Yalantis/uCrop · warning

save_cimg(): Unable to save compressed data in file '%s' unl

Error message

save_cimg(): Unable to save compressed data in file '%s' unless zlib is enabled, saving them uncompressed.

What it means

CImgList<T>::save_cimg() with is_compressed set warns when the library was built without zlib (cimg_use_zlib undefined): compressed .cimgz output cannot be produced, so the data is written uncompressed instead. The file is still saved, but larger and with a different layout than a genuinely compressed .cimgz.

Source

Thrown at ucrop/src/main/jni/CImg.h:68153

    }

    //! Save list into a .cimg file.
    /**
       \param filename Filename to write data to.
       \param is_compressed Tells if data compression must be enabled.
    **/
    const CImgList<T>& save_cimg(const char *const filename, const bool is_compressed=false) const {
      return _save_cimg(0,filename,is_compressed);
    }

    const CImgList<T>& _save_cimg(std::FILE *const file, const char *const filename, const bool is_compressed) const {
      if (!file && !filename)
        throw CImgArgumentException(_cimglist_instance
                                    "save_cimg(): Specified filename is (null).",
                                    cimglist_instance);
#ifndef cimg_use_zlib
      if (is_compressed)
        cimg::warn(_cimglist_instance
                   "save_cimg(): Unable to save compressed data in file '%s' unless zlib is enabled, "
                   "saving them uncompressed.",
                   cimglist_instance,
                   filename?filename:"(FILE*)");
#endif
      const char *const ptype = pixel_type(), *const etype = cimg::endianness()?"big":"little";
      std::FILE *const nfile = file?file:cimg::fopen(filename,"wb");
      const bool is_bool = ptype==cimg::type<bool>::string();
      std::fprintf(nfile,"%u %s %s_endian\n",_width,ptype,etype);

      cimglist_for(*this,l) {
        const CImg<T>& img = _data[l];
        std::fprintf(nfile,"%u %u %u %u",img._width,img._height,img._depth,img._spectrum);
        if (img._data) {
          CImg<T> tmp;
          if (cimg::endianness()) { tmp = img; cimg::invert_endianness(tmp._data,tmp.size()); }
          const CImg<T>& ref = cimg::endianness()?tmp:img;
          bool failed_to_compress = true;

View on GitHub (pinned to f788b534b4)

Solutions

1. Recompile defining cimg_use_zlib and link zlib (-lz, install zlib dev package)
2. Save uncompressed: pass is_compressed=false or use the .cimg extension
3. Read back accordingly — a file saved this way is uncompressed despite any .cimgz naming

Example fix

// before
images.save_cimg("out.cimgz", true); // warns without zlib
// after
#ifdef cimg_use_zlib
images.save_cimg("out.cimgz", true);
#else
images.save_cimg("out.cimg", false);
#endif

#ifdef cimg_use_zlib
images.save_cimg("out.cimgz", true);
#else
images.save_cimg("out.cimg", false);
#endif

When it happens

Trigger: Calling save_cimg(filename, ..., is_compressed=true) (or saving with a .cimgz filename requesting compression) in a build without cimg_use_zlib.

Common situations: Default header-only CImg builds without zlib linked; porting code that used a zlib-enabled build to one without; toolchains missing zlib dev headers so cimg_use_zlib was never defined.

Related errors

• load_cimg(): Unable to load compressed data from file '%s' u
• save_cimg(): Failed to save compressed data for file '%s', s
• get_serialize(): Failed to save compressed data, saving them
• load_bmp(): Unable to load compressed data from '(*FILE)' in
• save_jxl(): Failed to set lossy compression level when savin

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/ee6c66cc028afa18. Report an issue: GitHub.