Yalantis/uCrop · error · CImgInstanceException

atN(): Empty instance.

Error message

atN(): Empty instance.

What it means

Non-const CImgList<T>::atN(pos,x=0,y=0,z=0,c=0) accesses a writable pixel of the image at index pos, with all of x,y,z,c defaulting to 0 (Neumann boundary clamping on pos). It throws CImgInstanceException when the list instance is empty.

Source

Thrown at ucrop/src/main/jni/CImg.h:65232

    }

    //! Access to pixel value with Dirichlet boundary conditions for the coordinate (\c pos) \const.
    T atN(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:(*this)(pos,x,y,z,c);
    }

    //! Return pixel value with Neumann boundary conditions for the coordinate (\c pos).
    /**
       \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& atN(const int pos, const int x=0, const int y=0, const int z=0, const int c=0) {
      if (is_empty())
        throw CImgInstanceException(_cimglist_instance
                                    "atN(): Empty instance.",
                                    cimglist_instance);
      return _atN(pos,x,y,z,c);
    }

    //! Return pixel value with Neumann boundary conditions for the coordinate (\c pos) \const.
    T atN(const int pos, const int x=0, const int y=0, const int z=0, const int c=0) const {
      if (is_empty())
        throw CImgInstanceException(_cimglist_instance
                                    "atN(): Empty instance.",
                                    cimglist_instance);
      return _atN(pos,x,y,z,c);
    }

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

View on GitHub (pinned to f788b534b4)

Solutions

1. Check list.is_empty() (or size()>0) before calling atN.
2. Ensure the load/insert path succeeded before pixel access.
3. If you only ever use one image, consider CImg<T> instead of CImgList and check the image is non-empty too.
4. Wrap in try/catch on CImgInstanceException where emptiness is an expected input.

Example fix

// before
CImgList<T> list = load(path); // may leave list empty on failure
list.atN(0) = 0;
// after
CImgList<T> list;
list.load(path);
if (!list.is_empty()) {
  list.atN(0) = 0;
}

// C++
if (list.is_empty()) return; // before list.atN(p)

template <typename T>
bool usable(const CImgList<T>& l) { return l.size() > 0; }

// C++
try {
  list.atN(0) = 0;
} catch (CImgInstanceException& e) {
  // list empty: load/reinit or skip
}

When it happens

Trigger: Calling list.atN(p) (often effectively list.atN(0)) on an empty CImgList to read/write pixel (0,0,0,0) of image p.

Common situations: Quick pixel probes on lists that were never loaded; code assuming at least one image exists after processing; lists emptied by prior remove/clear calls.

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

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/22e4e0a8f5278c91. Report an issue: GitHub.