使用 Lockbits 更改 alpha 系数

Question

我写了一个函数来改变图像的 alpha 系数。我使用 setpixel 和 getpixel，这非常慢。我发现 Lockbits 方法更快。我怎样才能用 Lockbits 做到这一点？ 这是我当前的代码：

 using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Drawing.Imaging;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        private static Image Tran(Image s,int alpha)
        {
            int x = 0, y = 0;
            Bitmap tImage = new Bitmap(s);
            for (x = 0; x < tImage.Width; x++)
            {
                for (y = 0; y < tImage.Height; y++)
                {

                    tImage.SetPixel(x, y, Color.FromArgb(alpha, tImage.GetPixel(x, y).R, tImage.GetPixel(x, y).G, tImage.GetPixel(x, y).B));
                }
            }
            return tImage;

        }
        public Form1()
        {
            InitializeComponent();
            trackBar1.TickStyle = TickStyle.Both;
            trackBar1.Orientation = Orientation.Vertical;
            trackBar1.Minimum = 0;
            trackBar1.Maximum = 255;
            trackBar1.Height = 101;
            trackBar1.Value = 255;
            pictureBox1.Image = Image.FromFile("C:\\Users\\rati\\Desktop\\water.jpg");
            pictureBox1.SizeMode = PictureBoxSizeMode.StretchImage;
        }
        private void trackBar1_Scroll(object sender, EventArgs e)
        {
            pictureBox1.Image = ChangeAlpha(pictureBox1.Image, trackBar1.Value);

            textBox1.Text = trackBar1.Value.ToString();

        }


    }
}

Answer 1

您可以更改图像的不透明度，方法是使用新的ColorMatrix 在新位图中绘制图像，并为其Matrix33 分配一个介于 0 和 1 之间的浮点值作为其新的 alpha 值：

public Image ChangeAlpha(Image img, int value)
{
    if (value < 0 || value > 255)
        throw new Exception("value must be between 0 and 255");

    Bitmap bmp = new Bitmap(img.Width, img.Height); // Determining Width and Height of Source Image
    Graphics graphics = Graphics.FromImage(bmp);
    ColorMatrix colormatrix = new ColorMatrix();
    colormatrix.Matrix33 = value / 255f;
    ImageAttributes imgAttribute = new ImageAttributes();
    imgAttribute.SetColorMatrix(colormatrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);
    graphics.DrawImage(img, new Rectangle(0, 0, bmp.Width, bmp.Height), 0, 0, img.Width, img.Height, GraphicsUnit.Pixel, imgAttribute);
    graphics.Dispose();   // Releasing all resource used by graphics 
    return bmp;
}

这是一个示例用法：

private void button1_Click(object sender, EventArgs e)
{
    int opacityvalue = 127;
    var img = ChangeAlpha(Image.FromFile(@"d:\1.png"), opacityvalue);
    img.Save(@"d:\2.png");
}

别忘了添加using System.Drawing; 和using System.Drawing.Imaging;。

你可以在下面看到value=127调用函数之前和之后：

编辑

如果您想在PictureBox 中查看结果，请注意使用 2 个不同的图片框，一个用于原始图像，一个用于更改图像：

private void trackBar1_Scroll(object sender, EventArgs e)
{
    this.pictureBox2.Image = ChangeAlpha(this.pictureBox1.Image, this.trackBar1.Value);
}

正如我在您的代码中看到的那样，当您更改不透明度时，您再次将结果设置为 PictureBox1 的图像并再次对结果应用不透明度。换句话说，您将不透明度应用到您一遍又一遍地对其应用不透明度的图像上。

Answer 2

public Form1()
{
    ...
    originalImage = new Bitmap("C:\\Users\\rati\\Desktop\\water.jpg");
    pictureBox1.Image = originalImage;
    ...
}

// Add an extra field to the Form class.
Bitmap originalImage;

void trackBar1_Scroll(object sender, EventArgs e)
{
    pictureBox1.Image = ChangeAlpha((byte)trackBar1.Value);
    textBox1.Text = trackBar1.Value.ToString();
}

Image ChangeAlpha(byte alpha)
{
    Bitmap bmp = new Bitmap(originalImage);

    Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
    BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.ReadWrite, bmp.PixelFormat);

    IntPtr ptr = bmpData.Scan0;

    int bytes = Math.Abs(bmpData.Stride) * bmp.Height;
    byte[] rgbValues = new byte[bytes];

    Marshal.Copy(ptr, rgbValues, 0, bytes);

    // Set every fourth value to alpha. A 32bpp bitmap will change transparency.
    for (int counter = 3; counter < rgbValues.Length; counter += 4)
        rgbValues[counter] = alpha;

    // Also you can try parallelize loop.
    //int length = rgbValues.Length / 4;
    //Parallel.For(3, length, counter => rgbValues[counter * 4 - 1] = alpha);

    Marshal.Copy(rgbValues, 0, ptr, bytes);
    bmp.UnlockBits(bmpData);

    return bmp;
}