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.