Yalantis/uCrop · error · CImgInstanceException

CMYKtoCMY(): Instance is not a CMYK image.

Error message

CMYKtoCMY(): Instance is not a CMYK image.

What it means

get_CMYKtoCMY() converts a 4-channel CMYK image to a new 3-channel CMY image; it requires _spectrum==4. CImg throws CImgInstanceException when the image does not have four channels, since it reads four source planes including the K (black) plane.

Solutions

1. Guard with if (img.spectrum()==4) before calling.
2. For RGB input, call RGBtoCMY() instead of CMYKtoCMY().
3. Verify the source really is CMYK from the loader/ICC profile, not just channel count.
4. Catch CImgInstanceException and route by spectrum.

Example fix

// before
img.CMYKtoCMY(); // img is RGB, spectrum=3
// after
if (img.spectrum()==4) img.CMYKtoCMY();
else if (img.spectrum()==3) img.RGBtoCMY();

if (img.spectrum()!=4) throw std::runtime_error("CMYKtoCMY requires 4-channel CMYK image");
CImg<float> cmy = img.get_CMYKtoCMY();

bool isCMYK(const cimg_library::CImg<T>& img) { return img.spectrum()==4; }

try {
  img.CMYKtoCMY();
} catch (const cimg_library::CImgInstanceException& e) {
  std::cerr << "Need CMYK (4 channels), got " << img.spectrum() << std::endl;
}

When it happens

Trigger: Calling img.CMYKtoCMY() or img.get_CMYKtoCMY() on RGB (3-channel), grayscale, or empty images.

Common situations: Mixed print pipelines where RGB files reach a CMYK-only code path; images whose alpha or extra channel made the count 3 or 5; incorrect loading order.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• CMYtoCMYK(): Instance is not a CMY image.
• CMYtoRGB(): Instance is not a CMY image.
• RGBtoCMY(): Instance is not a RGB image.
• RGBtoXYZ(): Instance is not a RGB image.
• RGBtoYCbCr(): Instance is not a RGB image.

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/fbbd743181d890c6. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:37500

        if (K>=255) C = M = Y = 0;
        else { const Tfloat K1 = 255 - K; C = 255*(C - K)/K1; M = 255*(M - K)/K1; Y = 255*(Y - K)/K1; }
        pd1[N] = (Tfloat)cimg::cut(C,0,255),
        pd2[N] = (Tfloat)cimg::cut(M,0,255),
        pd3[N] = (Tfloat)cimg::cut(Y,0,255),
        pd4[N] = (Tfloat)cimg::cut(K,0,255);
      }
      return res;
    }

    //! Convert pixel values from CMYK to CMY color spaces.
    CImg<T>& CMYKtoCMY() {
      return get_CMYKtoCMY().move_to(*this);
    }

    //! Convert pixel values from CMYK to CMY color spaces \newinstance.
    CImg<Tfloat> get_CMYKtoCMY() const {
      if (_spectrum!=4)
        throw CImgInstanceException(_cimg_instance
                                    "CMYKtoCMY(): Instance is not a CMYK image.",
                                    cimg_instance);

      CImg<Tfloat> res(_width,_height,_depth,3);
      const T *ps1 = data(0,0,0,0), *ps2 = data(0,0,0,1), *ps3 = data(0,0,0,2), *ps4 = data(0,0,0,3);
      Tfloat *pd1 = res.data(0,0,0,0), *pd2 = res.data(0,0,0,1), *pd3 = res.data(0,0,0,2);
      const longT whd = (longT)width()*height()*depth();
      cimg_pragma_openmp(parallel for cimg_openmp_if_size(whd,1024))
      for (longT N = 0; N<whd; ++N) {
        const Tfloat
          C = (Tfloat)ps1[N],
          M = (Tfloat)ps2[N],
          Y = (Tfloat)ps3[N],
          K = (Tfloat)ps4[N],
          K1 = 1 - K/255,
          nC = C*K1 + K,
          nM = M*K1 + K,
          nY = Y*K1 + K;

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