Fast way of calculating differences between two bitmaps [duplicate]

Question

This question already has answers here:

Closed 9 years ago.

Possible Duplicate:
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?

Answer 1

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.

Answer 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.

Answer 3

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.)