Adding fade Animation to the scrollview

Here is the simple usage of mapping the values to get fade effect while scrolling, I am not going to write whole code. Here is the logic...

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView

{

    mPreviousTouchPoint = scrollView.contentOffset;

}

- (void)scrollViewDidScroll:(UIScrollView *)sender
{
if(animation == fadeIn)
{
        pageWidth = self.pageSize.size.width;
        page = floor((self.scrollView.contentOffset.x - pageWidth) / pageWidth) + 1;
     
                 //Mapping values from 0-1024(subview frame) to 0-1(alpha range)
        float NewMax = 1;
        float NewMin = 0;
        float OldMax = pageWidth*(page+1);
        float OldMin = pageWidth*page;
     
        int OldValue = mScrollView.contentOffset.x;//doubth
        float newValue = (((OldValue - OldMin) * (NewMax - NewMin)) / (OldMax - OldMin)) + NewMin;
     
               //If it is scrolling to right applying animation to next page
        page = page+1;
     
                 //If it is scrolling to left applying animation to previous page
        if(mPreviousTouchPoint.x > mScrollView.contentOffset.x)
            page = page - 1;
     
        if(page < 0 || page >= mNoOfPages)
           return;
     
                //Applying alpha to the subview, where mSubviewsArray contains all the subviews of a       //scrollview
         UIView *nextPage = [mSubviewsArray objectAtIndex:page];
        if(nextPage)
        {
          if(mPreviousTouchPoint.x > mScrollView.contentOffset.x)
                nextPage.alpha = 1-newValue;
            else
                nextPage.alpha = newValue;
        }
}
}

#Some Coolest thinks i have learnt while developing an application

1. Mapping between two range
   //NewValue = (((OldValue - OldMin) * (NewMax - NewMin)) / (OldMax - OldMin)) + NewMin

+(NSInteger)convertValue:(NSInteger)OldValue newRange:(NSRange)newRange oldRange:(NSRange)oldRange

{

    NSInteger NewValue = (((OldValue - oldRange.location) * (newRange.length - newRange.location)) / (oldRange.length - oldRange.location)) + newRange.location;

    return NewValue;

}

 2. To check if a point is inside a convex polygon
// wn_PnPoly(): winding number test for a point in a polygon
//      Input:   P = a point,
//               V[] = vertex points of a polygon V[n+1] with V[n]=V[0]
//      Return:  wn = the winding number (=0 only if P is outside V[])
//http://www.softsurfer.com/Archive/algorithm_0103/algorithm_0103.htm

// isLeft(): tests if a point is Left|On|Right of an infinite line.
//    Input:  three points P0, P1, and P2
//    Return: >0 for P2 left of the line through P0 and P1
//            =0 for P2 on the line
//            <0 for P2 right of the line
//    See: the January 2001 Algorithm "Area of 2D and 3D Triangles and Polygons"
inline int isLeft( point P0, point P1, point P2 )
{
    return ( (P1.mX - P0.mX) * (P2.mY - P0.mY)
            - (P2.mX - P0.mX) * (P1.mY - P0.mY) );
}

FFBool isPointInsidePolyGon( point P, point* V, int n )
{
    int    wn = 0;    // the winding number counter

  // loop through all edges of the polygon
    for (int i=0; i<n; i++) {   // edge from V[i] to V[i+1]
        if (V[i].mY <= P.mY) {         // start y <= P.y
            if (V[i+1].mY > P.mY)      // an upward crossing
                if (isLeft( V[i], V[i+1], P) > 0)  // P left of edge
                    ++wn;            // have a valid up intersect
        }
        else {                       // start y > P.y (no test needed)
            if (V[i+1].mY <= P.mY)     // a downward crossing
                if (isLeft( V[i], V[i+1], P) < 0)  // P right of edge
                    --wn;            // have a valid down intersect
        }
    }
    return wn;
}