Yalantis/uCrop · error · CImgInstanceException

RGBtoHSI(): Instance is not a RGB image.

Error message

RGBtoHSI(): Instance is not a RGB image.

What it means

CImg<T>::RGBtoHSI() converts pixel values from the RGB color space to HSI and requires exactly 3 channels (_spectrum == 3), one each for R, G and B. If the image has a different channel count the conversion is meaningless, so it throws CImgInstanceException. Note it checks only channel count — it cannot verify the data actually is RGB, so calling it on a 3-channel BGR or Lab image produces wrong colors, not an exception.

Source

Thrown at ucrop/src/main/jni/CImg.h:37068

      cimg_pragma_openmp(parallel for cimg_openmp_if_size(size(),32))
      cimg_rofoff(*this,off) {
        const Tfloat
          val = (Tfloat)_data[off]/255,
          sval = (Tfloat)(val<=0.0031308f?val*12.92f:1.055f*std::pow(val,0.416667f) - 0.055f);
        _data[off] = (T)cimg::cut(sval*255,0,255);
      }
      return *this;
    }

    //! Convert pixel values from RGB to sRGB color spaces \newinstance.
    CImg<Tfloat> get_RGBtosRGB() const {
      return CImg<Tfloat>(*this,false).RGBtosRGB();
    }

    //! Convert pixel values from RGB to HSI color spaces.
    CImg<T>& RGBtoHSI() {
      if (_spectrum!=3)
        throw CImgInstanceException(_cimg_instance
                                    "RGBtoHSI(): Instance is not a RGB image.",
                                    cimg_instance);

      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,256))
      for (longT N = 0; N<whd; ++N) {
        const Tfloat
          R = (Tfloat)p1[N],
          G = (Tfloat)p2[N],
          B = (Tfloat)p3[N],
          m = cimg::min(R,G,B),
          M = cimg::max(R,G,B),
          C = M - m,
          sum = R + G + B,
          H = 60*(C==0?0:M==R?cimg::mod((G - B)/C,(Tfloat)6):M==G?(B - R)/C + 2:(R - G)/C + 4),
          S = sum<=0?0:1 - 3*m/sum,
          I = sum/(3*255);

View on GitHub (pinned to f788b534b4)

Solutions

1. Check img.spectrum() == 3 before the call; if grayscale, replicate channels (e.g. img.get_resize(..., -100, -100, -100, 3) or construct a 3-channel copy).
2. If RGBA, drop the alpha channel with img.get_channels(0,2) first.
3. Then call RGBtoHSI on the 3-channel result.
4. Audit the load path to force 3-channel loading where possible.

Example fix

// before
CImg<unsigned char> img("photo.png"); // RGBA, spectrum=4
img.RGBtoHSI(); // throws
// after
CImg<unsigned char> img("photo.png");
if (img.spectrum() == 4) img.channels(0,2);
if (img.spectrum() == 1) { unsigned char g = img(0,0,0,0); img.resize(-100,-100,-100,3); } // replicate as needed
img.RGBtoHSI();

bool isRgbLike(const CImg<T>& img) { return img.spectrum() == 3; }
template<typename T> bool isRgbLike(const CImg<T>& img) { return img.spectrum() == 3; }
if (isRgbLike(img)) img.RGBtoHSI();

template<typename T> bool isRgb(const CImg<T>& img) { return img.spectrum() == 3; }

try {
  img.RGBtoHSI();
} catch (const CImgInstanceException& e) {
  std::fprintf(stderr, "RGBtoHSI requires 3 channels, image has %d\n", img.spectrum());
}

When it happens

Trigger: Calling img.RGBtoHSI() on a grayscale image (spectrum=1), an RGBA image (spectrum=4), or any image whose spectrum != 3.

Common situations: Grayscale-loaded photos or masks that were never converted to RGB; RGBA images loaded from PNG with alpha and not reduced to 3 channels; assuming a loaded image is RGB without checking channels().

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

• HSItoRGB(): Instance is not a HSI image.
• RGBtoHSL(): Instance is not a RGB image.
• HSLtoRGB(): Instance is not a HSL image.
• RGBtoHSV(): Instance is not a RGB image.
• HSVtoRGB(): Instance is not a HSV image.

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/6ed6eb957850af06. Report an issue: GitHub.