C Buffer to C# Bitmap Object

Question

I have a buffer (uint8[] of BGR pixel data) in C holding a video frame. A pointer to this buffer is passed back by the C code to C# code as an IntPtr. I require to add a text overlay to the each frame and then pass on a pointer to the frame for further processing. I believe what I need to do (in C#) is to copy each frame to a bitmap object, get the device context of the bitmap and use then use TextOut (etc) to write text to the bitmap. I would then copy the modified bitmap frame data back to my original array.

My question is twofold:

1. Is this the best approach?
2. What is the best (fastest) way to copy the data from my IntPtr to a bitmap object.

Thanks.

Answer 1

The fastest way is by not copying the data. That requires that your data is in a supported pixel format, BGR sounds a bit scary but odds are high it is actually PixelFormat.Format24bppRgb.

Which then allows you to use the Bitmap(int, int, int, PixelFormat, IntPtr constructor).

Answer 2

I can't comment on your approach, but the fastest way to copy data using two pointers would be to do a Platform Invoke call to the memcpy function in msvcrt.dll

Code example below, taken from the WriteableBitmapEx source

internal static class NativeMethods
{
    internal static unsafe void CopyUnmanagedMemory(byte* srcPtr, int srcOffset, 
                                        byte* dstPtr, int dstOffset, int count)
    {
        srcPtr += srcOffset;
        dstPtr += dstOffset;

        memcpy(dstPtr, srcPtr, count);
    }

    // Win32 memory copy function
    [DllImport("msvcrt.dll", EntryPoint = "memcpy", 
          CallingConvention = CallingConvention.Cdecl, SetLastError = false)]
    private static extern unsafe byte* memcpy(byte* dst, byte* src, int count);
}

To convert an IntPtr to byte* simply use

unsafe 
{
    IntPtr myPtr;
    byte* bytePtr = (byte*)myPtr.ToPointer();
}