Yalantis/uCrop · warning

save_magick(): Instance is multispectral, only the three…

Error message

save_magick(): Instance is multispectral, only the three first channels will be saved in file '%s'.

What it means

Warning from CImg<T>::save_magick() when the image has more than 3 channels (_spectrum>3). The Magick::Image created here is TrueColor RGB (3 channels), so channels beyond the first three (e.g. alpha plus extra bands, multispectral data) are dropped. The save still completes.

Solutions

  1. Reduce to RGB explicitly before saving (e.g. get_channels(0,2)) or extract the bands you need
  2. Save extra channels separately as individual images
  3. Use a format/backend that supports N channels (save_tiff, or .cimg)
  4. Ignore if dropping alpha/extra bands is acceptable

Example fix

// before
rgba_hyperspectral.save_magick("out.png"); // channels 3+ dropped
// after
rgba_hyperspectral.get_channels(0,2).save_magick("out.png");
Defensive patterns

Strategy: validation

Validate before calling

if (img.spectrum() > 3) img = img.get_channels(0,2); // then save_magick

Try / catch

// warning only; guard by spectrum check before calling save_magick

Prevention

When it happens

Trigger: Calling save_magick() on an image whose _spectrum exceeds 3, such as 4-channel RGBA-plus data or N-band hyperspectral images.

Common situations: Saving remote-sensing/hyperspectral imagery, or RGBA images where a 4th channel (alpha) is silently dropped in TrueColorType mode.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/5a065fe08d564da4. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:61990

      \param bytes_per_pixel Force the number of bytes per pixel for the saving, when possible.
    **/
    const CImg<T>& save_magick(const char *const filename, const unsigned int bytes_per_pixel=0) const {
      if (!filename)
        throw CImgArgumentException(_cimg_instance
                                    "save_magick(): Specified filename is (null).",
                                    cimg_instance);
      if (is_empty()) { cimg::fempty(0,filename); return *this; }

#ifdef cimg_use_magick
      double stmin, stmax = (double)max_min(stmin);
      if (_depth>1)
        cimg::warn(_cimg_instance
                   "save_magick(): Instance is volumetric, only the first slice will be saved in file '%s'.",
                   cimg_instance,
                   filename);

      if (_spectrum>3)
        cimg::warn(_cimg_instance
                   "save_magick(): Instance is multispectral, only the three first channels will be "
                   "saved in file '%s'.",
                   cimg_instance,
                   filename);

      if (stmin<0 || (bytes_per_pixel==1 && stmax>=256) || stmax>=65536)
        cimg::warn(_cimg_instance
                   "save_magick(): Instance has pixel values in [%g,%g], probable type overflow in file '%s'.",
                   cimg_instance,
                   stmin,stmax,filename);

      Magick::Image image(Magick::Geometry(_width,_height),"black");
      image.type(Magick::TrueColorType);
      image.depth(bytes_per_pixel?(8*bytes_per_pixel):(stmax>=256?16:8));
      const T
        *ptr_r = data(0,0,0,0),
        *ptr_g = _spectrum>1?data(0,0,0,1):0,
        *ptr_b = _spectrum>2?data(0,0,0,2):0;

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