What causes "sort(): Invalid specified axis '%c' (should be { x | y | z | c })." in Yalantis/uCrop?

Pass exactly one of 'x','y','z','c' (lowercase)

Yalantis/uCrop · error · CImgArgumentException

sort(): Invalid specified axis '%c

Error message

sort(): Invalid specified axis '%c' (should be { x | y | z | c }).

What it means

CImg<T>::sort(axis) sorts pixel values along one of the four image axes identified by the characters 'x','y','z','c'. Any other axis character is rejected by CImgArgumentException at the switch default.

Solutions

Pass exactly one of 'x','y','z','c' (lowercase)
Validate/normalize the axis character before the call and reject invalid input early
Map indices 0-3 to 'x','y','z','c' if your config stores numeric axes

Example fix

// before
img.sort(userAxis); // userAxis == 'X'
// after
char ax = std::tolower(userAxis);
if (ax!='x' && ax!='y' && ax!='z' && ax!='c') throw std::invalid_argument("axis");
img.sort(ax, true);

Defensive patterns

Strategy: validation

Validate before calling

if (axis!='x' && axis!='y' && axis!='z' && axis!='c') throw std::invalid_argument("sort axis must be x, y, z or c");

Type guard

bool isValidAxis(char a) { return a=='x' || a=='y' || a=='z' || a=='c'; }

Try / catch

try { img.sort(axis, true); } catch (CImgArgumentException& e) { /* normalize axis or report invalid axis */ }

Prevention

Always pass lowercase axis chars
Whitelist axis chars when they come from config or CLI
Map numeric axes 0-3 to 'x','y','z','c' explicitly

When it happens

Trigger: Calling sort(axis, is_increasing) with axis not in {'x','y','z','c'} — e.g. 'X', 'w', 0, or a variable holding an unexpected char.

Common situations: Typo or case mismatch ('X' vs 'x'); passing an axis taken from user config or CLI without validation; confusion between axis chars and axis indices in other APIs.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/2f26ccd2926400fb. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:33347

        perm.assign(_height);
        get_crop(0,0,0,0,0,_height - 1,0,0).sort(perm,is_increasing);
        CImg<T> img(*this,false);
        cimg_forXYZC(*this,x,y,z,c) (*this)(x,y,z,c) = img(x,perm[y],z,c);
      } break;
      case 'z' : {
        perm.assign(_depth);
        get_crop(0,0,0,0,0,0,_depth - 1,0).sort(perm,is_increasing);
        CImg<T> img(*this,false);
        cimg_forXYZC(*this,x,y,z,c) (*this)(x,y,z,c) = img(x,y,perm[z],c);
      } break;
      case 'c' : {
        perm.assign(_spectrum);
        get_crop(0,0,0,0,0,0,0,_spectrum - 1).sort(perm,is_increasing);
        CImg<T> img(*this,false);
        cimg_forXYZC(*this,x,y,z,c) (*this)(x,y,z,c) = img(x,y,z,perm[c]);
      } break;
      default :
        throw CImgArgumentException(_cimg_instance
                                    "sort(): Invalid specified axis '%c' "
                                    "(should be { x | y | z | c }).",
                                    cimg_instance,axis);
      }
      return *this;
    }

    //! Sort pixel values \newinstance.
    CImg<T> get_sort(const bool is_increasing=true, const char axis=0) const {
      return (+*this).sort(is_increasing,axis);
    }

    template<typename t>
    CImg<T>& _quicksort(const long indm, const long indM, CImg<t>& permutations,
                        const bool is_increasing, const bool is_permutations) {
      if (indm<indM) {
        const long mid = (indm + indM)/2;
        if (is_increasing) {

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