Yalantis/uCrop · error · CImgIOException

save_video(): File '%s', unable to initialize video writer w

Error message

save_video(): File '%s', unable to initialize video writer with codec '%c%c%c%c'.

What it means

After constructing cv::VideoWriter with the requested codec fourcc, fps and frame size, save_video() checks isOpened(). OpenCV failing to open the writer causes this IO exception listing the filename and codec characters.

Source

Thrown at ucrop/src/main/jni/CImg.h:68599

                                        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);
          }
          CImg<charT>::string(filename).move_to(filenames[index]);
          sizes(index,0) = W;
          sizes(index,1) = H;
          cimg::mutex(9,0);
        }

        if (!is_empty()) {
          const unsigned int W = sizes(index,0), H = sizes(index,1);
          cimg::mutex(9);
          cimglist_for(*this,l) {
            CImg<T> &src = _data[l];
            if (src.is_empty())
              cimg::warn(_cimglist_instance
                         "save_video(): Skip empty frame %d for file '%s'.",

View on GitHub (pinned to f788b534b4)

Solutions

Use a codec the OpenCV backend supports (e.g. 'mp4v' with .mp4, 'MJPG' with .avi) or install OpenH264/ffmpeg for h264
Verify the output directory exists and is writable
Check fps>0 and that WxH match the actual frame size
Test writing a tiny video with cv::VideoWriter directly to isolate backend issues

Example fix

// before
frames.save_video("out.mp4", 25, "h264"); // fails without H264 encoder
// after
frames.save_video("out.mp4", 25, "mp4v"); // widely supported fourcc

Defensive patterns

Strategy: fallback

Validate before calling

// C++
std::string ext = extension_of(fname);
bool okPair = (codec == "mp4v" && ext == ".mp4") || (codec == "MJPG" && ext == ".avi");
if (!okPair) codec = defaultCodecFor(ext);

Try / catch

try { frames.save_video(fname, fps, "h264"); }
catch (CImgIOException& e) { frames.save_video(fname, fps, "mp4v"); }

Prevention

Use fourcc/extension combos known to work (MJPG+.avi, mp4v+.mp4)
Ensure the OpenCV build has a video backend and H264 support if needed
Pass fps>0 and correct WxH
Test writer initialization once at app startup

When it happens

Trigger: cv::VideoWriter cannot open: unsupported codec fourcc for the extension (e.g. 'h264' without OpenH264/ffmpeg backend), invalid fps, unsupported resolution, unwritable/invalid output path, or missing OpenCV video backend.

Common situations: Using 'h264' codec with a stock OpenCV build lacking H264 encoder support; mismatched extension/codec (e.g. .avi with mp4v unavailable); fps of 0; target directory missing.

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/fcab1ac2873a066f. Report an issue: GitHub.