Yalantis/uCrop · error · CImgIOException

save_webp(): Failed to encode image to file

Error message

save_webp(): Failed to encode image to file '%s'.

What it means

After WebPEncodeRGB/WebPEncodeRGBA returns, CImg checks whether the encoder produced any data (imgData is null). A null buffer means libwebp failed to encode the image (e.g. invalid dimensions, out of memory, or unsupported pixel setup), so CImg throws CImgIOException before writing the file.

Solutions

1. Verify _width>0 and _height>0 before saving; a zero-dimension image cannot be encoded.
2. Check available memory and image size; reduce image dimensions (resize) before WebP encoding.
3. Lower the quality parameter is unlikely to help; instead confirm the webp build is intact and dimensions are sane, then retry or use another format (save_png).

Example fix

// before
img.save_webp("huge.webp");
// after
if (img.width() == 0 || img.height() == 0) throw std::runtime_error("empty image");
CImg<float> small = img.get_resize(4096, 4096, -100, -100);
small.save_webp("huge.webp");

if (img.is_empty() || img.width() == 0 || img.height() == 0) {
  throw std::runtime_error("cannot save empty image as webp");
}

try {
  img.save_webp("out.webp");
} catch (const cimg_library::CImgIOException &e) {
  std::fprintf(stderr, "webp encode failed: %s\n", e.what());
  img.save_png("out.png"); // fallback format
}

When it happens

Trigger: Calling save_webp() when WebPEncodeRGB/RGBA returns size==0 and imgData==nullptr — typically from enormous width*height (memory allocation failure in libwebp) or invalid image dimensions.

Common situations: Saving extremely large images that exceed memory limits, images with zero width/height, or environments where the webp encoder cannot allocate the output buffer. Sometimes seen with corrupted/undimensioned image instances.

Understand the failure class

Background: "failed to write file", "Could not save figure", "Error saving remote file" — file write failed: causes and fixes across languages and libraries — this error's family across 38 libraries.

Related errors

• save_webp(): WebP only supports (A)RGB colorspace.
• load_webp(): Does not support animated WebP
• load_webp(): Failed to decode image
• save_jpeg(): Specified filename is (null).
• save_webp(): Specified filename is (null).

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/ebd9adc7849b9ae2. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:61627

      uint8_t *ptr = rgbaBuffer._data;
      cimg_forY(*this,y) {
        cimg_forX(*this,x) {
          *(ptr++) = (T)*(ptr_r++);
          *(ptr++) = (T)*(ptr_g++);
          *(ptr++) = (T)*(ptr_b++);
          if (ptr_a) *(ptr++) = (T)*(ptr_a++);
        }
      }
      uint8_t *imgData = NULL;
      const int stride = _width*_spectrum*sizeof(uint8_t);
      size_t size = 0;
      if (_spectrum==3) {
        size = WebPEncodeRGB(rgbaBuffer._data, _width, _height, stride, (float)quality, &imgData);
      } else {
        size = WebPEncodeRGBA(rgbaBuffer._data, _width, _height, stride, (float)quality, &imgData);
      }
      if (!imgData) {
        throw CImgIOException(_cimg_instance
                              "save_webp(): Failed to encode image to file '%s'.",
                              cimg_instance,
                              filename);
      }
      cimg::fwrite(imgData, size, file);
      cimg::fclose(file);
      WebPFree(imgData);
      return *this;
#endif
    }

    //! Save image as a JPEG file.
    /**
      \param filename Filename, as a C-string.
      \param quality Image quality (in %)
    **/
    const CImg<T>& save_jpeg(const char *const filename, const unsigned int quality=100) const {
      return _save_jpeg(0,filename,quality);

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