Allow an Image to be accessed by several threads

Question

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

Answer 1

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.

Here is a working example.

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;
        }
    }
}

Input Image:

Output Image:

Some rough tests:

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

• GetPixel/SetPixel - Single Core - 131 sec.
• LockBits - Single Core - 4.5 sec.
• LockBits - Dual Core - 3 sec.