Parallel.For statement return "System.InvalidOperationException" with a Bitmap Processing

Question

Well, I have a code to apply a Rain Bow filter in "x" image, I have to do in two ways: Sequential & parallel, my sequential code is working without problems, but the parallel section doesn't work. And I have no idea, why?.

Code

public static Bitmap RainbowFilterParallel(Bitmap bmp)
    {

        Bitmap temp = new Bitmap(bmp.Width, bmp.Height);
        int raz = bmp.Height / 4;

        Parallel.For(0, bmp.Width, i =>
        {
            Parallel.For(0, bmp.Height, x =>
            {

                if (i < (raz))
                {
                    temp.SetPixel(i, x, Color.FromArgb(bmp.GetPixel(i, x).R / 5, bmp.GetPixel(i, x).G, bmp.GetPixel(i, x).B));
                }
                else if (i < (raz * 2))
                {
                    temp.SetPixel(i, x, Color.FromArgb(bmp.GetPixel(i, x).R, bmp.GetPixel(i, x).G / 5, bmp.GetPixel(i, x).B));
                }
                else if (i < (raz * 3))
                {
                    temp.SetPixel(i, x, Color.FromArgb(bmp.GetPixel(i, x).R, bmp.GetPixel(i, x).G, bmp.GetPixel(i, x).B / 5));
                }
                else if (i < (raz * 4))
                {
                    temp.SetPixel(i, x, Color.FromArgb(bmp.GetPixel(i, x).R / 5, bmp.GetPixel(i, x).G, bmp.GetPixel(i, x).B / 5));
                }
                else
                {
                    temp.SetPixel(i, x, Color.FromArgb(bmp.GetPixel(i, x).R / 5, bmp.GetPixel(i, x).G / 5, bmp.GetPixel(i, x).B / 5));
                }

            });

        });
        return temp;
    }

Besides, In a moments the program return the same error but says "The object is already in use".

PS. I'm beginner with c#, and I Searched this topic in another post and I found nothing.

Thank you very much in advance

Answer 1

As commenter Ron Beyer points out, using the SetPixel() and GetPixel() methods is very slow. Each call to one of those methods involves a lot of overhead in the transition between your managed code down to the actual binary buffer that the Bitmap object represents. There are a lot of layers there, and the video driver typically gets involved which requires transitions between user and kernel level execution.

But besides being slow, these methods also make the object "busy", and so if an attempt to use the bitmap (including calling one of those methods) is made between the time one of those methods is called and when it returns (i.e. while the call is in progress), an error occurs with the exception you saw.

Since the only way that parallelizing your current code would be helpful is if these method calls could occur concurrently, and since they simply cannot, this approach isn't going to work.

On the other hand, using the LockBits() method is not only guaranteed to work, there's a very good chance that you will find the performance is so much better using LockBits() that you don't even need to parallelize the algorithm. But should you decide you do, because of the way LockBits() works — you gain access to a raw buffer of bytes that represents the bitmap image — you can easily parallelize the algorithm and take advantage of multiple CPU cores (if present).

Note that when using LockBits() you will be working with the Bitmap object at a level that you might not be accustomed to. If you are not already knowledgeable with how bitmaps really work "under the hood", you will have to familiarize yourself with the way that bitmaps are actually stored in memory. This includes understanding what the different pixel formats mean, how to interpret and modify pixels for a given format, and how a bitmap is laid out in memory (e.g. the order of rows, which can vary depending on the bitmap, as well as the "stride" of the bitmap).

These things are not terribly hard to learn, but it will require patience. It is well worth the effort though, if performance is your goal.