C# Opacity Of Picturebox

This article should help you:

http://howtoideas.net/how-to-set-image-opacity-using-c[^]

Abhinav is right.

But look at this problem from the other hand: in very many cases you don't need to use PictureBox at all. Good chances are, this is your case, too. Let me put this is this way: whenever using PictureBox makes sense, you don't need transparency. Whenever you need transparency, you don't need a PictureBox at all.

Find the explanation of what to do in my past answer here:
How do I clear a panel from old drawing[^].

Now, you will achieve transparency simply by modifying alpha channel in your colors. To make it effective you need to use System.Drawing.Bitmap.LockBits and either Marshall or unsafe (using pointers), see http://msdn.microsoft.com/en-us/library/system.drawing.bitmap.lockbits.aspx[^].

—SA

You may use this code block to perform image opacity on an pictureBox control in windows

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
namespace WinTest
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            Bitmap pic = new Bitmap(WinTest.Properties.Resources.untitled);
            for (int w = 0; w < pic.Width; w++)
            {
                for (int h = 0; h < pic.Height; h++)
                {
                    Color c = pic.GetPixel(w, h);
                    Color newC = Color.FromArgb(50, c);
                    pic.SetPixel(w, h, newC);
                }
            }
            pictureBox1.Image = pic;
        }

    }
}

As yogiCsharp said:

private void Opacity(ref Bitmap image)
        {
            for (int w = 0; w < image.Width; w++)
            {
                for (int h = 0; h < image.Height; h++)
                {
                    Color c = image.GetPixel(w, h);
                    if (c != Color.Transparent) /*<- it only change colours different than transparency color.*/
                    {
                        Color newC = Color.FromArgb(c.A/2, c.R/2,c.G/2,c.B); /*<- this gives real opacity.*/
                        image.SetPixel(w, h, newC);
                    }
                }
            }
        }

As you can see, in the Color NewC line, I'm using half Alpha (A channel of the pixel), half Red (R channel of the image), half Green (G channel of the image), half Blue (B Channel of the image).
Also is better, so you can put the opacity in percetange on the oppacity parameter:

private void Opacity(ref Bitmap image, int opacity)
        {
            for (int w = 0; w < image.Width; w++)
            {
                for (int h = 0; h < image.Height; h++)
                {
                    Color c = image.GetPixel(w, h);
                    if (c != Color.Transparent) /*<- it only change colours different than transparency color.*/
                    {
                        Color newC = Color.FromArgb(c.A*(opacity/100), c.R*(opacity/100),c.G*(opacity/100),c.B*(opacity/100)); /*<- this gives real opacity.*/
                        image.SetPixel(w, h, newC);
                    }
                }
            }
        }

Sorry for my english Spanish native speacker.