anchor.diff:      dst = ".cache/xfeatures2d/boostdesc";
     };
   
  +  # See opencv_contrib/modules/face/CMakeLists.txt
  +  face = {
  +    src = fetchFromGitHub {
  +      owner  = "opencv";
  +      repo   = "opencv_3rdparty";
  +      rev    = "8afa57abc8229d611c4937165d20e2a2d9fc5a12";
  +      sha256 = "061lsvqdidq9xa2hwrcvwi9ixflr2c2lfpc8drr159g68zi8bp4v";
  +    };
  +    files = {
  +      "face_landmark_model.dat" = "7505c44ca4eb54b4ab1e4777cb96ac05";
  +    };
  +    dst = ".cache/data";
  +  };
  +
     # See opencv/cmake/OpenCVDownload.cmake
     installExtraFiles = extra : with lib; ''
       mkdir -p "${extra.dst}"
  
positive.code:     ++ lib.optionals (enableFfmpeg && stdenv.isDarwin)
                       [ VideoDecodeAcceleration bzip2 ]
      ++ lib.optionals enableGStreamer (with gst_all_1; [ gstreamer gst-plugins-base ])
      ++ lib.optional enableEigen eigen
      ++ lib.optional enableOpenblas openblas
      # There is seemingly no compile-time flag for Tesseract.  It's
  
negative.code:     # tesseract & leptonica.
      ++ lib.optionals enableTesseract [ tesseract leptonica ]
      ++ lib.optional enableCuda cudatoolkit
      ++ lib.optionals stdenv.isDarwin [ AVFoundation Cocoa QTKit ]
      ++ lib.optionals enableDocs [ doxygen graphviz-nox ];
  
  