Memory is not freed after converting BitmapImage

Question

I got a problem with the following c# (test-)code:

    public static void TestBitmap2ByteArray(BitmapImage bitmap)
    {
        JpegBitmapEncoder encoder = new JpegBitmapEncoder();
        MemoryStream memstream = new MemoryStream();
        encoder.Frames.Add(BitmapFrame.Create(bitmap));
        encoder.Save(memstream);

        memstream.Close();
    }

Each time I call the function, memory is allocated and not freed again. In the real project, the method is called very often and the application runs out of memory.

This is a stripped down version of the code, not returning anything.

I am using Visual Studio2010 and .net 3.5 SP1.

Help is appreciated. Thank You.

Answer 1

As described in my other answer a better way to solve this is to access the bitmap data directly. BitmapImage inherits from BitmapSource. BitmapSource is great for this and also works with WPF binding.

I am using BitmapSource myself to manipulate images that are bound directly to WPF (MVVM style). Basically I create a region in memory and point BitmapSource to it. This allows me to read/write pixels directly to memory and invalidate the BitmapSource so that WPF redraws the image. I have a standard "Bitmap" object I use for this. The direct data access makes it super-fast. (Seriously, no problem at all modifying all bits in 4 images at 30fps .. haven't tried at higher speeds as it hasn't been required.)

Sample usage can be found on my blog. But basically you do this:

unsafe {
   byte* imgBytePtr = (byte*)myBitmap.ImageData;
   Int32* imgInt32Ptr = (Int32*)myBitmap.ImageData;
   int height = (int)myBitmap.BitmapSource.Height;
   int width = (int)myBitmap.BitmapSource.Width;
   int bpp = myBitmap.BytesPerPixel;

   // Note: No need to iterate just for copy. A Marshal.Copy() at this point can copy all the bytes into a byte-array if you want.
   // But the best would be if your application could do its work directly in the imgBytePtr[]-array.

   for (int x = 0; x < height; x++)
   {
      for (int y = 0; y < width; y++)
      {
         // Get bytes into RGBA values
         int bytePos = x * (width * bpp) + (y * bpp);
         byte R = imgBytePtr[bytePos + 0];
         byte B = imgBytePtr[bytePos + 1];
         byte G = imgBytePtr[bytePos + 2];
         byte A = imgBytePtr[bytePos + 3];

         // Alternatively get Int32 value of color
         int intPos = x * width + y;
         int intColor = imgIntPtr[intPos];


         // Examples of manipulating data         

         // Remove blue
         imgBytePtr[bytePos + 1] = 0;
         // Alternative remove blue by bitmask
         imgIntPtr[intPos] = imgIntPtr[intPos] & 0xFF00FFFF; 

      }
   }
}
// Now execute Invalidate() and WPF will automagically update bound picture object :)

This makes a BitmapSource, if you need BitmapImage instead you can see if you can change it to work.

    /// 
    /// This object holds a byte array of the picture as well as a BitmapSource for WPF objects to bind to. Simply call .Invalidate() to update GUI.
    /// 
    public class Bitmap : IDisposable
    {
        // some ideas/code borowed from CL NUI sample CLNUIImage.cs
        [DllImport("kernel32.dll", SetLastError = true)]
        static extern IntPtr CreateFileMapping(IntPtr hFile, IntPtr lpFileMappingAttributes, uint flProtect, uint dwMaximumSizeHigh, uint dwMaximumSizeLow, string lpName);
        [DllImport("kernel32.dll", SetLastError = true)]
        static extern IntPtr MapViewOfFile(IntPtr hFileMappingObject, uint dwDesiredAccess, uint dwFileOffsetHigh, uint dwFileOffsetLow, uint dwNumberOfBytesToMap);
        [DllImport("kernel32.dll", SetLastError = true)]
        static extern bool UnmapViewOfFile(IntPtr hMap);
        [DllImport("kernel32.dll", SetLastError = true)]
        static extern bool CloseHandle(IntPtr hHandle);

        private IntPtr _section = IntPtr.Zero;
        public IntPtr ImageData { get; private set; }
        public InteropBitmap BitmapSource { get; private set; }
        public int BytesPerPixel = 3;

        /// 
        /// Initializes an empty Bitmap
        /// 
        /// Image width
        /// Image height
        /// Image format
        public Bitmap(int width, int height, PixelFormat pixelFormat)
        {
            BytesPerPixel = pixelFormat.BitsPerPixel / 8;
            uint imageSize = (uint)width * (uint)height * (uint)BytesPerPixel;
            // create memory section and map
            _section = CreateFileMapping(new IntPtr(-1), IntPtr.Zero, 0x04, 0, imageSize, null);
            ImageData = MapViewOfFile(_section, 0xF001F, 0, 0, imageSize);
            BitmapSource = Imaging.CreateBitmapSourceFromMemorySection(_section, width, height, pixelFormat, width * BytesPerPixel, 0) as InteropBitmap;
        }

        /// 
        /// Invalidates the bitmap causing a redraw
        /// 
        public void Invalidate()
        {
            BitmapSource.Invalidate();
        }

        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        protected void Dispose(bool disposing)
        {
            if (disposing)
            {
                // free managed resources
            }
            // free native resources if there are any.
            if (ImageData != IntPtr.Zero)
            {
                UnmapViewOfFile(ImageData);
                ImageData = IntPtr.Zero;
            }
            if (_section != IntPtr.Zero)
            {
                CloseHandle(_section);
                _section = IntPtr.Zero;
            }
        }
    }

Answer 2

You are accessing the bitmap data by saving it to a stream. This causes overhead. A much better way to do it is by using LockBits. This will give you direct access to the bytes in the image and you can easily access it both as *byte[] and *UInt32[] (note: requires unsafe{}).

If you still need to copy the bytes to a byte array then Marshal.Copy can be used, but if your intent is to make modifications directly to the image you are free to do so. The LockBits link has a sample showing the use of Marshal.Copy.

If you need some samples of how to process picture I have released some sample code on image processing for the Kinect.