Fast way of calculating differences between two bitmaps

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What is the fastest way I can compare two equal-size bitmaps to determine whether they are identical?

I'm trying to efficiently calculate the differences between two bitmaps and set any matching pixels black. I've tried this:

for (int x = 0; x < 1280; x++)
{
    for (int y = 0; y < 720; y++)
    {
        if (bitmap.GetPixel(x, y) == bitmap2.GetPixel(x, y))
        {
            bitmap2.SetPixel(x, y, Color.Black);
        }
    }
}

But it turns out that GetPixel and SetPixel are slow so this doesn't really work well enough. Anyone know an alternative (faster) way of doing this?

I have edited your title. Please see, "[Should questions include “tags” in their titles?](http://meta.stackexchange.com/questions/19190/)", where the consensus is "no, they should not". — John Saunders, Jan 08 '13 at 02:49
Sorry, my bad. At least I know now. Pretty sure I've done it in the past as well so oops! — Joey Morani, Jan 08 '13 at 02:55
@JoeyMorani: thanks for letting me know. I think I've got them all now. — John Saunders, Jan 08 '13 at 03:17

Andrew Marshall · Accepted Answer · 2013-01-08T03:21:07.287

This method uses unsafe code, assuming bitmaps are the same size and are 4 bytes per pixel.

Rectangle bounds = new Rectangle(0,0,bitmapA.Width,bitmapA.Height);
var bmpDataA = bitmapA.LockBits(bounds, ImageLockMode.ReadWrite, bitmapA.PixelFormat);
var bmpDataB = bitmapB.LockBits(bounds, ImageLockMode.ReadWrite, bitmapB.PixelFormat);

const int height = 720;
int npixels = height * bmpDataA.Stride/4;
unsafe {
    int * pPixelsA = (int*)bmpDataA.Scan0.ToPointer();
    int * pPixelsB = (int*)bmpDataB.Scan0.ToPointer();

    for ( int i = 0; i < npixels; ++i ) {
        if (pPixelsA[i] != pPixelsB[i]) {
             pPixelsB[i] = Color.Black.ToArgb();
        }
    }
}
bitmapA.UnlockBits(bmpDataA);
bitmapB.UnlockBits(bmpDataB);

For a safe method, copy the pixel data to an array buffer for processing using the InteropServices.Marshal.Copy methods.

score 2 · Answer 2 · edited May 23 '17 at 10:34

2

Raw bitmap data and LockBitmap.

http://www.codeproject.com/Tips/240428/Work-with-bitmap-faster-with-Csharp

Question (missing examples). What is the fastest way I can compare two equal-size bitmaps to determine whether they are identical?

Forget if you turn debug mode off the speed increase. abaut 10x but lockbit is still faster.

edited May 23 '17 at 10:34

Community

1
1

answered Jan 08 '13 at 02:48

score 1 · Answer 3 · answered Jan 08 '13 at 02:48

Almost sure this has been answered before. You should be using:

Bitmap.LockBits

Also accessing Width and Height (or the other properties with the same info) is also slow so copy them to a local variable if you want to use them in a loop (instead of say the 720 and 1280 in your example.)

Fast way of calculating differences between two bitmaps

3 Answers3