Yalantis/uCrop · error · CImgInstanceException

atNX(): Empty instance.

Error message

atNX(): Empty instance.

What it means

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

Source

Thrown at ucrop/src/main/jni/CImg.h:65177

    }

    //! Access to pixel value with Dirichlet boundary conditions for the 2 coordinates (\c pos,\c x) \const.
    T atNX(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].atX(x,y,z,c,out_value);
    }

    //! Access to pixel value with Neumann boundary conditions for the 2 coordinates (\c pos, \c x).
    /**
       \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& atNX(const int pos, const int x, const int y=0, const int z=0, const int c=0) {
      if (is_empty())
        throw CImgInstanceException(_cimglist_instance
                                    "atNX(): Empty instance.",
                                    cimglist_instance);

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

    //! Access to pixel value with Neumann boundary conditions for the 2 coordinates (\c pos, \c x) \const.
    T atNX(const int pos, const int x, const int y=0, const int z=0, const int c=0) const {
      if (is_empty())
        throw CImgInstanceException(_cimglist_instance
                                    "atNX(): Empty instance.",
                                    cimglist_instance);

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

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

View on GitHub (pinned to f788b534b4)

Solutions

1. Check list.is_empty() before the write loop.
2. Verify the list size matches expectations (e.g. >=1 image) before indexing.
3. Populate or re-load the list if the earlier load failed silently.
4. Catch CImgInstanceException around batch writes if input can legitimately be empty.

Example fix

// before
CImgList<T> list; // never filled
list.atNX(0, x) = value;
// after
if (list.size() > 0) {
  list.atNX(0, x) = value;
}

// C++
if (list.size() > 0) { /* atNX write is safe */ }

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

// C++
try {
  list.atNX(p, x) = v;
} catch (CImgInstanceException& e) {
  // empty list handling
}

When it happens

Trigger: Calling list.atNX(p,x) to write a pixel on a CImgList whose is_empty() is true (no images ever inserted or all removed).

Common situations: Row-based pixel processing on lists that failed to populate; mutating code paths downstream of a conditional load; lists emptied by filters before the write loop.

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.
• atNXY(): 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/5f83611a7603403c. Report an issue: GitHub.