entries(n) = Color.FromArgb(255, c)

n += 1

Next c

'Set the newly selected transparency

ncp.Entries(CurrentEntry) = Color.FromArgb(0, cp.Entries(CurrentEntry))

're-insert the palette

bm.Palette = ncp

'now to copy the actual bitmap data

'lock the source and destination bits

Dim src As BitmapData = CType(_gifImage, Bitmap).LockBits(New Rectangle(0, 0, _gifImage.Width, _gifImage.Height), ImageLockMode.ReadOnly, _gifImage.PixelFormat)

Dim dst As BitmapData = bm.LockBits(New Rectangle(0, 0, bm.Width, bm.Height), ImageLockMode.WriteOnly, bm.PixelFormat)

If (True) Then

'steps through each pixel

Dim y As Integer

For y = 0 To _gifImage.Height - 1

Dim x As Integer

For x = 0 To _gifImage.Width - 1

'transferring the bytes

Marshal.WriteByte(dst.Scan0, dst.Stride * y + x, Marshal.ReadByte(src.Scan0, src.Stride * y + x))

Next x

Next y

End If

'all done, unlock the bitmaps

CType(_gifImage, Bitmap).UnlockBits(src)

bm.UnlockBits(dst)

'clear out the picturebox

Me.pictureBox1.Image = Nothing

_gifImage.Dispose()

'set the new image in place

_gifImage = bm

cp = _gifImage.Palette

Me.pictureBox1.Image = _gifImage

End If

End Sub 'panel1_Click

Private Sub button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles button1.Click

Dim dlg As New OpenFileDialog

dlg.Filter = "GIF files|*.GIF"

If dlg.ShowDialog() = DialogResult.OK Then

_gifImage = Image.FromFile(dlg.FileName)

Me.pictureBox1.Image = _gifImage

cp = _gifImage.Palette

Me.panel1.Invalidate()

End If

End Sub 'button1_Click

Private Sub button2_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles button2.Click

Dim dlg As New SaveFileDialog

dlg.Filter = "GIF files|*.gif"

dlg.DefaultExt = ".gif"

dlg.AddExtension = True

If dlg.ShowDialog() = DialogResult.OK Then

_gifImage.Save(dlg.FileName, ImageFormat.Gif)

End If

End Sub 'button2_Click

Private Sub button3_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles button3.Click

Application.Exit()

End Sub 'button3_Click

Private showTrans As Boolean

Private Sub timer1_Tick(ByVal sender As Object, ByVal e As System.EventArgs) Handles timer1.Tick

showTrans ^= True

Dim g As Graphics = Me.panel1.CreateGraphics()

'I do this rather than invalidate the panel because

'the panel draws its background ans so flickers horribly.

Dim pe As New PaintEventArgs(g, New Rectangle(0, 0, Me.panel1.Width, Me.panel1.Height))

Me.panel1_Paint(Me, pe)

g.Dispose()

End Sub 'timer1_Tick

End Class 'Form1

End Namespace 'TransparentGifCreator

Figure 2 shows the application in action.

Figure 2: Before and after changing the transparent colour.

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