Yalantis/uCrop · error · CImgInstanceException

CImgList< >::FFT(): Specified real part (%u,%u,%u,%u,%p)…

Error message

CImgList<%s>::FFT(): Specified real part (%u,%u,%u,%u,%p) and imaginary part (%u,%u,%u,%u,%p) have different dimensions.

What it means

The static CImg<T>::FFT(real, imag) requires the real and imaginary parts to have identical width, height, depth and spectrum (is_sameXYZC). If imag was not empty (so it was not auto-zero-filled) but its dimensions differ from real, this CImgInstanceException is thrown, printing both shapes and data pointers. FFT pairs must be congruent complex arrays.

Solutions

Make imag the same size as real before calling FFT (imag.assign(real.width(), real.height(), real.depth(), real.spectrum(), (T)0) then copy).
Pass an empty (default-constructed) imag if you only have real data; it is auto-assigned to real's dimensions.
Resize/crop the smaller part to match the other (CImg::resize).
Verify with real.is_sameXYZC(imag) before calling FFT.

Example fix

// before
CImg<float> real(256,256), imag(128,128);
CImg<float>::FFT(real, imag); // dimension mismatch
// after
CImg<float> real(256,256), imag(256,256,1,1,0);
CImg<float>::FFT(real, imag);

Defensive patterns

Strategy: validation

Validate before calling

if (!real.is_sameXYZC(imag))
  imag.assign(real.width(), real.height(), real.depth(), real.spectrum(), (T)0); // zero-fill

Type guard

bool isCompatiblePair(const CImg<T>& r, const CImg<T>& i) { return i.is_empty() || r.is_sameXYZC(i); }

Try / catch

try { CImg<float>::FFT(real, imag); }
catch (CImgInstanceException& e) { /* log both shapes, re-assign imag to real's dims */ }

Prevention

Pass an empty imag when you only have real data (auto zero-filled)
After cropping/resizing one part, resize the other identically
Call is_sameXYZC() as an assertion before FFT

When it happens

Trigger: Calling CImg<T>::FFT(real, imag) where both images are non-empty but sized differently, e.g. real is 256x256 and imag is 128x128, or they differ only in spectrum/depth; also when a previous FFT's output (interleaved spectrum layout) is passed back with mismatched cropping.

Common situations: Loading real and imaginary parts from separate files of different sizes, cropping one part but not the other, or mixing results from images of different resolutions.

Related errors

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/53deb697bda4989e. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:47934

#endif
    }

    //! Compute n-D Fast Fourier Transform.
    /**
       \param[in,out] real Real part of the pixel values.
       \param[in,out] imag Imaginary part of the pixel values.
       \param is_inverse Tells if the forward (\c false) or inverse (\c true) FFT is computed.
       \param nb_threads Number of parallel threads used for the computation.
         Use \c 0 to set this to the number of available cpus.
    **/
    static void FFT(CImg<T>& real, CImg<T>& imag, const bool is_inverse=false,
                    const unsigned int nb_threads=0) {
      if (!real)
        throw CImgInstanceException("CImgList<%s>::FFT(): Empty specified real part.",
                                    pixel_type());
      if (!imag) imag.assign(real._width,real._height,real._depth,real._spectrum,(T)0);
      if (!real.is_sameXYZC(imag))
        throw CImgInstanceException("CImgList<%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);
      cimg::unused(nb_threads);
#ifdef cimg_use_fftw3
      cimg::mutex(12);
#ifndef cimg_use_fftw3_singlethread
      fftw_plan_with_nthreads(nb_threads?nb_threads:cimg::nb_cpus());
#endif
      fftw_complex *data_in = (fftw_complex*)fftw_malloc(sizeof(fftw_complex)*real._width*real._height*real._depth);
      if (!data_in)
        throw CImgInstanceException("CImgList<%s>::FFT(): Failed to allocate memory (%s) "
                                    "for computing FFT of image (%u,%u,%u,%u).",
                                    pixel_type(),
                                    cimg::strbuffersize(sizeof(fftw_complex)*real._width*
                                                        real._height*real._depth*real._spectrum),
                                    real._width,real._height,real._depth,real._spectrum);

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