允许多个线程访问图像 [英] Allow an Image to be accessed by several threads

问题描述

我正在尝试在C＃中进行一些图像处理。
我想使用一些线程在我的图像中的几个区域上进行并行计算。
线程实际上是在Bitmap对象中获取和设置像素。 2个线程绝对没有机会访问同一像素，所以这不是问题。

I'm trying to do some image processing in C#. I want to use some threads to do parallel computations on several zones in my image. Threads are actually getting and setting pixels in a Bitmap object. There is absolutely no chance for 2 threads to access the same pixel, so that's not the problem.

问题是C＃不允许我在同一个线程上启动多个线程Bitmap对象，即使我确定不会同时读取和修改相同的像素。

The problem is that C# doesnt allow me to start several threads on the same Bitmap object, even if i'm sure that the same pixel won't be read and modified simultaneously.

有没有办法避免C＃引发此错误？或者在我的Bitmap对象上运行多个线程是不可能的？

Is there any way to avoid C# to raise this error ? Or is it just impossible to run several threads on my Bitmap object ?

谢谢，

Pierre-Olivier

Pierre-Olivier

推荐答案

使用 LockBits （也比 GetPixel快得多& SetPixel ）您可以将图像的像素复制到缓冲区，在其上运行并行线程，然后将缓冲区复制回来。

Using LockBits (which is also much faster than GetPixel & SetPixel) you can copy the image's pixels to a buffer, run parallel threads on it, and then copy the buffer back.

这是一个有效的例子。

void Test()
{
    string inputFile = @"e:\temp\a.jpg";
    string outputFile = @"e:\temp\b.jpg";

    Bitmap bmp = Bitmap.FromFile(inputFile) as Bitmap;

    var rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
    var data = bmp.LockBits(rect, ImageLockMode.ReadWrite, bmp.PixelFormat);
    var depth = Bitmap.GetPixelFormatSize(data.PixelFormat) / 8; //bytes per pixel

    var buffer = new byte[data.Width * data.Height * depth];

    //copy pixels to buffer
    Marshal.Copy(data.Scan0, buffer, 0, buffer.Length);

    Parallel.Invoke(
        () => {
            //upper-left
            Process(buffer, 0, 0, data.Width / 2, data.Height / 2, data.Width, depth);
        },
        () => {
            //lower-right
            Process(buffer, data.Width / 2, data.Height / 2, data.Width, data.Height, data.Width, depth);
        }
    );

    //Copy the buffer back to image
    Marshal.Copy(buffer, 0, data.Scan0, buffer.Length);

    bmp.UnlockBits(data);

    bmp.Save(outputFile, ImageFormat.Jpeg);
}

void Process(byte[] buffer, int x, int y, int endx, int endy, int width, int depth)
{
    for (int i = x; i < endx; i++)
    {
        for (int j = y; j < endy; j++)
        {
            var offset = ((j * width) + i) * depth;
            // Dummy work    
            // To grayscale (0.2126 R + 0.7152 G + 0.0722 B)
            var b = 0.2126 * buffer[offset + 0] + 0.7152 * buffer[offset + 1] + 0.0722 * buffer[offset + 2];
            buffer[offset + 0] = buffer[offset + 1] = buffer[offset + 2] = (byte)b;
        }
    }
}

输入图片：

输出图片：

一些粗略的测试：

在双核2.1GHz机器上将（ 41兆像素，[7152x5368] ）图像转换为灰度

Converting a (41 MegaPixel, [7152x5368]) image to a gray scale on a dual core 2.1GHz machine

• GetPixel / SetPixel - 单核 - 131秒。


• LockBits - 单核 - 4.5秒。


• LockBits - 双核 - 3秒

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