```
#define CV_DESCALE(x,n) (((x) + (1 << ((n)-1))) >> (n))
float a = prevPt.x - iprevPt.x;
float b = prevPt.y - iprevPt.y;
const int W_BITS = 14, W_BITS1 = 14;
const float FLT_SCALE = 1.f/(1 << 20);
int iw00 = cvRound((1.f - a)*(1.f - b)*(1 << W_BITS));
int iw01 = cvRound(a*(1.f - b)*(1 << W_BITS));
int iw10 = cvRound((1.f - a)*b*(1 << W_BITS));
int iw11 = (1 << W_BITS) - iw00 - iw01 - iw10;
……
for( ; x < winSize.width*cn; x++, dsrc += 2, dIptr += 2 )
{
int ival = CV_DESCALE(src[x]*iw00 + src[x+cn]*iw01 +
src[x+stepI]*iw10 + src[x+stepI+cn]*iw11, W_BITS1-5);
int ixval = CV_DESCALE(dsrc[0]*iw00 + dsrc[cn2]*iw01 +
dsrc[dstep]*iw10 + dsrc[dstep+cn2]*iw11, W_BITS1);
int iyval = CV_DESCALE(dsrc[1]*iw00 + dsrc[cn2+1]*iw01 + dsrc[dstep+1]*iw10 +
dsrc[dstep+cn2+1]*iw11, W_BITS1);
Iptr[x] = (short)ival;
dIptr[0] = (short)ixval;
dIptr[1] = (short)iyval;
iA11 += (itemtype)(ixval*ixval);
iA12 += (itemtype)(ixval*iyval);
iA22 += (itemtype)(iyval*iyval);
}
```

I encountered a question while studying the **LK sparse optical flow algorithm** in OpenCV and reading the source code. In the code snippet, the coefficient iw is left-shifted by **W_BITS units (14)** for convenience in calculation. However, in the for loop, the value of ival is scaled and rounded using **CV_DESCALE**. I don’t understand why the second parameter is **W_BITS - 5 (9)**, and why do we need to subtract 5?