Question

The problem is pretty basic.
(I am puzzled why the search didn’t find anything)

I have a rectangular “picture” that stores it’s pixel color line
after line in a std::vector

I want to copy a rectangular region out of that picture.

How would I elegantly code this in c++?

My first try:

 template <class  T>    std::vector<T> copyRectFromVector(const std::vector<T>& vec, std::size_t startx,  std::size_t starty, std::size_t endx, std::size_t endy, std::size_t fieldWidth, std::size_t fieldHeight)
    {
     using namespace std;
    vector<T> ret((endx-startx)*(endy-starty)+10);  // 10: chickenfactor

    // checks if the given parameters make sense:
    if (vec.size() < fieldWidth*endy)
    {
        cerr << "Error: CopyRectFromVector: vector to small to contain rectangular region!" << std::endl;
        return ret;
    }

    // do the copying line by line:
    vector<T>::const_iterator vecIt = vec.begin();
    vector<T>::forward_iterator retIt = ret.end();

    vecIt += startx + (starty*fieldWidth);
     for(int i=starty; i < endy; ++i)
    {
            std::copy(vecIt, vecIt + endx - startx, retIt);
        }
        return ret;
}

does not even compile…..

Addit: Clarification:
I know how to do this “by hand”. It is not a problem as such. But I would love some c++ stl iterator magic that does the same, but faster and… more c++ stylish.

Addition: I give the algorithm the pictureDataVector, the width and height of the picture and a rectangle denoting the region that I want to copy out of the picture.
The return value should be a new vector with the contents of the rectangle.

Think of it as opening your favorite image editor, and copy a rectangular region out of that.
The Picture is stored as a long 1D array(vector) of pixelcolors.

Answer 1

Your question asks for a C++ way of copying a rectangular field of elements in some container. You have a fairly close example of doing so and will get more in the answers. Let’s generalize, though:

You want an iterator that travels a rectangular range of elements over some range of elements. So, how about write a sort of adapter that sits on any container and provides this special iterator.

Gonna go broad strokes with the code here:

vector<pixels> my_picture;
point selTopLeft(10,10), selBotRight(40, 50);
int picWidth(640), picHeight(480);
rectangular_selection<vector<pixels> > selection1(my_picture.begin(),
  my_picture.end(), picWidth, picHeight, selTopLeft, selBotRight);

// Now you can use stl algorithms on your rectangular range
vector<pixels> rect_copy = std::copy(selection1.begin(), selection1.end());
// or maybe you don't want to copy, you want 
// to modify the selection in place
std::for_each (selection1.begin(), selection1.end(), invert_color);

I’m sure this is totally do-able, but I’m not comfortable coding stl-style template stuff off-the-cuff. If I have some time and you’re interested, I may re-edit a rough-draft later, since this is an interesting concept.

See this SO question’s answer for inspiration.