anchor.diff:  , enableFfmpeg    ? false, ffmpeg
   , enableGStreamer ? false, gst_all_1
   , enableTesseract ? false, tesseract, leptonica
  +, enableTbb       ? false, tbb
   , enableOvis      ? false, ogre
   , enableGPhoto2   ? false, libgphoto2
   , enableDC1394    ? false, libdc1394
  
positive.code:     (opencvFlag "OPENEXR" enableEXR)
      (opencvFlag "CUDA" enableCuda)
      (opencvFlag "CUBLAS" enableCuda)
    ] ++ lib.optionals enableCuda [
      "-DCUDA_FAST_MATH=ON"
      "-DCUDA_HOST_COMPILER=${cudatoolkit.cc}/bin/cc"
  
negative.code:     # simply enabled automatically if contrib is built, and it detects
      # tesseract & leptonica.
      ++ lib.optionals enableTesseract [ tesseract leptonica ]
      ++ lib.optional enableTbb tbb
      ++ lib.optional enableCuda cudatoolkit
      ++ lib.optionals stdenv.isDarwin [ AVFoundation Cocoa QTKit VideoDecodeAcceleration bzip2 ]
      ++ lib.optionals enableDocs [ doxygen graphviz-nox ];
  