fix: WITHOUT_FACERECOGNITION build flag

to avoid double loading of protobuf, implicitly loaded by opencv,
we need to deactivate the build of two face detection plugins that
use opencv (which loads protobuf implicitly). This fixes bug #185

BUILD.md

@@ -10,6 +10,10 @@ The presence of optional libraries on the system will trigger compilation of ext
 
   + [Cairo](http://cairographics.org) required for cairo- filters and mixers
 
+## Optional build flags
+
+  + `-DWITHOUT_FACERECOGNITION=ON` - Disable face recognition plugins (facedetect and facebl0r) to avoid protobuf conflicts with applications like MLT
+
 It is recommended to use a separate `build` sub-folder.
 
 ```
@@ -18,6 +22,13 @@ cd build && cmake ../
 make
 ```
 
+To disable face recognition plugins (recommended when using with MLT):
+```
+mkdir -p build
+cd build && cmake -DWITHOUT_FACERECOGNITION=ON ../
+make
+```
+
 Also ninja and nmake are supported through cmake:
 ```
 cmake -G 'Ninja' ../

CMakeLists.txt

@@ -8,6 +8,8 @@ set (VERSION 2.5.1)
 include(GNUInstallDirs)
 
 option (WITHOUT_OPENCV "Disable plugins dependent upon OpenCV" OFF)
+option (WITHOUT_FACERECOGNITION "Disable facedetect plugin to avoid protobuf conflicts" OFF)
+
 if (NOT WITHOUT_OPENCV)
   find_package (OpenCV)
 endif ()

src/filter/facebl0r/CMakeLists.txt

@@ -1,3 +1,8 @@
+# facebl0r filter - requires OpenCV (and protobuf)
+# This filter can cause conflicts with applications that also use protobuf
+# such as MLT, so it can be disabled with -DWITHOUT_FACERECOGNITION=ON
+
+if (OpenCV_FOUND AND NOT WITHOUT_FACERECOGNITION)
   set (SOURCES facebl0r.cpp)
   set (TARGET facebl0r)
 
@@ -11,3 +16,4 @@ set_target_properties (${TARGET} PROPERTIES PREFIX "")
   target_link_libraries(${TARGET} ${OpenCV_LIBS})
 
   install (TARGETS ${TARGET} LIBRARY DESTINATION ${LIBDIR})
+endif()

src/filter/facedetect/CMakeLists.txt

@@ -1,3 +1,8 @@
+# facedetect filter - requires OpenCV (and protobuf)
+# This filter can cause conflicts with applications that also use protobuf
+# such as MLT, so it can be disabled with -DWITHOUT_FACERECOGNITION=ON
+
+if (OpenCV_FOUND AND NOT WITHOUT_FACERECOGNITION)
   set (SOURCES facedetect.cpp)
   set (TARGET facedetect)
 
@@ -11,3 +16,4 @@ set_target_properties (${TARGET} PROPERTIES PREFIX "")
   target_link_libraries(${TARGET} ${OpenCV_LIBS})
 
   install (TARGETS ${TARGET} LIBRARY DESTINATION ${LIBDIR})
+endif()