Yalantis/uCrop · error · CImgInstanceException

magnitude(): Empty instance.

Error message

magnitude(): Empty instance.

What it means

Instance guard in CImg<T>::magnitude(): the image is treated as a matrix/vector to compute its Lp-norm, which requires allocated pixel data; the instance is empty so the norm is undefined and the method refuses to proceed.

Solutions

1. Check is_empty() before calling magnitude()
2. Ensure the image was loaded/populated (size() > 0)
3. Fix upstream zero-size image creation
4. Catch CImgInstanceException if the empty case is expected

Example fix

// before
double m = img.magnitude(2);
// after
if (!img.is_empty()) {
  double m = img.magnitude(2);
}

if (img.is_empty()) throw std::runtime_error("image empty before magnitude()");

bool usable = !img.is_empty() && img.size() > 0;

try { double m = img.magnitude(2); } catch (const CImgInstanceException& e) { /* handle empty */ }

When it happens

Trigger: Calling CImg<T>::magnitude(magnitude_type) on an empty instance — default construction, zero dimensions, or a failed load/processing step.

Common situations: Gradient/edge-strength measurements on unloaded images; norm computations in pipelines where an earlier step yielded an empty buffer.

Related errors

• _cimg_instance "kth_smallest(): Empty instance."
• _cimg_instance "max_min(): Empty instance."
• _cimg_instance "maxabs(): Empty instance."
• _cimg_instance "median(): Empty instance."
• _cimg_instance "min_max(): Empty instance."

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/b95124ead0ecd8af. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:32535

    //@}
    //-------------------------------------
    //
    //! \name Vector / Matrix Operations
    //@{
    //-------------------------------------

    //! Compute norm of the image, viewed as a matrix.
    /**
       \param magnitude_type Can be:
       - \c 0: L0-norm
       - \c 1: L1-norm
       - \c 2: L2-norm
       - \c p>2 : Lp-norm
       - \c ~0U: Linf-norm
    **/
    double magnitude(const float magnitude_type=2) const {
      if (is_empty())
        throw CImgInstanceException(_cimg_instance
                                    "magnitude(): Empty instance.",
                                    cimg_instance);
      const ulongT siz = size();
      double res = 0;
      if (magnitude_type==2) { // L2
        cimg_pragma_openmp(parallel for reduction(+:res) cimg_openmp_if_size(size(),8192))
        for (longT off = 0; off<(longT)siz; ++off) res+=(double)cimg::sqr(_data[off]);
        res = (double)std::sqrt(res);
      } else if (magnitude_type==1) { // L1
        cimg_pragma_openmp(parallel for reduction(+:res) cimg_openmp_if_size(size(),8192))
        for (longT off = 0; off<(longT)siz; ++off) res+=(double)cimg::abs(_data[off]);
      } else if (!magnitude_type) { // L0
        cimg_pragma_openmp(parallel for reduction(+:res) cimg_openmp_if_size(size(),8192))
        for (longT off = 0; off<(longT)siz; ++off) res+=(double)(_data[off]?1:0);
      } else if (cimg::type<float>::is_inf(magnitude_type)) { // L-inf
        cimg_for(*this,ptrs,T) { const double val = (double)cimg::abs(*ptrs); if (val>res) res = val; }
      } else { // L-p
        cimg_pragma_openmp(parallel for reduction(+:res) cimg_openmp_if_size(size(),8192))

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