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

Check img.is_empty() (or img.size()>0) before calling max()

Yalantis/uCrop · error · CImgInstanceException

_cimg_instance "max(): Empty instance."

Error message

_cimg_instance "max(): Empty instance."

What it means

Instance guard in CImg<T>::max(): the method must return a reference to the largest pixel, but the image is empty (no pixels allocated), so no maximum exists to reference. It is the standard empty-instance check shared by the min/max family.

Solutions

Check img.is_empty() (or img.size()>0) before calling max()
Fix the load/crop logic that yielded an empty image

Example fix

// before
float v = img.max();
// after
if (!img.is_empty()) { float v = img.max(); } else { /* handle empty */ }

Defensive patterns

Strategy: validation

Validate before calling

if (!img.is_empty()) { ... img.max() ... }

Try / catch

try { T v = img.max(); } catch (const CImgInstanceException& e) { /* handle empty image */ }

Prevention

Check load return state and is_empty() before normalization code
Avoid default-constructed images flowing into stats without guards

When it happens

Trigger: Calling img.max() on a default-constructed CImg or an image that became empty after assignment/crop/failed load.

Common situations: Computing normalization ranges on images that failed to load; optional overlay images never initialized; ROI cropping with empty intersection.

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

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/16eb408bc4479bf1. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:31807

      if (is_empty())
        throw CImgInstanceException(_cimg_instance
                                    "minabs(): Empty instance.",
                                    cimg_instance);
      const T *ptr_minabs = _data;
      T minabs_value = *ptr_minabs;
      cimg_for(*this,ptrs,T) {
        const T ma = cimg::abs(*ptrs);
        if (ma<minabs_value) { minabs_value = ma; ptr_minabs = ptrs; }
      }
      return *ptr_minabs;
    }

    //! Return a reference to the maximum pixel value.
    /**
     **/
    T& max() {
      if (is_empty())
        throw CImgInstanceException(_cimg_instance
                                    "max(): Empty instance.",
                                    cimg_instance);
      T *ptr_max = _data;
      T max_value = *ptr_max;
      cimg_for(*this,ptrs,T) if (*ptrs>max_value) max_value = *(ptr_max=ptrs);
      return *ptr_max;
    }

    //! Return a reference to the maximum pixel value \const.
    const T& max() const {
      if (is_empty())
        throw CImgInstanceException(_cimg_instance
                                    "max(): Empty instance.",
                                    cimg_instance);
      const T *ptr_max = _data;
      T max_value = *ptr_max;
      cimg_for(*this,ptrs,T) if (*ptrs>max_value) max_value = *(ptr_max=ptrs);
      return *ptr_max;

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