Yalantis/uCrop · error · CImgInstanceException

CImg<%s>::FFT(): Specified real part is empty.

Error message

CImg<%s>::FFT(): Specified real part is empty.

What it means

CImg::FFT(real, imag, axis, is_inverse) is a static two-image FFT: `real` holds the real component and must be a non-empty image. If real is empty (default-constructed or assigned()), the function throws CImgInstanceException immediately, before any FFT work.

Source

Thrown at ucrop/src/main/jni/CImg.h:47656

      \param is_inverse Tells if the forward (\c false) or inverse (\c true) FFT is computed.
    **/
    CImgList<Tfloat> get_FFT(const bool is_inverse=false) const {
      CImgList<Tfloat> res(*this,CImg<Tfloat>());
      CImg<Tfloat>::FFT(res[0],res[1],is_inverse);
      return res;
    }

    //! Compute 1D Fast Fourier Transform, along a specified axis.
    /**
       \param[in,out] real Real part of the pixel values.
       \param[in,out] imag Imaginary part of the pixel values.
       \param axis Axis along which the FFT is computed.
       \param is_inverse Tells if the forward (\c false) or inverse (\c true) FFT is computed.
    **/
    static void FFT(CImg<T>& real, CImg<T>& imag, const char axis, const bool is_inverse=false,
                    const unsigned int nb_threads=0) {
      if (!real)
        throw CImgInstanceException("CImg<%s>::FFT(): Specified real part is empty.",
                                    pixel_type());
      if (!imag) imag.assign(real._width,real._height,real._depth,real._spectrum,(T)0);
      if (!real.is_sameXYZC(imag))
        throw CImgInstanceException("CImg<%s>::FFT(): Specified real part (%u,%u,%u,%u,%p) and "
                                    "imaginary part (%u,%u,%u,%u,%p) have different dimensions.",
                                    pixel_type(),
                                    real._width,real._height,real._depth,real._spectrum,real._data,
                                    imag._width,imag._height,imag._depth,imag._spectrum,imag._data);
      const char _axis = cimg::lowercase(axis);
      if (_axis!='x' && _axis!='y' && _axis!='z')
        throw CImgArgumentException("CImgList<%s>::FFT(): Invalid specified axis '%c' for real and imaginary parts "
                                    "(%u,%u,%u,%u) "
                                    "(should be { x | y | z }).",
                                    pixel_type(),axis,
                                    real._width,real._height,real._depth,real._spectrum);
      cimg::unused(nb_threads);
#ifdef cimg_use_fftw3
      cimg::mutex(12);

View on GitHub (pinned to f788b534b4)

Solutions

1. Ensure `real` is a valid, non-empty image: construct with dimensions or load successfully before calling FFT.
2. Check `if (real)` (operator bool) before calling FFT and handle the empty case.
3. If the real part should be zero, assign it explicitly: real.assign(w,h,d,s,0).
4. Verify that the preceding load/crop that produced `real` succeeded.

Example fix

// before
CImg<float> real, imag;
CImg<float>::FFT(real, imag, 'x'); // real is empty -> throws
// after
CImg<float> real(img.width(), img.height(), img.depth(), img.spectrum(), 0);
real.copy(img);
CImg<float>::FFT(real, imag, 'x');

if (!real) { /* load failed or image empty: abort or assign dims */ }

bool fftRealReady(const CImg<T>& real) {
  return (bool)real && real.size() > 0;
}

try {
  CImg<T>::FFT(real, imag, 'x');
} catch (const CImgInstanceException& e) {
  // handle empty real part: load/assign before retrying
}

When it happens

Trigger: Calling CImg<T>::FFT(real, imag, 'x') where real was default-constructed, or real.assign() was called, or a failed load left the image empty (operator! true).

Common situations: Image load failure (missing file) silently yielding an empty CImg that is then passed to FFT; forgetting to size the real part; using a moved-from image; FFT in a processing chain downstream of a failed crop.

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

• CImgList<%s>::FFT(): Invalid specified axis '%c' for real an
• cimg::fopen(): Specified file path is (null).
• cimg::fopen(): File '%s', specified mode is (null).
• cimg::fread(): Invalid reading request of %u %s%s from file
• cimg::fwrite(): Invalid writing request of %u %s%s from buff

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/f875502f012e46e9. Report an issue: GitHub.