Yalantis/uCrop · error · CImgInstanceException

atNXY(): Empty instance.

Error message

atNXY(): Empty instance.

What it means

Non-const CImgList<T>::atNXY(pos,x,y,z=0,c=0) accesses a writable pixel with Neumann boundary conditions on coordinates (pos,x,y) (z and c default to 0). It throws CImgInstanceException when the list instance is empty.

Source

Thrown at ucrop/src/main/jni/CImg.h:65122

    }

    //! Access to pixel value with Dirichlet boundary conditions for the 3 coordinates (\c pos, \c x,\c y) \const.
    T atNXY(const int pos, const int x, const int y, const int z, const int c, const T& out_value) const {
      return (pos<0 || pos>=width())?out_value:_data[pos].atXY(x,y,z,c,out_value);
    }

    //! Access to pixel value with Neumann boundary conditions for the 3 coordinates (\c pos, \c x,\c y).
    /**
       \param pos Index of the image element to access.
       \param x X-coordinate of the pixel value.
       \param y Y-coordinate of the pixel value.
       \param z Z-coordinate of the pixel value.
       \param c C-coordinate of the pixel value.
       \note <tt>list.atNXYZ(p,x,y,z,c);</tt> is equivalent to <tt>list[p].atXYZ(x,y,z,c);</tt>.
    **/
    T& atNXY(const int pos, const int x, const int y, const int z=0, const int c=0) {
      if (is_empty())
        throw CImgInstanceException(_cimglist_instance
                                    "atNXY(): Empty instance.",
                                    cimglist_instance);

      return _atNXY(pos,x,y,z,c);
    }

    //! Access to pixel value with Neumann boundary conditions for the 3 coordinates (\c pos, \c x,\c y) \const.
    T atNXY(const int pos, const int x, const int y, const int z=0, const int c=0) const {
      if (is_empty())
        throw CImgInstanceException(_cimglist_instance
                                    "atNXY(): Empty instance.",
                                    cimglist_instance);

      return _atNXY(pos,x,y,z,c);
    }

    T& _atNXY(const int pos, const int x, const int y, const int z=0, const int c=0) {
      return _data[cimg::cut(pos,0,width() - 1)].atXY(x,y,z,c);

View on GitHub (pinned to f788b534b4)

Solutions

1. Check list.is_empty() before any atNXY call.
2. Confirm the list-populating call succeeded and returned a nonzero size.
3. If editing a single image, extract it (or use a CImg directly) and check is_empty() on the image instead.
4. Catch CImgInstanceException if empty lists are a normal input condition.

Example fix

// before
CImgList<unsigned char> frames;
frames.atNXY(0, x, y) = 255;
// after
CImgList<unsigned char> frames;
if (!frames.load(path) || frames.is_empty()) return;
frames.atNXY(0, x, y, 0, 0) = 255;

// C++
if (list.is_empty()) return; // before atNXY writes

template <typename T>
bool writable(const CImgList<T>& l) { return !l.is_empty(); }

// C++
try {
  list.atNXY(p, x, y) = v;
} catch (CImgInstanceException& e) {
  // empty list: re-load or skip
}

When it happens

Trigger: Calling list.atNXY(p,x,y) on a CImgList with is_empty()==true, e.g. an unloaded or cleared list.

Common situations: 2D pixel edits on a list-based image set whose load failed; code after remove_image() removed all entries; lists returned empty by a filtering routine.

Understand the failure class

Background: EmptyResultError / "no results found": when an API or scraper succeeds but returns zero rows — this error's family across 9 libraries.

Related errors

• atNXYZC(): Empty instance.
• atNXYZ(): Empty instance.
• atNX(): Empty instance.
• atN(): Empty instance.
• cimg::mod(): Specified modulo value is 0.

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/0438533431a84373. Report an issue: GitHub.