What causes "trace(): Empty instance." in Yalantis/uCrop?

Assign the image before calling trace(), e.g. img.assign(w,h,1,1) with real data or load from file.

Yalantis/uCrop · error · CImgInstanceException

trace(): Empty instance.

Error message

trace(): Empty instance.

What it means

CImg::trace() computes the trace of the image viewed as a square matrix (sum of diagonal elements (k,k)). It throws CImgInstanceException when the image instance is empty (no pixel data), because a trace is undefined for a zero-sized matrix.

Solutions

Assign the image before calling trace(), e.g. img.assign(w,h,1,1) with real data or load from file.
Check img.is_empty() before calling trace().
If the source may be an unloaded file, verify the load succeeded (img.width()>0) before matrix operations.

Example fix

// before
CImg<float> m; // default constructed, empty
double t = m.trace();
// after
CImg<float> m(3,3,1,1, data3x3);
if (!m.is_empty()) double t = m.trace();

Defensive patterns

Strategy: validation

Validate before calling

if (img.is_empty()) { /* handle: load failed or unassigned */ } else double t = img.trace();

Type guard

bool usableMatrix = !img.is_empty();

Try / catch

try { double t = img.trace(); } catch (CImgInstanceException& e) { std::cerr << e.what() << '\n'; }

Prevention

Check is_empty() after any load() call before doing math
Never do matrix ops on default-constructed CImg objects
Assign matrices with explicit dimensions and data

When it happens

Trigger: Calling img.trace() on a default-constructed CImg (CImg<T> img;), on an image after assign() with zero dimensions, or on a CImg that failed to load a file and remained empty.

Common situations: Loading an image file that does not exist or fails to decode, then doing matrix math on the result; building a matrix dynamically and forgetting the assign() step; using a moved-from CImg instance.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/5ef7fa90aa297136. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:32566

        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))
        for (longT off = 0; off<(longT)siz; ++off)
          res+=(double)std::pow((double)cimg::abs(_data[off]),(double)magnitude_type);
        res = (double)std::pow(res,1.0/magnitude_type);
      }
      return res;
    }

    //! Compute the trace of the image, viewed as a matrix.
    /**
     **/
    double trace() const {
      if (is_empty())
        throw CImgInstanceException(_cimg_instance
                                    "trace(): Empty instance.",
                                    cimg_instance);
      double res = 0;
      cimg_forX(*this,k) res+=(double)(*this)(k,k);
      return res;
    }

    //! Compute the determinant of the image, viewed as a matrix.
    /**
     **/
    double det() const {
      if (is_empty() || _width!=_height || _depth!=1 || _spectrum!=1)
        throw CImgInstanceException(_cimg_instance
                                    "det(): Instance is not a square matrix.",
                                    cimg_instance);

      switch (_width) {
      case 1 : return (double)((*this)(0,0));

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