multithreading - C# Threaded image processing

Question

                    for (int x = 0; x < blockCountX; x++)
                    {
                        for (int y = 0; y < blockCountY; y++)
                        {
                            //get blocks from image to new image and send to threaded processor
                            imageBlocks[x, y] = image.Clone(clipRectangle, System.Drawing.Imaging.PixelFormat.Format8bppIndexed);
                            System.Threading.Thread t = new System.Threading.Thread(new System.Threading.ParameterizedThreadStart(ThreadedFromHeightMap));
                            t.Start(imageBlocks[x,y]);
                            clipRectangle.Offset(0, IMAGEBLOCKSIZE);
                        }
                        clipRectangle.Offset(IMAGEBLOCKSIZE, clipRectangle.Location.Y * -1);
                    }
                    break;
            }
        }
        private void ThreadedFromHeightMap(object Image)
        {
            Bitmap image = (Bitmap)Image;
            int width = image.Width;
            int height = image.Height;

            for (int x = 0; x < width; x++)
            {
                for (int y = 0; y < height; y++)
                {
                    map.Hexes.Add(new Point(x, y), new Hex(image.GetPixel(x, y).B));
                    //tempHexes.Enqueue(new Hex(image.GetPixel(x, y).B));
                }
            }
        }

I'm trying to take pixel data from a 2048 x 2048 8bpp grayscale heightmap and building a hex map with corresponding height values. I'm storing the hexes in a Dictionary collection. All told there's ~4 million hexes in the collection.

To do this efficiently I'm breaking the image up into 256 x 256 chunks and passing that image to another thread which will parse it and add the hexes to the collection. This is were it all goes haywire. Instead of one image with (0,0) as its top left corner, I now have 64 image chunks that have (0,0) as their top left corner. However I'm using the pixel location as an index for the dictionary. This crashes when the second thread tries to add another value with index (0,0).

How do I mitigate this problem? I've thought about building a class that just has an image member and a chunk number member and pass that to the thread so I can adjust the pixel location depending on what chunk the thread is working on but that seems less than optimal.

(I realize the Dictionary I'm using isn't thread safe. I've since fixed that.)

Answer 1

May I recommend a few things?

1. Forget about image.GetPixel(), which is horribly slow; work directly with the bitmap data, and the performance of your algorithm will improve by so much, that you will not need to run parallel threads to improve its efficiency. See MSDN: http://msdn.microsoft.com/en-us/library/system.drawing.imaging.bitmapdata.aspx

2. If you insist on parallel threads, make use of the threadpool, instead of spawning 64 threads. (See MSDN: http://msdn.microsoft.com/en-us/library/3dasc8as(v=vs.80).aspx)

3. If you insist on spawning many threads, do NOT spawn more threads than the cores of your CPU. I do not suppose you have 64 cores on your machine, do you?

4. If you insist on spawning many threads, you will, of course, need to pass the location of each tile to the thread, so that you know exactly where that tile should be placed when you reconstruct the big picture. That's not less than optimal, it is necessary.