CODEWITHSUNDEEP
c#c++computer-visionpinvokeunmanaged
Franz Gsell
Franz Gsell

Reputation: 1565

C# unsafe performance vs unmanaged PInvoke call

I am running an application which is working with bitmap images. Now I am looking for a fast way to swap the "Red" and "Blue" values of a "Format24bppRgb" bitmap image. In my C# code my first try was to use an unsafe code fragment:

var bmpData = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height),
    ImageLockMode.ReadWrite, bmp.PixelFormat);
unsafe
{
    byte* array = (byte*)bmpData.Scan0.ToPointer();
    byte temp;
    for (int x = 0; x < bmp.Width * bmp.Height * 3; x = x + 3) {
        temp = *(array + x + 2);
        *(array + x + 2) = *(array + x);
        *(array + x) = temp;
    }
}

for my used bitmap sizes this takes around 50-70ms. Now I tried to do the work in an external library (based on C++) with a pinvoke call:

[DllImport("ByteSwitch.dll")] 
public static extern IntPtr ChangeRB(IntPtr data, int width, int height);

data = ChangeRB(bmpData.Scan0, bmp.Width, bmp.Height);

Which is defined as follows:

extern "C" __declspec(dllexport) void* ChangeRB(void* xArray, int xHeight, int xWidth);

void* ChangeRB(void* array, int height, int width)
{
    unsigned char* _array = (unsigned char*)array;
    char temp;
    for (int x = 0; x < height * width * 3; x = x + 3)
    {
        temp = _array[x + 2];
        _array[x + 2] = _array[x];
        _array[x] = temp;
    }
    return _array;
}

and this call takes around 1ms! So I cannot explain the huge performance difference here - or is it really the case that the unmanaged pinvoke is so much faster than the "unsafe" code fragment?

Upvotes: 1

Views: 1002

Answers (1)

Simon Mourier
Simon Mourier

Reputation: 139316

The performance problem doesn't come from interop nor from C#, it comes from the fact you use bitmap's Width and Height in the loop. Both internally call a GDI Plus API:

public int Width {
    get {
        int width; 
 
        int status = SafeNativeMethods.Gdip.GdipGetImageWidth(new HandleRef(this, nativeImage), out width);
 
        if (status != SafeNativeMethods.Gdip.Ok)
            throw SafeNativeMethods.Gdip.StatusException(status);
 
        return width;
    }
}

And note you don't do this in the C/C++ case... you pass precomputed height and width. So, if you change the C# version for this:

unsafe
{
    byte* array = (byte*)bmpData.Scan0.ToPointer();
    byte temp;
    var max = bmp.Width * bmp.Height * 3;
    for (int x = 0; x < max; x = x + 3) {
        temp = *(array + x + 2);
        *(array + x + 2) = *(array + x);
        *(array + x) = temp;
    }
}

It may run globally even faster. You can also use a safe version like this:

private static void ChangeSafe(Bitmap bmp, BitmapData bmpData)
{
    var array = bmpData.Scan0;
    byte temp;
    var max = bmp.Width * bmp.Height * 3;
    for (var x = 0; x < max; x = x + 3)
    {
        temp = Marshal.ReadByte(array + x + 2);
        Marshal.WriteByte(array + x + 2, Marshal.ReadByte(array + x));
        Marshal.WriteByte(array + x, temp);
    }
}

It's marginally slower but avoids the need for unsafe code.

Upvotes: 6

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