Void colorReduce (Mat image, int div=64) 
{
Int n=static_cast & lt; Int> (log (static_cast & lt; Double> (div))/log (2.0) + 0.5); 
Uchar mask=0 XFF & lt; Uchar div2=div & gt;> 1; 
//the iterator 
Mat_ & lt; Vec3d & gt; : : iterator it=image. BeginMat_ & lt; Vec3d & gt; : : iterator it_end=image. EndFor (; it !=it_end; + + it) 
{
for (int i=0; I & lt; 3; I++) 
{
(* it) [I] & amp;=mask; 
//error: here expressions must have integral or not distinguish range enum type 
(* it) [I] +=div2; 
} 
} 
}