What causes "save_video(): Frame [0] is an empty image." in Yalantis/uCrop?

Validate frames[0].width()>0 && frames[0].height()>0 before saving

Yalantis/uCrop · error · CImgInstanceException

save_video(): Frame [0] is an empty image.

Error message

save_video(): Frame [0] is an empty image.

What it means

Guard in CImgList<T>::save_video(): the first frame of the list (element [0]) is an empty image; the video writer needs valid frame dimensions derived from frame [0] to initialize the stream, so an empty first frame aborts saving.

Solutions

Validate frames[0].width()>0 && frames[0].height()>0 before saving
Skip or replace empty frames when populating the list
Only append frames after confirming the capture succeeded

Example fix

// before
frames.push_back(CImg<unsigned char>()); // empty placeholder
frames.save_video("out.avi", 25);
// after
if (!frame.is_empty()) frames.push_back(frame);
frames.save_video("out.avi", 25);

Defensive patterns

Strategy: validation

Validate before calling

// C++
bool valid = !frames.is_empty() && frames[0].width() > 0 && frames[0].height() > 0;
if (!valid) return;

Type guard

bool hasSizedFrame(const CImgList<T>& l) { return !l.is_empty() && l[0].width() > 0 && l[0].height() > 0; }

Try / catch

try { frames.save_video(fname, fps, codec); }
catch (CImgInstanceException& e) { log("first frame empty, skipping save"); }

Prevention

Never append default-constructed CImg placeholders
Drop frames where width()==0 || height()==0 at capture time
Assert frame dimensions when appending
Validate capture device output before recording

When it happens

Trigger: Calling save_video() where the first frame in the list is a 0-width or 0-height image (default-constructed CImg, or failed frame allocation).

Common situations: Appending default-constructed CImg placeholders into the list; camera delivering an empty first frame before streaming starts.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/d6e3ba4278f31da7. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:68584

        if (index<0) {
          if (!filename)
            throw CImgArgumentException(_cimglist_instance
                                        "save_video(): No already open video writer found. You must specify a "
                                        "non-(null) filename argument for the first call.",
                                        cimglist_instance);
          else { cimg::mutex(9); cimglist_for(filenames,l) if (!filenames[l]) { index = l; break; } cimg::mutex(9,0); }
          if (index<0)
            throw CImgIOException(_cimglist_instance
                                  "save_video(): File '%s', no video writer slots available. "
                                  "You have to release some of your previously opened videos.",
                                  cimglist_instance,filename);
          if (is_empty())
            throw CImgInstanceException(_cimglist_instance
                                        "save_video(): Instance list is empty.",
                                        cimglist_instance);
          const unsigned int W = _data?_data[0]._width:0, H = _data?_data[0]._height:0;
          if (!W || !H)
            throw CImgInstanceException(_cimglist_instance
                                        "save_video(): Frame [0] is an empty image.",
                                        cimglist_instance);
          const char
            *const _codec = codec && *codec?codec:"h264",
            codec0 = cimg::uppercase(_codec[0]),
            codec1 = _codec[0]?cimg::uppercase(_codec[1]):0,
            codec2 = _codec[1]?cimg::uppercase(_codec[2]):0,
            codec3 = _codec[2]?cimg::uppercase(_codec[3]):0;
          cimg::mutex(9);
          writers[index] = new cv::VideoWriter(filename,_cimg_fourcc(codec0,codec1,codec2,codec3),fps,cv::Size(W,H));
          if (!writers[index]->isOpened()) {
            delete writers[index];
            writers[index] = 0;
            cimg::mutex(9,0);
            throw CImgIOException(_cimglist_instance
                                  "save_video(): File '%s', unable to initialize video writer with codec '%c%c%c%c'.",
                                  cimglist_instance,filename,
                                  codec0,codec1,codec2,codec3);

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