save_png(): Instance is multispectral, only the three first

Error message

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

What it means

Warning from CImg<T>::save_png() when the image has more than 4 channels (_spectrum>4). PNG supports at most 4 channels (RGBA), so channels beyond the first four are dropped from the output. The save continues normally.

Source

Thrown at ucrop/src/main/jni/CImg.h:62091

#if defined __GNUC__
      const char *volatile nfilename = filename; // Use 'volatile' to avoid (wrong) g++ warning
      std::FILE *volatile nfile = file?file:cimg::fopen(nfilename,"wb");
      volatile double stmin, stmax = (double)max_min(stmin);
#else
      const char *nfilename = filename;
      std::FILE *nfile = file?file:cimg::fopen(nfilename,"wb");
      double stmin, stmax = (double)max_min(stmin);
#endif

      if (_depth>1)
        cimg::warn(_cimg_instance
                   "save_png(): Instance is volumetric, only the first slice will be saved in file '%s'.",
                   cimg_instance,
                   filename);

      if (_spectrum>4)
        cimg::warn(_cimg_instance
                   "save_png(): 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_png(): Instance has pixel values in [%g,%g], probable type overflow in file '%s'.",
                   cimg_instance,
                   stmin,stmax,filename);

      // Setup PNG structures for write.
      png_voidp user_error_ptr = 0;
      png_error_ptr user_error_fn = 0, user_warning_fn = 0;
      png_structp png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,user_error_ptr, user_error_fn,
                                                    user_warning_fn);
      if (!png_ptr){
        if (!file) cimg::fclose(nfile);
        throw CImgIOException(_cimg_instance

View on GitHub (pinned to f788b534b4)

Solutions

Select the channels to keep before saving: img.get_channels(0,3)
Save extra channels as separate PNG images
Use a format supporting arbitrary channel counts (save_tiff, .cimg, PFM)
Ignore if the first four channels are all you need

Example fix

// before
multispectral.save_png("out.png"); // bands 5+ dropped
// after
multispectral.get_channels(0,3).save_png("out.png");

Defensive patterns

Strategy: validation

Validate before calling

if (img.spectrum() > 4) img = img.get_channels(0,3); // then save_png

Try / catch

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

Prevention

Check img.spectrum() before saving to PNG
PNG supports at most 4 channels; use TIFF for more

When it happens

Trigger: Calling save_png() on an image with _spectrum>4, e.g. multispectral/hyperspectral imagery with many bands, or a tensor whose channel dimension was repurposed.

Common situations: Saving 5+ band satellite imagery or scientific channel stacks to PNG, where all bands beyond RGBA silently disappear.

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/7058e1c0c2e13a94. Report an issue: GitHub.