Using pointer to array in unsafe C#

Question

In C, I can define a pointer to an array like this:

char b1[SOME_SIZE];
char (*b3)[3]=(char(*)[3])b1;

so that b3[i][j] == b1[i*3+j].

Can I declare such a pointer, b3, in unsafe C#?

My intention is to access bitmap channels as:

///...
unsafe {
    //...
    byte *target; //8bpp
    byte (*source)[3]; //24bpp
    //...
    target[x]=(source[x][0]+source[x][1]+source[x][2])/3;
    //...

I hope this way, using source[x][ch] instead of source[x*3+ch] to get some compiler optimization.

Hope-based optimization doesn't make any sense really. You can make a pointer byte*[] but it's probably not what you intend to have - look in the docs. — BartoszKP
– BartoszKP, Commented Dec 11, 2013 at 10:12
@BartoszKP Maybe you're right, I'm probably over-optimizing! My point is that in C/C++, well made declarations give often better performance (and legibility) against heavy code, as compiler can optimize former but not latter. — LS_ᴅᴇᴠ
– LS_ᴅᴇᴠ, Commented Dec 11, 2013 at 10:29
In C# this is the recommended way to work with bitmaps in terms of performance. — BartoszKP
– BartoszKP, Commented Dec 11, 2013 at 10:56
I know, but I'm trying to get a clearer code using pointer to array. — LS_ᴅᴇᴠ
– LS_ᴅᴇᴠ, Commented Dec 11, 2013 at 11:20
In your question you say that you want to "get some compiler optimization", not "get a clearer code". Make up your mind, you can't have both. — BartoszKP
– BartoszKP, Commented Dec 11, 2013 at 11:21

Erti-Chris Eelmaa · Accepted Answer · 2013-12-13 10:38:45Z

7

+25

[StructLayout(LayoutKind.Sequential, Pack = 1)]
struct FastPixel
{
    public readonly byte R;
    public readonly byte G;
    public readonly byte B;
}


private static void Main()
{
    unsafe
    {
        // 8-bit.
        byte[] b1 =
        {
            0x1, 0x2, 0x3,
            0x6, 0x7, 0x8,
            0x12, 0x13, 0x14
        };


        fixed (byte* buffer = b1)
        {
            var fastPixel = (FastPixel*) buffer;
            var pixelSize = Marshal.SizeOf(typeof (FastPixel));

            var bufferLength = b1.Length / pixelSize;
            for (var i = 0; i < bufferLength; i++)
            {
                Console.WriteLine("AVERAGE {0}", (fastPixel[i].R + fastPixel[i].G + fastPixel[i].B)/pixelSize);
            }
        }
    }
}

}

This should be pretty much identical what you have. Note that I don't expect any performance gains. This is not micro-optimization, it's nano-optimization.

If dealing with huge images, look into parallel programming & SSE and how cache lines work(they have saved me actually 3-4 seconds - crazy right?!)

answered Dec 13, 2013 at 10:38

Erti-Chris Eelmaa

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2 Comments

LS_ᴅᴇᴠ Over a year ago

Thanks, but that's what I'm actually using. I'm looking for an array pointer, so I don't even need do define a struct.

LS_ᴅᴇᴠ Over a year ago

About optimization gain: I will benchmark b1[x*3+ch] against fastpixel[x].R to check...

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