Problem runing bitmap with Scan0 and System.IntPtr

Question

hi guys, i got a problem hope somebody can help me: i got b which is the bitmap (entire picture 640*480), but i just need to scan a little part from that bitmap, this dimension is 110*110, but the following code is not doing what i need! help me pls! note:i dont want to cut the region 110*110 to scan. BitmapData bmData = b.LockBits(new Rectangle(0, 0, 110, 110), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb); int stride = bmData.Stride; // bytes in a row 3*b.Width System.IntPtr Scan0 = bmData.Scan0; byte * p = (byte *)(void *)Scan0; //int nOffset = stride - b.Width*3; int nOffset = stride - 110*3; for(int y=0;y < 110;++y) { for(int x=0; x < 110; ++x ) { do something... p += 3; --->why+3? } p += nOffset; --->what this line does? } b.UnlockBits(bmData);