Yalantis/uCrop · error · CImgInstanceException

LabtoXYZ(): Instance is not a Lab image.

Error message

LabtoXYZ(): Instance is not a Lab image.

What it means

LabtoXYZ() converts pixel values from CIE Lab back to CIE XYZ and requires the image to hold exactly 3 Lab channels (L, a, b). It throws CImgInstanceException when _spectrum != 3, since it accesses three channel pointers directly. This guards against running the inverse transform on images not produced by XYZtoLab (or hand-built with the right layout).

Source

Thrown at ucrop/src/main/jni/CImg.h:37633

          fX = (Tfloat)_cimg_Labf(X),
          fY = (Tfloat)_cimg_Labf(Y),
          fZ = (Tfloat)_cimg_Labf(Z);
        p1[N] = (T)cimg::cut(116*fY - 16,0,100);
        p2[N] = (T)(500*(fX - fY));
        p3[N] = (T)(200*(fY - fZ));
      }
      return *this;
    }

    //! Convert pixel values from XYZ to Lab color spaces \newinstance.
    CImg<Tfloat> get_XYZtoLab(const bool use_D65=true) const {
      return CImg<Tfloat>(*this,false).XYZtoLab(use_D65);
    }

    //! Convert pixel values from Lab to XYZ color spaces.
    CImg<T>& LabtoXYZ(const bool use_D65=true) {
      if (_spectrum!=3)
        throw CImgInstanceException(_cimg_instance
                                    "LabtoXYZ(): Instance is not a Lab image.",
                                    cimg_instance);
      const CImg<Tfloat> white = CImg<Tfloat>(1,1,1,3,255).RGBtoXYZ(use_D65);
      T *p1 = data(0,0,0,0), *p2 = data(0,0,0,1), *p3 = data(0,0,0,2);
      const longT whd = (longT)width()*height()*depth();
      cimg_pragma_openmp(parallel for cimg_openmp_if_size(whd,128))
      for (longT N = 0; N<whd; ++N) {
        const Tfloat
          L = (Tfloat)p1[N],
          a = (Tfloat)p2[N],
          b = (Tfloat)p3[N],
          cY = (L + 16)/116,
          cZ = cY - b/200,
          cX = a/500 + cY,
          X = (Tfloat)(24389*cX>216?cX*cX*cX:(116*cX - 16)*27/24389),
          Y = (Tfloat)(27*L>216?cY*cY*cY:27*L/24389),
          Z = (Tfloat)(24389*cZ>216?cZ*cZ*cZ:(116*cZ - 16)*27/24389);
        p1[N] = (T)(X*white[0]);

View on GitHub (pinned to f788b534b4)

Solutions

1. Verify img.spectrum()==3 immediately before the call and fix the pipeline so Lab data is actually 3-channel.
2. If the image is still RGB, call RGBtoXYZ() then XYZtoLab() before LabtoXYZ() — LabtoXYZ only reverses XYZtoLab.
3. Drop or split off alpha channels (spectrum==4) before conversion.
4. Guard with try/catch on CImgInstanceException for defensive native code.

Example fix

// before
texture.LabtoXYZ(); // texture is RGBA, spectrum==4 -> throws
// after
CImg<float> lab = texture.get_channels(0,2); // drop alpha
lab.LabtoXYZ();

if (img.spectrum()!=3) { /* repair before LabtoXYZ */ }

template<class T> bool has3Channels(const cimg_library::CImg<T>& i){ return i.spectrum()==3; }

try { img.LabtoXYZ(); } catch (cimg_library::CImgInstanceException& e) { /* recover */ }

When it happens

Trigger: Calling img.LabtoXYZ() on an image with spectrum != 3 — typically a grayscale image, an RGBA image, or an image that was never converted to Lab. Also from get_LabtoXYZ() with the same invalid instance.

Common situations: Undoing a Lab conversion on a copy whose alpha channel was re-added; loading a Lab-looking TIFF with 1 or 4 channels; applying LabtoXYZ to an image still in RGB (forgot RGBtoXYZ first).

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

• XYZtoLab(): Instance is not a XYZ image.
• XYZtoxyY(): Instance is not a XYZ image.
• xyYtoXYZ(): Instance is not a xyY image.
• RGBtoYCbCr(): Instance is not a RGB image.
• YCbCrtoRGB(): Instance is not a YCbCr image.

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/890a540ab1ee8f00. Report an issue: GitHub.