How do I get a .NET Bitmap and an OpenCV image to point to the same chunk of memory?

Question

How do I get a .NET Bitmap and an OpenCV image to point to the same chunk of memory? I know that I can copy from one to the other, but I would prefer that they just point to the same pixels.

Bonus points for a solution using Emgu.

Answer 1

Following is just a theory and might not work, I'm not very experienced with opencv/emgucv

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Both System.Drawing.Bitmap and Emgu.CV.Image have constructors taking scan0 as an argument. You can allocate memory for image and pass this pointer to that constructors. memory can be allocated by var ptr = Marshal.AllocHGlobal(stride*height) (dont forget to free ofc) or by allocating a managed array of sufficent size and aquiring its address. Address can be aquired following way:

var array = new byte[stride*height];
var gch = GCHandle.Alloc(array, GCHandleType.Pinned);
var ptr = gch.AddrOfPinnedObject();

This also "pins" array, so it cannot be moved by garbage collector and address won't change.

We are going to use these constructors:

Bitmap(int width, int height, int stride, PixelFormat format, IntPtr scan0);
Image<Bgr, Byte>(int width, int height, int stride, IntPtr scan0);

width and height arguments are self-explanatory. format for bitmap is PixelFormat.Format24bppRgb. stride is amount of bytes for single line of image, it also must be aligned (be a multiple of 4). You can get it this way:

var stride = (width * 3);
var align = stride % 4;
if(align != 0) stride += 4 - align;

And scan0 is a pointer to our memory block.

So I suggest that after creating System.Drawing.Bitmap and Emgu.CV.Image using these constructors will use the same memory block. If not, it means that opencv or Bitmap or both copy provided memory block and possible solution (if it exists) is definetly not worth efforts.

Answer 2

From the Emgu.CV source:

// Summary:
//     The Get property provide a more efficient way to convert Image<Gray, Byte>,
//     Image<Bgr, Byte> and Image<Bgra, Byte> into Bitmap such that the image data
//     is shared with Bitmap. If you change the pixel value on the Bitmap, you change
//     the pixel values on the Image object as well! For other types of image this
//     property has the same effect as ToBitmap() Take extra caution not to use
//     the Bitmap after the Image object is disposed The Set property convert the
//     bitmap to this Image type.
public Bitmap Bitmap { get; set; }

So basically, for specific types of image, you can just use the .Bitmap property of the CV image, which will then point directly to the CV image data.

Example:

string filename;
// ... point filename to your file of choice ...

Image<Bgr, byte> cvImg  = new Image<Bgr, byte>(filename);
Bitmap           netBmp = cvImg.Bitmap;

Answer 3

Not sure if this helps, but you can get the pointer from an Emgu image using .Ptr

e.g. IntPtr ip= img1.Ptr;