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 why+3? } p += nOffset; --->what this line does? } b.UnlockBits(bmData);

shabonaa · Answer

p += 3; that is becuase u have a byte pointer and u use 24bpprgb that is stored in 3 byte for each pixel p+=nOffset; All bitmaps are word aligned and so there can be a difference between the size of a row and the number of pixels in it. This padding must be skipped if we try and access it we will not simply fail we will crash. We therefore calculate the offset we need to jump at the end of each row and store it as nOffset. you can refer to the www.thecodeproject.com there is article for Christian Graus with title  Image Processing for Dummies with C# and GDI+ Part 1 - Per Pixel Filters if u don't want to cut the region u may read the pixels in the region and store them in a new bitmap

erichood · Answer

for(int x=0; x why+3? } Because you've defined the pointer as a byte but the bitmap as 24bpp which is 3 bytes. So each pixel uses 3 bytes. p += nOffset; --->what this line does? Each row is 640*3 bytes long. To get to the beginning of the next row from the x=110 you need to add (640-110)*3 to p.