Question

Here is what I’m doing. My application takes points from the user while dragging and in real time displays a filled polygon.

It basically adds the mouse position on MouseMove. This point is a USERPOINT and has bezier handles because eventually I will do bezier and this is why I must transfer them into a vector.

So basically MousePos -> USERPOINT. USERPOINT gets added to a std::vector<USERPOINT> . Then in my UpdateShape() function, I do this:

DrawingPoints is defined like this:

std::vector<std::vector<GLdouble>> DrawingPoints;


Contour[i].DrawingPoints.clear();



 for(unsigned int x = 0; x < Contour[i].UserPoints.size() - 1; ++x)
         SetCubicBezier(
             Contour[i].UserPoints[x],
             Contour[i].UserPoints[x + 1],
             i);

SetCubicBezier() currently looks like this:

void OGLSHAPE::SetCubicBezier(USERFPOINT &a,USERFPOINT &b, int &currentcontour )
{
std::vector<GLdouble> temp(2);

    if(a.RightHandle.x == a.UserPoint.x && a.RightHandle.y == a.UserPoint.y 
        && b.LeftHandle.x == b.UserPoint.x && b.LeftHandle.y == b.UserPoint.y )
    {

        temp[0] = (GLdouble)a.UserPoint.x;
        temp[1] = (GLdouble)a.UserPoint.y;

        Contour[currentcontour].DrawingPoints.push_back(temp);

        temp[0] = (GLdouble)b.UserPoint.x;
        temp[1] = (GLdouble)b.UserPoint.y;


        Contour[currentcontour].DrawingPoints.push_back(temp);

    }
    else
    {
         //do cubic bezier calculation
        }

So for the reason of cubic bezier, I need to make USERPOINTS into GlDouble[2] (since GLUTesselator takes in a static array of double.

So I did some profiling. At ~ 100 points, the code:

 for(unsigned int x = 0; x < Contour[i].UserPoints.size() - 1; ++x)
         SetCubicBezier(
             Contour[i].UserPoints[x],
             Contour[i].UserPoints[x + 1],
             i);

Took 0 ms to execute. then around 120, it jumps to 16ms and never looks back. I’m positive this is due to std::vector. What can I do to make it stay at 0ms. I don’t mind using lots of memory while generating the shape then removing the excess when the shape is finalized, or something like this.

Answer 1

0ms is no time…nothing executes in no time. This should be your first indicator that you might want to check your timing methods over timing results.

Namely, timers typically don’t have good resolution. Your pre-16ms results are probably just actually 1ms – 15ms being incorrectly reported at 0ms. In any case, if we could tell you how to keep it at 0ms, we’d be rich and famous.

Instead, find out which parts of the loop take the longest, and optimize those. Don’t work towards an arbitrary time measure. I’d recommend getting a good profiler to get accurate results. Then you don’t need to guess what’s slow (something in the loop), but can actually see what part is slow.